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| author | Mike Rapoport <rppt@linux.ibm.com> | 2022-06-27 09:00:26 +0300 |
|---|---|---|
| committer | akpm <akpm@linux-foundation.org> | 2022-06-27 12:52:53 -0700 |
| commit | ee65728e103bb7dd99d8604bf6c7aa89c7d7e446 (patch) | |
| tree | 356a37c67d23c69cf8de83120d08048276cb5bfc /Documentation/mm | |
| parent | 46a3b1125308f8f90a065eeecfafd2a96b01a36c (diff) | |
| download | linux-ee65728e103bb7dd99d8604bf6c7aa89c7d7e446.tar.bz2 | |
docs: rename Documentation/vm to Documentation/mm
so it will be consistent with code mm directory and with
Documentation/admin-guide/mm and won't be confused with virtual machines.
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Suggested-by: Matthew Wilcox <willy@infradead.org>
Tested-by: Ira Weiny <ira.weiny@intel.com>
Acked-by: Jonathan Corbet <corbet@lwn.net>
Acked-by: Wu XiangCheng <bobwxc@email.cn>
Diffstat (limited to 'Documentation/mm')
44 files changed, 5561 insertions, 0 deletions
diff --git a/Documentation/mm/active_mm.rst b/Documentation/mm/active_mm.rst new file mode 100644 index 000000000000..6f8269c284ed --- /dev/null +++ b/Documentation/mm/active_mm.rst @@ -0,0 +1,91 @@ +.. _active_mm: + +========= +Active MM +========= + +:: + + List: linux-kernel + Subject: Re: active_mm + From: Linus Torvalds <torvalds () transmeta ! com> + Date: 1999-07-30 21:36:24 + + Cc'd to linux-kernel, because I don't write explanations all that often, + and when I do I feel better about more people reading them. + + On Fri, 30 Jul 1999, David Mosberger wrote: + > + > Is there a brief description someplace on how "mm" vs. "active_mm" in + > the task_struct are supposed to be used? (My apologies if this was + > discussed on the mailing lists---I just returned from vacation and + > wasn't able to follow linux-kernel for a while). + + Basically, the new setup is: + + - we have "real address spaces" and "anonymous address spaces". The + difference is that an anonymous address space doesn't care about the + user-level page tables at all, so when we do a context switch into an + anonymous address space we just leave the previous address space + active. + + The obvious use for a "anonymous address space" is any thread that + doesn't need any user mappings - all kernel threads basically fall into + this category, but even "real" threads can temporarily say that for + some amount of time they are not going to be interested in user space, + and that the scheduler might as well try to avoid wasting time on + switching the VM state around. Currently only the old-style bdflush + sync does that. + + - "tsk->mm" points to the "real address space". For an anonymous process, + tsk->mm will be NULL, for the logical reason that an anonymous process + really doesn't _have_ a real address space at all. + + - however, we obviously need to keep track of which address space we + "stole" for such an anonymous user. For that, we have "tsk->active_mm", + which shows what the currently active address space is. + + The rule is that for a process with a real address space (ie tsk->mm is + non-NULL) the active_mm obviously always has to be the same as the real + one. + + For a anonymous process, tsk->mm == NULL, and tsk->active_mm is the + "borrowed" mm while the anonymous process is running. When the + anonymous process gets scheduled away, the borrowed address space is + returned and cleared. + + To support all that, the "struct mm_struct" now has two counters: a + "mm_users" counter that is how many "real address space users" there are, + and a "mm_count" counter that is the number of "lazy" users (ie anonymous + users) plus one if there are any real users. + + Usually there is at least one real user, but it could be that the real + user exited on another CPU while a lazy user was still active, so you do + actually get cases where you have a address space that is _only_ used by + lazy users. That is often a short-lived state, because once that thread + gets scheduled away in favour of a real thread, the "zombie" mm gets + released because "mm_count" becomes zero. + + Also, a new rule is that _nobody_ ever has "init_mm" as a real MM any + more. "init_mm" should be considered just a "lazy context when no other + context is available", and in fact it is mainly used just at bootup when + no real VM has yet been created. So code that used to check + + if (current->mm == &init_mm) + + should generally just do + + if (!current->mm) + + instead (which makes more sense anyway - the test is basically one of "do + we have a user context", and is generally done by the page fault handler + and things like that). + + Anyway, I put a pre-patch-2.3.13-1 on ftp.kernel.org just a moment ago, + because it slightly changes the interfaces to accommodate the alpha (who + would have thought it, but the alpha actually ends up having one of the + ugliest context switch codes - unlike the other architectures where the MM + and register state is separate, the alpha PALcode joins the two, and you + need to switch both together). + + (From http://marc.info/?l=linux-kernel&m=93337278602211&w=2) diff --git a/Documentation/mm/arch_pgtable_helpers.rst b/Documentation/mm/arch_pgtable_helpers.rst new file mode 100644 index 000000000000..cbaee9e59241 --- /dev/null +++ b/Documentation/mm/arch_pgtable_helpers.rst @@ -0,0 +1,260 @@ +.. SPDX-License-Identifier: GPL-2.0 + +.. _arch_page_table_helpers: + +=============================== +Architecture Page Table Helpers +=============================== + +Generic MM expects architectures (with MMU) to provide helpers to create, access +and modify page table entries at various level for different memory functions. +These page table helpers need to conform to a common semantics across platforms. +Following tables describe the expected semantics which can also be tested during +boot via CONFIG_DEBUG_VM_PGTABLE option. All future changes in here or the debug +test need to be in sync. + + +PTE Page Table Helpers +====================== + ++---------------------------+--------------------------------------------------+ +| pte_same | Tests whether both PTE entries are the same | ++---------------------------+--------------------------------------------------+ +| pte_bad | Tests a non-table mapped PTE | ++---------------------------+--------------------------------------------------+ +| pte_present | Tests a valid mapped PTE | ++---------------------------+--------------------------------------------------+ +| pte_young | Tests a young PTE | ++---------------------------+--------------------------------------------------+ +| pte_dirty | Tests a dirty PTE | ++---------------------------+--------------------------------------------------+ +| pte_write | Tests a writable PTE | ++---------------------------+--------------------------------------------------+ +| pte_special | Tests a special PTE | ++---------------------------+--------------------------------------------------+ +| pte_protnone | Tests a PROT_NONE PTE | ++---------------------------+--------------------------------------------------+ +| pte_devmap | Tests a ZONE_DEVICE mapped PTE | ++---------------------------+--------------------------------------------------+ +| pte_soft_dirty | Tests a soft dirty PTE | ++---------------------------+--------------------------------------------------+ +| pte_swp_soft_dirty | Tests a soft dirty swapped PTE | ++---------------------------+--------------------------------------------------+ +| pte_mkyoung | Creates a young PTE | ++---------------------------+--------------------------------------------------+ +| pte_mkold | Creates an old PTE | ++---------------------------+--------------------------------------------------+ +| pte_mkdirty | Creates a dirty PTE | ++---------------------------+--------------------------------------------------+ +| pte_mkclean | Creates a clean PTE | ++---------------------------+--------------------------------------------------+ +| pte_mkwrite | Creates a writable PTE | ++---------------------------+--------------------------------------------------+ +| pte_wrprotect | Creates a write protected PTE | ++---------------------------+--------------------------------------------------+ +| pte_mkspecial | Creates a special PTE | ++---------------------------+--------------------------------------------------+ +| pte_mkdevmap | Creates a ZONE_DEVICE mapped PTE | ++---------------------------+--------------------------------------------------+ +| pte_mksoft_dirty | Creates a soft dirty PTE | ++---------------------------+--------------------------------------------------+ +| pte_clear_soft_dirty | Clears a soft dirty PTE | ++---------------------------+--------------------------------------------------+ +| pte_swp_mksoft_dirty | Creates a soft dirty swapped PTE | ++---------------------------+--------------------------------------------------+ +| pte_swp_clear_soft_dirty | Clears a soft dirty swapped PTE | ++---------------------------+--------------------------------------------------+ +| pte_mknotpresent | Invalidates a mapped PTE | ++---------------------------+--------------------------------------------------+ +| ptep_clear | Clears a PTE | ++---------------------------+--------------------------------------------------+ +| ptep_get_and_clear | Clears and returns PTE | ++---------------------------+--------------------------------------------------+ +| ptep_get_and_clear_full | Clears and returns PTE (batched PTE unmap) | ++---------------------------+--------------------------------------------------+ +| ptep_test_and_clear_young | Clears young from a PTE | ++---------------------------+--------------------------------------------------+ +| ptep_set_wrprotect | Converts into a write protected PTE | ++---------------------------+--------------------------------------------------+ +| ptep_set_access_flags | Converts into a more permissive PTE | ++---------------------------+--------------------------------------------------+ + + +PMD Page Table Helpers +====================== + ++---------------------------+--------------------------------------------------+ +| pmd_same | Tests whether both PMD entries are the same | ++---------------------------+--------------------------------------------------+ +| pmd_bad | Tests a non-table mapped PMD | ++---------------------------+--------------------------------------------------+ +| pmd_leaf | Tests a leaf mapped PMD | ++---------------------------+--------------------------------------------------+ +| pmd_huge | Tests a HugeTLB mapped PMD | ++---------------------------+--------------------------------------------------+ +| pmd_trans_huge | Tests a Transparent Huge Page (THP) at PMD | ++---------------------------+--------------------------------------------------+ +| pmd_present | Tests a valid mapped PMD | ++---------------------------+--------------------------------------------------+ +| pmd_young | Tests a young PMD | ++---------------------------+--------------------------------------------------+ +| pmd_dirty | Tests a dirty PMD | ++---------------------------+--------------------------------------------------+ +| pmd_write | Tests a writable PMD | ++---------------------------+--------------------------------------------------+ +| pmd_special | Tests a special PMD | ++---------------------------+--------------------------------------------------+ +| pmd_protnone | Tests a PROT_NONE PMD | ++---------------------------+--------------------------------------------------+ +| pmd_devmap | Tests a ZONE_DEVICE mapped PMD | ++---------------------------+--------------------------------------------------+ +| pmd_soft_dirty | Tests a soft dirty PMD | ++---------------------------+--------------------------------------------------+ +| pmd_swp_soft_dirty | Tests a soft dirty swapped PMD | ++---------------------------+--------------------------------------------------+ +| pmd_mkyoung | Creates a young PMD | ++---------------------------+--------------------------------------------------+ +| pmd_mkold | Creates an old PMD | ++---------------------------+--------------------------------------------------+ +| pmd_mkdirty | Creates a dirty PMD | ++---------------------------+--------------------------------------------------+ +| pmd_mkclean | Creates a clean PMD | ++---------------------------+--------------------------------------------------+ +| pmd_mkwrite | Creates a writable PMD | ++---------------------------+--------------------------------------------------+ +| pmd_wrprotect | Creates a write protected PMD | ++---------------------------+--------------------------------------------------+ +| pmd_mkspecial | Creates a special PMD | ++---------------------------+--------------------------------------------------+ +| pmd_mkdevmap | Creates a ZONE_DEVICE mapped PMD | ++---------------------------+--------------------------------------------------+ +| pmd_mksoft_dirty | Creates a soft dirty PMD | ++---------------------------+--------------------------------------------------+ +| pmd_clear_soft_dirty | Clears a soft dirty PMD | ++---------------------------+--------------------------------------------------+ +| pmd_swp_mksoft_dirty | Creates a soft dirty swapped PMD | ++---------------------------+--------------------------------------------------+ +| pmd_swp_clear_soft_dirty | Clears a soft dirty swapped PMD | ++---------------------------+--------------------------------------------------+ +| pmd_mkinvalid | Invalidates a mapped PMD [1] | ++---------------------------+--------------------------------------------------+ +| pmd_set_huge | Creates a PMD huge mapping | ++---------------------------+--------------------------------------------------+ +| pmd_clear_huge | Clears a PMD huge mapping | ++---------------------------+--------------------------------------------------+ +| pmdp_get_and_clear | Clears a PMD | ++---------------------------+--------------------------------------------------+ +| pmdp_get_and_clear_full | Clears a PMD | ++---------------------------+--------------------------------------------------+ +| pmdp_test_and_clear_young | Clears young from a PMD | ++---------------------------+--------------------------------------------------+ +| pmdp_set_wrprotect | Converts into a write protected PMD | ++---------------------------+--------------------------------------------------+ +| pmdp_set_access_flags | Converts into a more permissive PMD | ++---------------------------+--------------------------------------------------+ + + +PUD Page Table Helpers +====================== + ++---------------------------+--------------------------------------------------+ +| pud_same | Tests whether both PUD entries are the same | ++---------------------------+--------------------------------------------------+ +| pud_bad | Tests a non-table mapped PUD | ++---------------------------+--------------------------------------------------+ +| pud_leaf | Tests a leaf mapped PUD | ++---------------------------+--------------------------------------------------+ +| pud_huge | Tests a HugeTLB mapped PUD | ++---------------------------+--------------------------------------------------+ +| pud_trans_huge | Tests a Transparent Huge Page (THP) at PUD | ++---------------------------+--------------------------------------------------+ +| pud_present | Tests a valid mapped PUD | ++---------------------------+--------------------------------------------------+ +| pud_young | Tests a young PUD | ++---------------------------+--------------------------------------------------+ +| pud_dirty | Tests a dirty PUD | ++---------------------------+--------------------------------------------------+ +| pud_write | Tests a writable PUD | ++---------------------------+--------------------------------------------------+ +| pud_devmap | Tests a ZONE_DEVICE mapped PUD | ++---------------------------+--------------------------------------------------+ +| pud_mkyoung | Creates a young PUD | ++---------------------------+--------------------------------------------------+ +| pud_mkold | Creates an old PUD | ++---------------------------+--------------------------------------------------+ +| pud_mkdirty | Creates a dirty PUD | ++---------------------------+--------------------------------------------------+ +| pud_mkclean | Creates a clean PUD | ++---------------------------+--------------------------------------------------+ +| pud_mkwrite | Creates a writable PUD | ++---------------------------+--------------------------------------------------+ +| pud_wrprotect | Creates a write protected PUD | ++---------------------------+--------------------------------------------------+ +| pud_mkdevmap | Creates a ZONE_DEVICE mapped PUD | ++---------------------------+--------------------------------------------------+ +| pud_mkinvalid | Invalidates a mapped PUD [1] | ++---------------------------+--------------------------------------------------+ +| pud_set_huge | Creates a PUD huge mapping | ++---------------------------+--------------------------------------------------+ +| pud_clear_huge | Clears a PUD huge mapping | ++---------------------------+--------------------------------------------------+ +| pudp_get_and_clear | Clears a PUD | ++---------------------------+--------------------------------------------------+ +| pudp_get_and_clear_full | Clears a PUD | ++---------------------------+--------------------------------------------------+ +| pudp_test_and_clear_young | Clears young from a PUD | ++---------------------------+--------------------------------------------------+ +| pudp_set_wrprotect | Converts into a write protected PUD | ++---------------------------+--------------------------------------------------+ +| pudp_set_access_flags | Converts into a more permissive PUD | ++---------------------------+--------------------------------------------------+ + + +HugeTLB Page Table Helpers +========================== + ++---------------------------+--------------------------------------------------+ +| pte_huge | Tests a HugeTLB | ++---------------------------+--------------------------------------------------+ +| pte_mkhuge | Creates a HugeTLB | ++---------------------------+--------------------------------------------------+ +| huge_pte_dirty | Tests a dirty HugeTLB | ++---------------------------+--------------------------------------------------+ +| huge_pte_write | Tests a writable HugeTLB | ++---------------------------+--------------------------------------------------+ +| huge_pte_mkdirty | Creates a dirty HugeTLB | ++---------------------------+--------------------------------------------------+ +| huge_pte_mkwrite | Creates a writable HugeTLB | ++---------------------------+--------------------------------------------------+ +| huge_pte_wrprotect | Creates a write protected HugeTLB | ++---------------------------+--------------------------------------------------+ +| huge_ptep_get_and_clear | Clears a HugeTLB | ++---------------------------+--------------------------------------------------+ +| huge_ptep_set_wrprotect | Converts into a write protected HugeTLB | ++---------------------------+--------------------------------------------------+ +| huge_ptep_set_access_flags | Converts into a more permissive HugeTLB | ++---------------------------+--------------------------------------------------+ + + +SWAP Page Table Helpers +======================== + ++---------------------------+--------------------------------------------------+ +| __pte_to_swp_entry | Creates a swapped entry (arch) from a mapped PTE | ++---------------------------+--------------------------------------------------+ +| __swp_to_pte_entry | Creates a mapped PTE from a swapped entry (arch) | ++---------------------------+--------------------------------------------------+ +| __pmd_to_swp_entry | Creates a swapped entry (arch) from a mapped PMD | ++---------------------------+--------------------------------------------------+ +| __swp_to_pmd_entry | Creates a mapped PMD from a swapped entry (arch) | ++---------------------------+--------------------------------------------------+ +| is_migration_entry | Tests a migration (read or write) swapped entry | ++-------------------------------+----------------------------------------------+ +| is_writable_migration_entry | Tests a write migration swapped entry | ++-------------------------------+----------------------------------------------+ +| make_readable_migration_entry | Creates a read migration swapped entry | ++-------------------------------+----------------------------------------------+ +| make_writable_migration_entry | Creates a write migration swapped entry | ++-------------------------------+----------------------------------------------+ + +[1] https://lore.kernel.org/linux-mm/20181017020930.GN30832@redhat.com/ diff --git a/Documentation/mm/balance.rst b/Documentation/mm/balance.rst new file mode 100644 index 000000000000..6a1fadf3e173 --- /dev/null +++ b/Documentation/mm/balance.rst @@ -0,0 +1,102 @@ +.. _balance: + +================ +Memory Balancing +================ + +Started Jan 2000 by Kanoj Sarcar <kanoj@sgi.com> + +Memory balancing is needed for !__GFP_ATOMIC and !__GFP_KSWAPD_RECLAIM as +well as for non __GFP_IO allocations. + +The first reason why a caller may avoid reclaim is that the caller can not +sleep due to holding a spinlock or is in interrupt context. The second may +be that the caller is willing to fail the allocation without incurring the +overhead of page reclaim. This may happen for opportunistic high-order +allocation requests that have order-0 fallback options. In such cases, +the caller may also wish to avoid waking kswapd. + +__GFP_IO allocation requests are made to prevent file system deadlocks. + +In the absence of non sleepable allocation requests, it seems detrimental +to be doing balancing. Page reclamation can be kicked off lazily, that +is, only when needed (aka zone free memory is 0), instead of making it +a proactive process. + +That being said, the kernel should try to fulfill requests for direct +mapped pages from the direct mapped pool, instead of falling back on +the dma pool, so as to keep the dma pool filled for dma requests (atomic +or not). A similar argument applies to highmem and direct mapped pages. +OTOH, if there is a lot of free dma pages, it is preferable to satisfy +regular memory requests by allocating one from the dma pool, instead +of incurring the overhead of regular zone balancing. + +In 2.2, memory balancing/page reclamation would kick off only when the +_total_ number of free pages fell below 1/64 th of total memory. With the +right ratio of dma and regular memory, it is quite possible that balancing +would not be done even when the dma zone was completely empty. 2.2 has +been running production machines of varying memory sizes, and seems to be +doing fine even with the presence of this problem. In 2.3, due to +HIGHMEM, this problem is aggravated. + +In 2.3, zone balancing can be done in one of two ways: depending on the +zone size (and possibly of the size of lower class zones), we can decide +at init time how many free pages we should aim for while balancing any +zone. The good part is, while balancing, we do not need to look at sizes +of lower class zones, the bad part is, we might do too frequent balancing +due to ignoring possibly lower usage in the lower class zones. Also, +with a slight change in the allocation routine, it is possible to reduce +the memclass() macro to be a simple equality. + +Another possible solution is that we balance only when the free memory +of a zone _and_ all its lower class zones falls below 1/64th of the +total memory in the zone and its lower class zones. This fixes the 2.2 +balancing problem, and stays as close to 2.2 behavior as possible. Also, +the balancing algorithm works the same way on the various architectures, +which have different numbers and types of zones. If we wanted to get +fancy, we could assign different weights to free pages in different +zones in the future. + +Note that if the size of the regular zone is huge compared to dma zone, +it becomes less significant to consider the free dma pages while +deciding whether to balance the regular zone. The first solution +becomes more attractive then. + +The appended patch implements the second solution. It also "fixes" two +problems: first, kswapd is woken up as in 2.2 on low memory conditions +for non-sleepable allocations. Second, the HIGHMEM zone is also balanced, +so as to give a fighting chance for replace_with_highmem() to get a +HIGHMEM page, as well as to ensure that HIGHMEM allocations do not +fall back into regular zone. This also makes sure that HIGHMEM pages +are not leaked (for example, in situations where a HIGHMEM page is in +the swapcache but is not being used by anyone) + +kswapd also needs to know about the zones it should balance. kswapd is +primarily needed in a situation where balancing can not be done, +probably because all allocation requests are coming from intr context +and all process contexts are sleeping. For 2.3, kswapd does not really +need to balance the highmem zone, since intr context does not request +highmem pages. kswapd looks at the zone_wake_kswapd field in the zone +structure to decide whether a zone needs balancing. + +Page stealing from process memory and shm is done if stealing the page would +alleviate memory pressure on any zone in the page's node that has fallen below +its watermark. + +watemark[WMARK_MIN/WMARK_LOW/WMARK_HIGH]/low_on_memory/zone_wake_kswapd: These +are per-zone fields, used to determine when a zone needs to be balanced. When +the number of pages falls below watermark[WMARK_MIN], the hysteric field +low_on_memory gets set. This stays set till the number of free pages becomes +watermark[WMARK_HIGH]. When low_on_memory is set, page allocation requests will +try to free some pages in the zone (providing GFP_WAIT is set in the request). +Orthogonal to this, is the decision to poke kswapd to free some zone pages. +That decision is not hysteresis based, and is done when the number of free +pages is below watermark[WMARK_LOW]; in which case zone_wake_kswapd is also set. + + +(Good) Ideas that I have heard: + +1. Dynamic experience should influence balancing: number of failed requests + for a zone can be tracked and fed into the balancing scheme (jalvo@mbay.net) +2. Implement a replace_with_highmem()-like replace_with_regular() to preserve + dma pages. (lkd@tantalophile.demon.co.uk) diff --git a/Documentation/mm/bootmem.rst b/Documentation/mm/bootmem.rst new file mode 100644 index 000000000000..eb2b31eedfa1 --- /dev/null +++ b/Documentation/mm/bootmem.rst @@ -0,0 +1,5 @@ +.. SPDX-License-Identifier: GPL-2.0 + +=========== +Boot Memory +=========== diff --git a/Documentation/mm/damon/api.rst b/Documentation/mm/damon/api.rst new file mode 100644 index 000000000000..08f34df45523 --- /dev/null +++ b/Documentation/mm/damon/api.rst @@ -0,0 +1,20 @@ +.. SPDX-License-Identifier: GPL-2.0 + +============= +API Reference +============= + +Kernel space programs can use every feature of DAMON using below APIs. All you +need to do is including ``damon.h``, which is located in ``include/linux/`` of +the source tree. + +Structures +========== + +.. kernel-doc:: include/linux/damon.h + + +Functions +========= + +.. kernel-doc:: mm/damon/core.c diff --git a/Documentation/mm/damon/design.rst b/Documentation/mm/damon/design.rst new file mode 100644 index 000000000000..0cff6fac6b7e --- /dev/null +++ b/Documentation/mm/damon/design.rst @@ -0,0 +1,176 @@ +.. SPDX-License-Identifier: GPL-2.0 + +====== +Design +====== + +Configurable Layers +=================== + +DAMON provides data access monitoring functionality while making the accuracy +and the overhead controllable. The fundamental access monitorings require +primitives that dependent on and optimized for the target address space. On +the other hand, the accuracy and overhead tradeoff mechanism, which is the core +of DAMON, is in the pure logic space. DAMON separates the two parts in +different layers and defines its interface to allow various low level +primitives implementations configurable with the core logic. We call the low +level primitives implementations monitoring operations. + +Due to this separated design and the configurable interface, users can extend +DAMON for any address space by configuring the core logics with appropriate +monitoring operations. If appropriate one is not provided, users can implement +the operations on their own. + +For example, physical memory, virtual memory, swap space, those for specific +processes, NUMA nodes, files, and backing memory devices would be supportable. +Also, if some architectures or devices support special optimized access check +primitives, those will be easily configurable. + + +Reference Implementations of Address Space Specific Monitoring Operations +========================================================================= + +The monitoring operations are defined in two parts: + +1. Identification of the monitoring target address range for the address space. +2. Access check of specific address range in the target space. + +DAMON currently provides the implementations of the operations for the physical +and virtual address spaces. Below two subsections describe how those work. + + +VMA-based Target Address Range Construction +------------------------------------------- + +This is only for the virtual address space monitoring operations +implementation. That for the physical address space simply asks users to +manually set the monitoring target address ranges. + +Only small parts in the super-huge virtual address space of the processes are +mapped to the physical memory and accessed. Thus, tracking the unmapped +address regions is just wasteful. However, because DAMON can deal with some +level of noise using the adaptive regions adjustment mechanism, tracking every +mapping is not strictly required but could even incur a high overhead in some +cases. That said, too huge unmapped areas inside the monitoring target should +be removed to not take the time for the adaptive mechanism. + +For the reason, this implementation converts the complex mappings to three +distinct regions that cover every mapped area of the address space. The two +gaps between the three regions are the two biggest unmapped areas in the given +address space. The two biggest unmapped areas would be the gap between the +heap and the uppermost mmap()-ed region, and the gap between the lowermost +mmap()-ed region and the stack in most of the cases. Because these gaps are +exceptionally huge in usual address spaces, excluding these will be sufficient +to make a reasonable trade-off. Below shows this in detail:: + + <heap> + <BIG UNMAPPED REGION 1> + <uppermost mmap()-ed region> + (small mmap()-ed regions and munmap()-ed regions) + <lowermost mmap()-ed region> + <BIG UNMAPPED REGION 2> + <stack> + + +PTE Accessed-bit Based Access Check +----------------------------------- + +Both of the implementations for physical and virtual address spaces use PTE +Accessed-bit for basic access checks. Only one difference is the way of +finding the relevant PTE Accessed bit(s) from the address. While the +implementation for the virtual address walks the page table for the target task +of the address, the implementation for the physical address walks every page +table having a mapping to the address. In this way, the implementations find +and clear the bit(s) for next sampling target address and checks whether the +bit(s) set again after one sampling period. This could disturb other kernel +subsystems using the Accessed bits, namely Idle page tracking and the reclaim +logic. DAMON does nothing to avoid disturbing Idle page tracking, so handling +the interference is the responsibility of sysadmins. However, it solves the +conflict with the reclaim logic using ``PG_idle`` and ``PG_young`` page flags, +as Idle page tracking does. + + +Address Space Independent Core Mechanisms +========================================= + +Below four sections describe each of the DAMON core mechanisms and the five +monitoring attributes, ``sampling interval``, ``aggregation interval``, +``update interval``, ``minimum number of regions``, and ``maximum number of +regions``. + + +Access Frequency Monitoring +--------------------------- + +The output of DAMON says what pages are how frequently accessed for a given +duration. The resolution of the access frequency is controlled by setting +``sampling interval`` and ``aggregation interval``. In detail, DAMON checks +access to each page per ``sampling interval`` and aggregates the results. In +other words, counts the number of the accesses to each page. After each +``aggregation interval`` passes, DAMON calls callback functions that previously +registered by users so that users can read the aggregated results and then +clears the results. This can be described in below simple pseudo-code:: + + while monitoring_on: + for page in monitoring_target: + if accessed(page): + nr_accesses[page] += 1 + if time() % aggregation_interval == 0: + for callback in user_registered_callbacks: + callback(monitoring_target, nr_accesses) + for page in monitoring_target: + nr_accesses[page] = 0 + sleep(sampling interval) + +The monitoring overhead of this mechanism will arbitrarily increase as the +size of the target workload grows. + + +Region Based Sampling +--------------------- + +To avoid the unbounded increase of the overhead, DAMON groups adjacent pages +that assumed to have the same access frequencies into a region. As long as the +assumption (pages in a region have the same access frequencies) is kept, only +one page in the region is required to be checked. Thus, for each ``sampling +interval``, DAMON randomly picks one page in each region, waits for one +``sampling interval``, checks whether the page is accessed meanwhile, and +increases the access frequency of the region if so. Therefore, the monitoring +overhead is controllable by setting the number of regions. DAMON allows users +to set the minimum and the maximum number of regions for the trade-off. + +This scheme, however, cannot preserve the quality of the output if the +assumption is not guaranteed. + + +Adaptive Regions Adjustment +--------------------------- + +Even somehow the initial monitoring target regions are well constructed to +fulfill the assumption (pages in same region have similar access frequencies), +the data access pattern can be dynamically changed. This will result in low +monitoring quality. To keep the assumption as much as possible, DAMON +adaptively merges and splits each region based on their access frequency. + +For each ``aggregation interval``, it compares the access frequencies of +adjacent regions and merges those if the frequency difference is small. Then, +after it reports and clears the aggregated access frequency of each region, it +splits each region into two or three regions if the total number of regions +will not exceed the user-specified maximum number of regions after the split. + +In this way, DAMON provides its best-effort quality and minimal overhead while +keeping the bounds users set for their trade-off. + + +Dynamic Target Space Updates Handling +------------------------------------- + +The monitoring target address range could dynamically changed. For example, +virtual memory could be dynamically mapped and unmapped. Physical memory could +be hot-plugged. + +As the changes could be quite frequent in some cases, DAMON allows the +monitoring operations to check dynamic changes including memory mapping changes +and applies it to monitoring operations-related data structures such as the +abstracted monitoring target memory area only for each of a user-specified time +interval (``update interval``). diff --git a/Documentation/mm/damon/faq.rst b/Documentation/mm/damon/faq.rst new file mode 100644 index 000000000000..dde7e2414ee6 --- /dev/null +++ b/Documentation/mm/damon/faq.rst @@ -0,0 +1,50 @@ +.. SPDX-License-Identifier: GPL-2.0 + +========================== +Frequently Asked Questions +========================== + +Why a new subsystem, instead of extending perf or other user space tools? +========================================================================= + +First, because it needs to be lightweight as much as possible so that it can be +used online, any unnecessary overhead such as kernel - user space context +switching cost should be avoided. Second, DAMON aims to be used by other +programs including the kernel. Therefore, having a dependency on specific +tools like perf is not desirable. These are the two biggest reasons why DAMON +is implemented in the kernel space. + + +Can 'idle pages tracking' or 'perf mem' substitute DAMON? +========================================================= + +Idle page tracking is a low level primitive for access check of the physical +address space. 'perf mem' is similar, though it can use sampling to minimize +the overhead. On the other hand, DAMON is a higher-level framework for the +monitoring of various address spaces. It is focused on memory management +optimization and provides sophisticated accuracy/overhead handling mechanisms. +Therefore, 'idle pages tracking' and 'perf mem' could provide a subset of +DAMON's output, but cannot substitute DAMON. + + +Does DAMON support virtual memory only? +======================================= + +No. The core of the DAMON is address space independent. The address space +specific monitoring operations including monitoring target regions +constructions and actual access checks can be implemented and configured on the +DAMON core by the users. In this way, DAMON users can monitor any address +space with any access check technique. + +Nonetheless, DAMON provides vma/rmap tracking and PTE Accessed bit check based +implementations of the address space dependent functions for the virtual memory +and the physical memory by default, for a reference and convenient use. + + +Can I simply monitor page granularity? +====================================== + +Yes. You can do so by setting the ``min_nr_regions`` attribute higher than the +working set size divided by the page size. Because the monitoring target +regions size is forced to be ``>=page size``, the region split will make no +effect. diff --git a/Documentation/mm/damon/index.rst b/Documentation/mm/damon/index.rst new file mode 100644 index 000000000000..48c0bbff98b2 --- /dev/null +++ b/Documentation/mm/damon/index.rst @@ -0,0 +1,29 @@ +.. SPDX-License-Identifier: GPL-2.0 + +========================== +DAMON: Data Access MONitor +========================== + +DAMON is a data access monitoring framework subsystem for the Linux kernel. +The core mechanisms of DAMON (refer to :doc:`design` for the detail) make it + + - *accurate* (the monitoring output is useful enough for DRAM level memory + management; It might not appropriate for CPU Cache levels, though), + - *light-weight* (the monitoring overhead is low enough to be applied online), + and + - *scalable* (the upper-bound of the overhead is in constant range regardless + of the size of target workloads). + +Using this framework, therefore, the kernel's memory management mechanisms can +make advanced decisions. Experimental memory management optimization works +that incurring high data accesses monitoring overhead could implemented again. +In user space, meanwhile, users who have some special workloads can write +personalized applications for better understanding and optimizations of their +workloads and systems. + +.. toctree:: + :maxdepth: 2 + + faq + design + api diff --git a/Documentation/mm/free_page_reporting.rst b/Documentation/mm/free_page_reporting.rst new file mode 100644 index 000000000000..8c05e62d8b2b --- /dev/null +++ b/Documentation/mm/free_page_reporting.rst @@ -0,0 +1,40 @@ +.. _free_page_reporting: + +===================== +Free Page Reporting +===================== + +Free page reporting is an API by which a device can register to receive +lists of pages that are currently unused by the system. This is useful in +the case of virtualization where a guest is then able to use this data to +notify the hypervisor that it is no longer using certain pages in memory. + +For the driver, typically a balloon driver, to use of this functionality +it will allocate and initialize a page_reporting_dev_info structure. The +field within the structure it will populate is the "report" function +pointer used to process the scatterlist. It must also guarantee that it can +handle at least PAGE_REPORTING_CAPACITY worth of scatterlist entries per +call to the function. A call to page_reporting_register will register the +page reporting interface with the reporting framework assuming no other +page reporting devices are already registered. + +Once registered the page reporting API will begin reporting batches of +pages to the driver. The API will start reporting pages 2 seconds after +the interface is registered and will continue to do so 2 seconds after any +page of a sufficiently high order is freed. + +Pages reported will be stored in the scatterlist passed to the reporting +function with the final entry having the end bit set in entry nent - 1. +While pages are being processed by the report function they will not be +accessible to the allocator. Once the report function has been completed +the pages will be returned to the free area from which they were obtained. + +Prior to removing a driver that is making use of free page reporting it +is necessary to call page_reporting_unregister to have the +page_reporting_dev_info structure that is currently in use by free page +reporting removed. Doing this will prevent further reports from being +issued via the interface. If another driver or the same driver is +registered it is possible for it to resume where the previous driver had +left off in terms of reporting free pages. + +Alexander Duyck, Dec 04, 2019 diff --git a/Documentation/mm/frontswap.rst b/Documentation/mm/frontswap.rst new file mode 100644 index 000000000000..feecc5e24477 --- /dev/null +++ b/Documentation/mm/frontswap.rst @@ -0,0 +1,266 @@ +.. _frontswap: + +========= +Frontswap +========= + +Frontswap provides a "transcendent memory" interface for swap pages. +In some environments, dramatic performance savings may be obtained because +swapped pages are saved in RAM (or a RAM-like device) instead of a swap disk. + +.. _Transcendent memory in a nutshell: https://lwn.net/Articles/454795/ + +Frontswap is so named because it can be thought of as the opposite of +a "backing" store for a swap device. The storage is assumed to be +a synchronous concurrency-safe page-oriented "pseudo-RAM device" conforming +to the requirements of transcendent memory (such as Xen's "tmem", or +in-kernel compressed memory, aka "zcache", or future RAM-like devices); +this pseudo-RAM device is not directly accessible or addressable by the +kernel and is of unknown and possibly time-varying size. The driver +links itself to frontswap by calling frontswap_register_ops to set the +frontswap_ops funcs appropriately and the functions it provides must +conform to certain policies as follows: + +An "init" prepares the device to receive frontswap pages associated +with the specified swap device number (aka "type"). A "store" will +copy the page to transcendent memory and associate it with the type and +offset associated with the page. A "load" will copy the page, if found, +from transcendent memory into kernel memory, but will NOT remove the page +from transcendent memory. An "invalidate_page" will remove the page +from transcendent memory and an "invalidate_area" will remove ALL pages +associated with the swap type (e.g., like swapoff) and notify the "device" +to refuse further stores with that swap type. + +Once a page is successfully stored, a matching load on the page will normally +succeed. So when the kernel finds itself in a situation where it needs +to swap out a page, it first attempts to use frontswap. If the store returns +success, the data has been successfully saved to transcendent memory and +a disk write and, if the data is later read back, a disk read are avoided. +If a store returns failure, transcendent memory has rejected the data, and the +page can be written to swap as usual. + +Note that if a page is stored and the page already exists in transcendent memory +(a "duplicate" store), either the store succeeds and the data is overwritten, +or the store fails AND the page is invalidated. This ensures stale data may +never be obtained from frontswap. + +If properly configured, monitoring of frontswap is done via debugfs in +the `/sys/kernel/debug/frontswap` directory. The effectiveness of +frontswap can be measured (across all swap devices) with: + +``failed_stores`` + how many store attempts have failed + +``loads`` + how many loads were attempted (all should succeed) + +``succ_stores`` + how many store attempts have succeeded + +``invalidates`` + how many invalidates were attempted + +A backend implementation may provide additional metrics. + +FAQ +=== + +* Where's the value? + +When a workload starts swapping, performance falls through the floor. +Frontswap significantly increases performance in many such workloads by +providing a clean, dynamic interface to read and write swap pages to +"transcendent memory" that is otherwise not directly addressable to the kernel. +This interface is ideal when data is transformed to a different form +and size (such as with compression) or secretly moved (as might be +useful for write-balancing for some RAM-like devices). Swap pages (and +evicted page-cache pages) are a great use for this kind of slower-than-RAM- +but-much-faster-than-disk "pseudo-RAM device". + +Frontswap with a fairly small impact on the kernel, +provides a huge amount of flexibility for more dynamic, flexible RAM +utilization in various system configurations: + +In the single kernel case, aka "zcache", pages are compressed and +stored in local memory, thus increasing the total anonymous pages +that can be safely kept in RAM. Zcache essentially trades off CPU +cycles used in compression/decompression for better memory utilization. +Benchmarks have shown little or no impact when memory pressure is +low while providing a significant performance improvement (25%+) +on some workloads under high memory pressure. + +"RAMster" builds on zcache by adding "peer-to-peer" transcendent memory +support for clustered systems. Frontswap pages are locally compressed +as in zcache, but then "remotified" to another system's RAM. This +allows RAM to be dynamically load-balanced back-and-forth as needed, +i.e. when system A is overcommitted, it can swap to system B, and +vice versa. RAMster can also be configured as a memory server so +many servers in a cluster can swap, dynamically as needed, to a single +server configured with a large amount of RAM... without pre-configuring +how much of the RAM is available for each of the clients! + +In the virtual case, the whole point of virtualization is to statistically +multiplex physical resources across the varying demands of multiple +virtual machines. This is really hard to do with RAM and efforts to do +it well with no kernel changes have essentially failed (except in some +well-publicized special-case workloads). +Specifically, the Xen Transcendent Memory backend allows otherwise +"fallow" hypervisor-owned RAM to not only be "time-shared" between multiple +virtual machines, but the pages can be compressed and deduplicated to +optimize RAM utilization. And when guest OS's are induced to surrender +underutilized RAM (e.g. with "selfballooning"), sudden unexpected +memory pressure may result in swapping; frontswap allows those pages +to be swapped to and from hypervisor RAM (if overall host system memory +conditions allow), thus mitigating the potentially awful performance impact +of unplanned swapping. + +A KVM implementation is underway and has been RFC'ed to lkml. And, +using frontswap, investigation is also underway on the use of NVM as +a memory extension technology. + +* Sure there may be performance advantages in some situations, but + what's the space/time overhead of frontswap? + +If CONFIG_FRONTSWAP is disabled, every frontswap hook compiles into +nothingness and the only overhead is a few extra bytes per swapon'ed +swap device. If CONFIG_FRONTSWAP is enabled but no frontswap "backend" +registers, there is one extra global variable compared to zero for +every swap page read or written. If CONFIG_FRONTSWAP is enabled +AND a frontswap backend registers AND the backend fails every "store" +request (i.e. provides no memory despite claiming it might), +CPU overhead is still negligible -- and since every frontswap fail +precedes a swap page write-to-disk, the system is highly likely +to be I/O bound and using a small fraction of a percent of a CPU +will be irrelevant anyway. + +As for space, if CONFIG_FRONTSWAP is enabled AND a frontswap backend +registers, one bit is allocated for every swap page for every swap +device that is swapon'd. This is added to the EIGHT bits (which +was sixteen until about 2.6.34) that the kernel already allocates +for every swap page for every swap device that is swapon'd. (Hugh +Dickins has observed that frontswap could probably steal one of +the existing eight bits, but let's worry about that minor optimization +later.) For very large swap disks (which are rare) on a standard +4K pagesize, this is 1MB per 32GB swap. + +When swap pages are stored in transcendent memory instead of written +out to disk, there is a side effect that this may create more memory +pressure that can potentially outweigh the other advantages. A +backend, such as zcache, must implement policies to carefully (but +dynamically) manage memory limits to ensure this doesn't happen. + +* OK, how about a quick overview of what this frontswap patch does + in terms that a kernel hacker can grok? + +Let's assume that a frontswap "backend" has registered during +kernel initialization; this registration indicates that this +frontswap backend has access to some "memory" that is not directly +accessible by the kernel. Exactly how much memory it provides is +entirely dynamic and random. + +Whenever a swap-device is swapon'd frontswap_init() is called, +passing the swap device number (aka "type") as a parameter. +This notifies frontswap to expect attempts to "store" swap pages +associated with that number. + +Whenever the swap subsystem is readying a page to write to a swap +device (c.f swap_writepage()), frontswap_store is called. Frontswap +consults with the frontswap backend and if the backend says it does NOT +have room, frontswap_store returns -1 and the kernel swaps the page +to the swap device as normal. Note that the response from the frontswap +backend is unpredictable to the kernel; it may choose to never accept a +page, it could accept every ninth page, or it might accept every +page. But if the backend does accept a page, the data from the page +has already been copied and associated with the type and offset, +and the backend guarantees the persistence of the data. In this case, +frontswap sets a bit in the "frontswap_map" for the swap device +corresponding to the page offset on the swap device to which it would +otherwise have written the data. + +When the swap subsystem needs to swap-in a page (swap_readpage()), +it first calls frontswap_load() which checks the frontswap_map to +see if the page was earlier accepted by the frontswap backend. If +it was, the page of data is filled from the frontswap backend and +the swap-in is complete. If not, the normal swap-in code is +executed to obtain the page of data from the real swap device. + +So every time the frontswap backend accepts a page, a swap device read +and (potentially) a swap device write are replaced by a "frontswap backend +store" and (possibly) a "frontswap backend loads", which are presumably much +faster. + +* Can't frontswap be configured as a "special" swap device that is + just higher priority than any real swap device (e.g. like zswap, + or maybe swap-over-nbd/NFS)? + +No. First, the existing swap subsystem doesn't allow for any kind of +swap hierarchy. Perhaps it could be rewritten to accommodate a hierarchy, +but this would require fairly drastic changes. Even if it were +rewritten, the existing swap subsystem uses the block I/O layer which +assumes a swap device is fixed size and any page in it is linearly +addressable. Frontswap barely touches the existing swap subsystem, +and works around the constraints of the block I/O subsystem to provide +a great deal of flexibility and dynamicity. + +For example, the acceptance of any swap page by the frontswap backend is +entirely unpredictable. This is critical to the definition of frontswap +backends because it grants completely dynamic discretion to the +backend. In zcache, one cannot know a priori how compressible a page is. +"Poorly" compressible pages can be rejected, and "poorly" can itself be +defined dynamically depending on current memory constraints. + +Further, frontswap is entirely synchronous whereas a real swap +device is, by definition, asynchronous and uses block I/O. The +block I/O layer is not only unnecessary, but may perform "optimizations" +that are inappropriate for a RAM-oriented device including delaying +the write of some pages for a significant amount of time. Synchrony is +required to ensure the dynamicity of the backend and to avoid thorny race +conditions that would unnecessarily and greatly complicate frontswap +and/or the block I/O subsystem. That said, only the initial "store" +and "load" operations need be synchronous. A separate asynchronous thread +is free to manipulate the pages stored by frontswap. For example, +the "remotification" thread in RAMster uses standard asynchronous +kernel sockets to move compressed frontswap pages to a remote machine. +Similarly, a KVM guest-side implementation could do in-guest compression +and use "batched" hypercalls. + +In a virtualized environment, the dynamicity allows the hypervisor +(or host OS) to do "intelligent overcommit". For example, it can +choose to accept pages only until host-swapping might be imminent, +then force guests to do their own swapping. + +There is a downside to the transcendent memory specifications for +frontswap: Since any "store" might fail, there must always be a real +slot on a real swap device to swap the page. Thus frontswap must be +implemented as a "shadow" to every swapon'd device with the potential +capability of holding every page that the swap device might have held +and the possibility that it might hold no pages at all. This means +that frontswap cannot contain more pages than the total of swapon'd +swap devices. For example, if NO swap device is configured on some +installation, frontswap is useless. Swapless portable devices +can still use frontswap but a backend for such devices must configure +some kind of "ghost" swap device and ensure that it is never used. + +* Why this weird definition about "duplicate stores"? If a page + has been previously successfully stored, can't it always be + successfully overwritten? + +Nearly always it can, but no, sometimes it cannot. Consider an example +where data is compressed and the original 4K page has been compressed +to 1K. Now an attempt is made to overwrite the page with data that +is non-compressible and so would take the entire 4K. But the backend +has no more space. In this case, the store must be rejected. Whenever +frontswap rejects a store that would overwrite, it also must invalidate +the old data and ensure that it is no longer accessible. Since the +swap subsystem then writes the new data to the read swap device, +this is the correct course of action to ensure coherency. + +* Why does the frontswap patch create the new include file swapfile.h? + +The frontswap code depends on some swap-subsystem-internal data +structures that have, over the years, moved back and forth between +static and global. This seemed a reasonable compromise: Define +them as global but declare them in a new include file that isn't +included by the large number of source files that include swap.h. + +Dan Magenheimer, last updated April 9, 2012 diff --git a/Documentation/mm/highmem.rst b/Documentation/mm/highmem.rst new file mode 100644 index 000000000000..c9887f241c6c --- /dev/null +++ b/Documentation/mm/highmem.rst @@ -0,0 +1,167 @@ +.. _highmem: + +==================== +High Memory Handling +==================== + +By: Peter Zijlstra <a.p.zijlstra@chello.nl> + +.. contents:: :local: + +What Is High Memory? +==================== + +High memory (highmem) is used when the size of physical memory approaches or +exceeds the maximum size of virtual memory. At that point it becomes +impossible for the kernel to keep all of the available physical memory mapped +at all times. This means the kernel needs to start using temporary mappings of +the pieces of physical memory that it wants to access. + +The part of (physical) memory not covered by a permanent mapping is what we +refer to as 'highmem'. There are various architecture dependent constraints on +where exactly that border lies. + +In the i386 arch, for example, we choose to map the kernel into every process's +VM space so that we don't have to pay the full TLB invalidation costs for +kernel entry/exit. This means the available virtual memory space (4GiB on +i386) has to be divided between user and kernel space. + +The traditional split for architectures using this approach is 3:1, 3GiB for +userspace and the top 1GiB for kernel space:: + + +--------+ 0xffffffff + | Kernel | + +--------+ 0xc0000000 + | | + | User | + | | + +--------+ 0x00000000 + +This means that the kernel can at most map 1GiB of physical memory at any one +time, but because we need virtual address space for other things - including +temporary maps to access the rest of the physical memory - the actual direct +map will typically be less (usually around ~896MiB). + +Other architectures that have mm context tagged TLBs can have separate kernel +and user maps. Some hardware (like some ARMs), however, have limited virtual +space when they use mm context tags. + + +Temporary Virtual Mappings +========================== + +The kernel contains several ways of creating temporary mappings. The following +list shows them in order of preference of use. + +* kmap_local_page(). This function is used to require short term mappings. + It can be invoked from any context (including interrupts) but the mappings + can only be used in the context which acquired them. + + This function should be preferred, where feasible, over all the others. + + These mappings are thread-local and CPU-local, meaning that the mapping + can only be accessed from within this thread and the thread is bound the + CPU while the mapping is active. Even if the thread is preempted (since + preemption is never disabled by the function) the CPU can not be + unplugged from the system via CPU-hotplug until the mapping is disposed. + + It's valid to take pagefaults in a local kmap region, unless the context + in which the local mapping is acquired does not allow it for other reasons. + + kmap_local_page() always returns a valid virtual address and it is assumed + that kunmap_local() will never fail. + + Nesting kmap_local_page() and kmap_atomic() mappings is allowed to a certain + extent (up to KMAP_TYPE_NR) but their invocations have to be strictly ordered + because the map implementation is stack based. See kmap_local_page() kdocs + (included in the "Functions" section) for details on how to manage nested + mappings. + +* kmap_atomic(). This permits a very short duration mapping of a single + page. Since the mapping is restricted to the CPU that issued it, it + performs well, but the issuing task is therefore required to stay on that + CPU until it has finished, lest some other task displace its mappings. + + kmap_atomic() may also be used by interrupt contexts, since it does not + sleep and the callers too may not sleep until after kunmap_atomic() is + called. + + Each call of kmap_atomic() in the kernel creates a non-preemptible section + and disable pagefaults. This could be a source of unwanted latency. Therefore + users should prefer kmap_local_page() instead of kmap_atomic(). + + It is assumed that k[un]map_atomic() won't fail. + +* kmap(). This should be used to make short duration mapping of a single + page with no restrictions on preemption or migration. It comes with an + overhead as mapping space is restricted and protected by a global lock + for synchronization. When mapping is no longer needed, the address that + the page was mapped to must be released with kunmap(). + + Mapping changes must be propagated across all the CPUs. kmap() also + requires global TLB invalidation when the kmap's pool wraps and it might + block when the mapping space is fully utilized until a slot becomes + available. Therefore, kmap() is only callable from preemptible context. + + All the above work is necessary if a mapping must last for a relatively + long time but the bulk of high-memory mappings in the kernel are + short-lived and only used in one place. This means that the cost of + kmap() is mostly wasted in such cases. kmap() was not intended for long + term mappings but it has morphed in that direction and its use is + strongly discouraged in newer code and the set of the preceding functions + should be preferred. + + On 64-bit systems, calls to kmap_local_page(), kmap_atomic() and kmap() have + no real work to do because a 64-bit address space is more than sufficient to + address all the physical memory whose pages are permanently mapped. + +* vmap(). This can be used to make a long duration mapping of multiple + physical pages into a contiguous virtual space. It needs global + synchronization to unmap. + + +Cost of Temporary Mappings +========================== + +The cost of creating temporary mappings can be quite high. The arch has to +manipulate the kernel's page tables, the data TLB and/or the MMU's registers. + +If CONFIG_HIGHMEM is not set, then the kernel will try and create a mapping +simply with a bit of arithmetic that will convert the page struct address into +a pointer to the page contents rather than juggling mappings about. In such a +case, the unmap operation may be a null operation. + +If CONFIG_MMU is not set, then there can be no temporary mappings and no +highmem. In such a case, the arithmetic approach will also be used. + + +i386 PAE +======== + +The i386 arch, under some circumstances, will permit you to stick up to 64GiB +of RAM into your 32-bit machine. This has a number of consequences: + +* Linux needs a page-frame structure for each page in the system and the + pageframes need to live in the permanent mapping, which means: + +* you can have 896M/sizeof(struct page) page-frames at most; with struct + page being 32-bytes that would end up being something in the order of 112G + worth of pages; the kernel, however, needs to store more than just + page-frames in that memory... + +* PAE makes your page tables larger - which slows the system down as more + data has to be accessed to traverse in TLB fills and the like. One + advantage is that PAE has more PTE bits and can provide advanced features + like NX and PAT. + +The general recommendation is that you don't use more than 8GiB on a 32-bit +machine - although more might work for you and your workload, you're pretty +much on your own - don't expect kernel developers to really care much if things +come apart. + + +Functions +========= + +.. kernel-doc:: include/linux/highmem.h +.. kernel-doc:: include/linux/highmem-internal.h diff --git a/Documentation/mm/hmm.rst b/Documentation/mm/hmm.rst new file mode 100644 index 000000000000..f2a59ed82ed3 --- /dev/null +++ b/Documentation/mm/hmm.rst @@ -0,0 +1,452 @@ +.. _hmm: + +===================================== +Heterogeneous Memory Management (HMM) +===================================== + +Provide infrastructure and helpers to integrate non-conventional memory (device +memory like GPU on board memory) into regular kernel path, with the cornerstone +of this being specialized struct page for such memory (see sections 5 to 7 of +this document). + +HMM also provides optional helpers for SVM (Share Virtual Memory), i.e., +allowing a device to transparently access program addresses coherently with +the CPU meaning that any valid pointer on the CPU is also a valid pointer +for the device. This is becoming mandatory to simplify the use of advanced +heterogeneous computing where GPU, DSP, or FPGA are used to perform various +computations on behalf of a process. + +This document is divided as follows: in the first section I expose the problems +related to using device specific memory allocators. In the second section, I +expose the hardware limitations that are inherent to many platforms. The third +section gives an overview of the HMM design. The fourth section explains how +CPU page-table mirroring works and the purpose of HMM in this context. The +fifth section deals with how device memory is represented inside the kernel. +Finally, the last section presents a new migration helper that allows +leveraging the device DMA engine. + +.. contents:: :local: + +Problems of using a device specific memory allocator +==================================================== + +Devices with a large amount of on board memory (several gigabytes) like GPUs +have historically managed their memory through dedicated driver specific APIs. +This creates a disconnect between memory allocated and managed by a device +driver and regular application memory (private anonymous, shared memory, or +regular file backed memory). From here on I will refer to this aspect as split +address space. I use shared address space to refer to the opposite situation: +i.e., one in which any application memory region can be used by a device +transparently. + +Split address space happens because devices can only access memory allocated +through a device specific API. This implies that all memory objects in a program +are not equal from the device point of view which complicates large programs +that rely on a wide set of libraries. + +Concretely, this means that code that wants to leverage devices like GPUs needs +to copy objects between generically allocated memory (malloc, mmap private, mmap +share) and memory allocated through the device driver API (this still ends up +with an mmap but of the device file). + +For flat data sets (array, grid, image, ...) this isn't too hard to achieve but +for complex data sets (list, tree, ...) it's hard to get right. Duplicating a +complex data set needs to re-map all the pointer relations between each of its +elements. This is error prone and programs get harder to debug because of the +duplicate data set and addresses. + +Split address space also means that libraries cannot transparently use data +they are getting from the core program or another library and thus each library +might have to duplicate its input data set using the device specific memory +allocator. Large projects suffer from this and waste resources because of the +various memory copies. + +Duplicating each library API to accept as input or output memory allocated by +each device specific allocator is not a viable option. It would lead to a +combinatorial explosion in the library entry points. + +Finally, with the advance of high level language constructs (in C++ but in +other languages too) it is now possible for the compiler to leverage GPUs and +other devices without programmer knowledge. Some compiler identified patterns +are only do-able with a shared address space. It is also more reasonable to use +a shared address space for all other patterns. + + +I/O bus, device memory characteristics +====================================== + +I/O buses cripple shared address spaces due to a few limitations. Most I/O +buses only allow basic memory access from device to main memory; even cache +coherency is often optional. Access to device memory from a CPU is even more +limited. More often than not, it is not cache coherent. + +If we only consider the PCIE bus, then a device can access main memory (often +through an IOMMU) and be cache coherent with the CPUs. However, it only allows +a limited set of atomic operations from the device on main memory. This is worse +in the other direction: the CPU can only access a limited range of the device +memory and cannot perform atomic operations on it. Thus device memory cannot +be considered the same as regular memory from the kernel point of view. + +Another crippling factor is the limited bandwidth (~32GBytes/s with PCIE 4.0 +and 16 lanes). This is 33 times less than the fastest GPU memory (1 TBytes/s). +The final limitation is latency. Access to main memory from the device has an +order of magnitude higher latency than when the device accesses its own memory. + +Some platforms are developing new I/O buses or additions/modifications to PCIE +to address some of these limitations (OpenCAPI, CCIX). They mainly allow +two-way cache coherency between CPU and device and allow all atomic operations the +architecture supports. Sadly, not all platforms are following this trend and +some major architectures are left without hardware solutions to these problems. + +So for shared address space to make sense, not only must we allow devices to +access any memory but we must also permit any memory to be migrated to device +memory while the device is using it (blocking CPU access while it happens). + + +Shared address space and migration +================================== + +HMM intends to provide two main features. The first one is to share the address +space by duplicating the CPU page table in the device page table so the same +address points to the same physical memory for any valid main memory address in +the process address space. + +To achieve this, HMM offers a set of helpers to populate the device page table +while keeping track of CPU page table updates. Device page table updates are +not as easy as CPU page table updates. To update the device page table, you must +allocate a buffer (or use a pool of pre-allocated buffers) and write GPU +specific commands in it to perform the update (unmap, cache invalidations, and +flush, ...). This cannot be done through common code for all devices. Hence +why HMM provides helpers to factor out everything that can be while leaving the +hardware specific details to the device driver. + +The second mechanism HMM provides is a new kind of ZONE_DEVICE memory that +allows allocating a struct page for each page of device memory. Those pages +are special because the CPU cannot map them. However, they allow migrating +main memory to device memory using existing migration mechanisms and everything +looks like a page that is swapped out to disk from the CPU point of view. Using a +struct page gives the easiest and cleanest integration with existing mm +mechanisms. Here again, HMM only provides helpers, first to hotplug new ZONE_DEVICE +memory for the device memory and second to perform migration. Policy decisions +of what and when to migrate is left to the device driver. + +Note that any CPU access to a device page triggers a page fault and a migration +back to main memory. For example, when a page backing a given CPU address A is +migrated from a main memory page to a device page, then any CPU access to +address A triggers a page fault and initiates a migration back to main memory. + +With these two features, HMM not only allows a device to mirror process address +space and keeps both CPU and device page tables synchronized, but also +leverages device memory by migrating the part of the data set that is actively being +used by the device. + + +Address space mirroring implementation and API +============================================== + +Address space mirroring's main objective is to allow duplication of a range of +CPU page table into a device page table; HMM helps keep both synchronized. A +device driver that wants to mirror a process address space must start with the +registration of a mmu_interval_notifier:: + + int mmu_interval_notifier_insert(struct mmu_interval_notifier *interval_sub, + struct mm_struct *mm, unsigned long start, + unsigned long length, + const struct mmu_interval_notifier_ops *ops); + +During the ops->invalidate() callback the device driver must perform the +update action to the range (mark range read only, or fully unmap, etc.). The +device must complete the update before the driver callback returns. + +When the device driver wants to populate a range of virtual addresses, it can +use:: + + int hmm_range_fault(struct hmm_range *range); + +It will trigger a page fault on missing or read-only entries if write access is +requested (see below). Page faults use the generic mm page fault code path just +like a CPU page fault. + +Both functions copy CPU page table entries into their pfns array argument. Each +entry in that array corresponds to an address in the virtual range. HMM +provides a set of flags to help the driver identify special CPU page table +entries. + +Locking within the sync_cpu_device_pagetables() callback is the most important +aspect the driver must respect in order to keep things properly synchronized. +The usage pattern is:: + + int driver_populate_range(...) + { + struct hmm_range range; + ... + + range.notifier = &interval_sub; + range.start = ...; + range.end = ...; + range.hmm_pfns = ...; + + if (!mmget_not_zero(interval_sub->notifier.mm)) + return -EFAULT; + + again: + range.notifier_seq = mmu_interval_read_begin(&interval_sub); + mmap_read_lock(mm); + ret = hmm_range_fault(&range); + if (ret) { + mmap_read_unlock(mm); + if (ret == -EBUSY) + goto again; + return ret; + } + mmap_read_unlock(mm); + + take_lock(driver->update); + if (mmu_interval_read_retry(&ni, range.notifier_seq) { + release_lock(driver->update); + goto again; + } + + /* Use pfns array content to update device page table, + * under the update lock */ + + release_lock(driver->update); + return 0; + } + +The driver->update lock is the same lock that the driver takes inside its +invalidate() callback. That lock must be held before calling +mmu_interval_read_retry() to avoid any race with a concurrent CPU page table +update. + +Leverage default_flags and pfn_flags_mask +========================================= + +The hmm_range struct has 2 fields, default_flags and pfn_flags_mask, that specify +fault or snapshot policy for the whole range instead of having to set them +for each entry in the pfns array. + +For instance if the device driver wants pages for a range with at least read +permission, it sets:: + + range->default_flags = HMM_PFN_REQ_FAULT; + range->pfn_flags_mask = 0; + +and calls hmm_range_fault() as described above. This will fill fault all pages +in the range with at least read permission. + +Now let's say the driver wants to do the same except for one page in the range for +which it wants to have write permission. Now driver set:: + + range->default_flags = HMM_PFN_REQ_FAULT; + range->pfn_flags_mask = HMM_PFN_REQ_WRITE; + range->pfns[index_of_write] = HMM_PFN_REQ_WRITE; + +With this, HMM will fault in all pages with at least read (i.e., valid) and for the +address == range->start + (index_of_write << PAGE_SHIFT) it will fault with +write permission i.e., if the CPU pte does not have write permission set then HMM +will call handle_mm_fault(). + +After hmm_range_fault completes the flag bits are set to the current state of +the page tables, ie HMM_PFN_VALID | HMM_PFN_WRITE will be set if the page is +writable. + + +Represent and manage device memory from core kernel point of view +================================================================= + +Several different designs were tried to support device memory. The first one +used a device specific data structure to keep information about migrated memory +and HMM hooked itself in various places of mm code to handle any access to +addresses that were backed by device memory. It turns out that this ended up +replicating most of the fields of struct page and also needed many kernel code +paths to be updated to understand this new kind of memory. + +Most kernel code paths never try to access the memory behind a page +but only care about struct page contents. Because of this, HMM switched to +directly using struct page for device memory which left most kernel code paths +unaware of the difference. We only need to make sure that no one ever tries to +map those pages from the CPU side. + +Migration to and from device memory +=================================== + +Because the CPU cannot access device memory directly, the device driver must +use hardware DMA or device specific load/store instructions to migrate data. +The migrate_vma_setup(), migrate_vma_pages(), and migrate_vma_finalize() +functions are designed to make drivers easier to write and to centralize common +code across drivers. + +Before migrating pages to device private memory, special device private +``struct page`` need to be created. These will be used as special "swap" +page table entries so that a CPU process will fault if it tries to access +a page that has been migrated to device private memory. + +These can be allocated and freed with:: + + struct resource *res; + struct dev_pagemap pagemap; + + res = request_free_mem_region(&iomem_resource, /* number of bytes */, + "name of driver resource"); + pagemap.type = MEMORY_DEVICE_PRIVATE; + pagemap.range.start = res->start; + pagemap.range.end = res->end; + pagemap.nr_range = 1; + pagemap.ops = &device_devmem_ops; + memremap_pages(&pagemap, numa_node_id()); + + memunmap_pages(&pagemap); + release_mem_region(pagemap.range.start, range_len(&pagemap.range)); + +There are also devm_request_free_mem_region(), devm_memremap_pages(), +devm_memunmap_pages(), and devm_release_mem_region() when the resources can +be tied to a ``struct device``. + +The overall migration steps are similar to migrating NUMA pages within system +memory (see :ref:`Page migration <page_migration>`) but the steps are split +between device driver specific code and shared common code: + +1. ``mmap_read_lock()`` + + The device driver has to pass a ``struct vm_area_struct`` to + migrate_vma_setup() so the mmap_read_lock() or mmap_write_lock() needs to + be held for the duration of the migration. + +2. ``migrate_vma_setup(struct migrate_vma *args)`` + + The device driver initializes the ``struct migrate_vma`` fields and passes + the pointer to migrate_vma_setup(). The ``args->flags`` field is used to + filter which source pages should be migrated. For example, setting + ``MIGRATE_VMA_SELECT_SYSTEM`` will only migrate system memory and + ``MIGRATE_VMA_SELECT_DEVICE_PRIVATE`` will only migrate pages residing in + device private memory. If the latter flag is set, the ``args->pgmap_owner`` + field is used to identify device private pages owned by the driver. This + avoids trying to migrate device private pages residing in other devices. + Currently only anonymous private VMA ranges can be migrated to or from + system memory and device private memory. + + One of the first steps migrate_vma_setup() does is to invalidate other + device's MMUs with the ``mmu_notifier_invalidate_range_start(()`` and + ``mmu_notifier_invalidate_range_end()`` calls around the page table + walks to fill in the ``args->src`` array with PFNs to be migrated. + The ``invalidate_range_start()`` callback is passed a + ``struct mmu_notifier_range`` with the ``event`` field set to + ``MMU_NOTIFY_MIGRATE`` and the ``owner`` field set to + the ``args->pgmap_owner`` field passed to migrate_vma_setup(). This is + allows the device driver to skip the invalidation callback and only + invalidate device private MMU mappings that are actually migrating. + This is explained more in the next section. + + While walking the page tables, a ``pte_none()`` or ``is_zero_pfn()`` + entry results in a valid "zero" PFN stored in the ``args->src`` array. + This lets the driver allocate device private memory and clear it instead + of copying a page of zeros. Valid PTE entries to system memory or + device private struct pages will be locked with ``lock_page()``, isolated + from the LRU (if system memory since device private pages are not on + the LRU), unmapped from the process, and a special migration PTE is + inserted in place of the original PTE. + migrate_vma_setup() also clears the ``args->dst`` array. + +3. The device driver allocates destination pages and copies source pages to + destination pages. + + The driver checks each ``src`` entry to see if the ``MIGRATE_PFN_MIGRATE`` + bit is set and skips entries that are not migrating. The device driver + can also choose to skip migrating a page by not filling in the ``dst`` + array for that page. + + The driver then allocates either a device private struct page or a + system memory page, locks the page with ``lock_page()``, and fills in the + ``dst`` array entry with:: + + dst[i] = migrate_pfn(page_to_pfn(dpage)); + + Now that the driver knows that this page is being migrated, it can + invalidate device private MMU mappings and copy device private memory + to system memory or another device private page. The core Linux kernel + handles CPU page table invalidations so the device driver only has to + invalidate its own MMU mappings. + + The driver can use ``migrate_pfn_to_page(src[i])`` to get the + ``struct page`` of the source and either copy the source page to the + destination or clear the destination device private memory if the pointer + is ``NULL`` meaning the source page was not populated in system memory. + +4. ``migrate_vma_pages()`` + + This step is where the migration is actually "committed". + + If the source page was a ``pte_none()`` or ``is_zero_pfn()`` page, this + is where the newly allocated page is inserted into the CPU's page table. + This can fail if a CPU thread faults on the same page. However, the page + table is locked and only one of the new pages will be inserted. + The device driver will see that the ``MIGRATE_PFN_MIGRATE`` bit is cleared + if it loses the race. + + If the source page was locked, isolated, etc. the source ``struct page`` + information is now copied to destination ``struct page`` finalizing the + migration on the CPU side. + +5. Device driver updates device MMU page tables for pages still migrating, + rolling back pages not migrating. + + If the ``src`` entry still has ``MIGRATE_PFN_MIGRATE`` bit set, the device + driver can update the device MMU and set the write enable bit if the + ``MIGRATE_PFN_WRITE`` bit is set. + +6. ``migrate_vma_finalize()`` + + This step replaces the special migration page table entry with the new + page's page table entry and releases the reference to the source and + destination ``struct page``. + +7. ``mmap_read_unlock()`` + + The lock can now be released. + +Exclusive access memory +======================= + +Some devices have features such as atomic PTE bits that can be used to implement +atomic access to system memory. To support atomic operations to a shared virtual +memory page such a device needs access to that page which is exclusive of any +userspace access from the CPU. The ``make_device_exclusive_range()`` function +can be used to make a memory range inaccessible from userspace. + +This replaces all mappings for pages in the given range with special swap +entries. Any attempt to access the swap entry results in a fault which is +resovled by replacing the entry with the original mapping. A driver gets +notified that the mapping has been changed by MMU notifiers, after which point +it will no longer have exclusive access to the page. Exclusive access is +guranteed to last until the driver drops the page lock and page reference, at +which point any CPU faults on the page may proceed as described. + +Memory cgroup (memcg) and rss accounting +======================================== + +For now, device memory is accounted as any regular page in rss counters (either +anonymous if device page is used for anonymous, file if device page is used for +file backed page, or shmem if device page is used for shared memory). This is a +deliberate choice to keep existing applications, that might start using device +memory without knowing about it, running unimpacted. + +A drawback is that the OOM killer might kill an application using a lot of +device memory and not a lot of regular system memory and thus not freeing much +system memory. We want to gather more real world experience on how applications +and system react under memory pressure in the presence of device memory before +deciding to account device memory differently. + + +Same decision was made for memory cgroup. Device memory pages are accounted +against same memory cgroup a regular page would be accounted to. This does +simplify migration to and from device memory. This also means that migration +back from device memory to regular memory cannot fail because it would +go above memory cgroup limit. We might revisit this choice latter on once we +get more experience in how device memory is used and its impact on memory +resource control. + + +Note that device memory can never be pinned by a device driver nor through GUP +and thus such memory is always free upon process exit. Or when last reference +is dropped in case of shared memory or file backed memory. diff --git a/Documentation/mm/hugetlbfs_reserv.rst b/Documentation/mm/hugetlbfs_reserv.rst new file mode 100644 index 000000000000..f143954e0d05 --- /dev/null +++ b/Documentation/mm/hugetlbfs_reserv.rst @@ -0,0 +1,596 @@ +.. _hugetlbfs_reserve: + +===================== +Hugetlbfs Reservation +===================== + +Overview +======== + +Huge pages as described at :ref:`hugetlbpage` are typically +preallocated for application use. These huge pages are instantiated in a +task's address space at page fault time if the VMA indicates huge pages are +to be used. If no huge page exists at page fault time, the task is sent +a SIGBUS and often dies an unhappy death. Shortly after huge page support +was added, it was determined that it would be better to detect a shortage +of huge pages at mmap() time. The idea is that if there were not enough +huge pages to cover the mapping, the mmap() would fail. This was first +done with a simple check in the code at mmap() time to determine if there +were enough free huge pages to cover the mapping. Like most things in the +kernel, the code has evolved over time. However, the basic idea was to +'reserve' huge pages at mmap() time to ensure that huge pages would be +available for page faults in that mapping. The description below attempts to +describe how huge page reserve processing is done in the v4.10 kernel. + + +Audience +======== +This description is primarily targeted at kernel developers who are modifying +hugetlbfs code. + + +The Data Structures +=================== + +resv_huge_pages + This is a global (per-hstate) count of reserved huge pages. Reserved + huge pages are only available to the task which reserved them. + Therefore, the number of huge pages generally available is computed + as (``free_huge_pages - resv_huge_pages``). +Reserve Map + A reserve map is described by the structure:: + + struct resv_map { + struct kref refs; + spinlock_t lock; + struct list_head regions; + long adds_in_progress; + struct list_head region_cache; + long region_cache_count; + }; + + There is one reserve map for each huge page mapping in the system. + The regions list within the resv_map describes the regions within + the mapping. A region is described as:: + + struct file_region { + struct list_head link; + long from; + long to; + }; + + The 'from' and 'to' fields of the file region structure are huge page + indices into the mapping. Depending on the type of mapping, a + region in the reserv_map may indicate reservations exist for the + range, or reservations do not exist. +Flags for MAP_PRIVATE Reservations + These are stored in the bottom bits of the reservation map pointer. + + ``#define HPAGE_RESV_OWNER (1UL << 0)`` + Indicates this task is the owner of the reservations + associated with the mapping. + ``#define HPAGE_RESV_UNMAPPED (1UL << 1)`` + Indicates task originally mapping this range (and creating + reserves) has unmapped a page from this task (the child) + due to a failed COW. +Page Flags + The PagePrivate page flag is used to indicate that a huge page + reservation must be restored when the huge page is freed. More + details will be discussed in the "Freeing huge pages" section. + + +Reservation Map Location (Private or Shared) +============================================ + +A huge page mapping or segment is either private or shared. If private, +it is typically only available to a single address space (task). If shared, +it can be mapped into multiple address spaces (tasks). The location and +semantics of the reservation map is significantly different for the two types +of mappings. Location differences are: + +- For private mappings, the reservation map hangs off the VMA structure. + Specifically, vma->vm_private_data. This reserve map is created at the + time the mapping (mmap(MAP_PRIVATE)) is created. +- For shared mappings, the reservation map hangs off the inode. Specifically, + inode->i_mapping->private_data. Since shared mappings are always backed + by files in the hugetlbfs filesystem, the hugetlbfs code ensures each inode + contains a reservation map. As a result, the reservation map is allocated + when the inode is created. + + +Creating Reservations +===================== +Reservations are created when a huge page backed shared memory segment is +created (shmget(SHM_HUGETLB)) or a mapping is created via mmap(MAP_HUGETLB). +These operations result in a call to the routine hugetlb_reserve_pages():: + + int hugetlb_reserve_pages(struct inode *inode, + long from, long to, + struct vm_area_struct *vma, + vm_flags_t vm_flags) + +The first thing hugetlb_reserve_pages() does is check if the NORESERVE +flag was specified in either the shmget() or mmap() call. If NORESERVE +was specified, then this routine returns immediately as no reservations +are desired. + +The arguments 'from' and 'to' are huge page indices into the mapping or +underlying file. For shmget(), 'from' is always 0 and 'to' corresponds to +the length of the segment/mapping. For mmap(), the offset argument could +be used to specify the offset into the underlying file. In such a case, +the 'from' and 'to' arguments have been adjusted by this offset. + +One of the big differences between PRIVATE and SHARED mappings is the way +in which reservations are represented in the reservation map. + +- For shared mappings, an entry in the reservation map indicates a reservation + exists or did exist for the corresponding page. As reservations are + consumed, the reservation map is not modified. +- For private mappings, the lack of an entry in the reservation map indicates + a reservation exists for the corresponding page. As reservations are + consumed, entries are added to the reservation map. Therefore, the + reservation map can also be used to determine which reservations have + been consumed. + +For private mappings, hugetlb_reserve_pages() creates the reservation map and +hangs it off the VMA structure. In addition, the HPAGE_RESV_OWNER flag is set +to indicate this VMA owns the reservations. + +The reservation map is consulted to determine how many huge page reservations +are needed for the current mapping/segment. For private mappings, this is +always the value (to - from). However, for shared mappings it is possible that +some reservations may already exist within the range (to - from). See the +section :ref:`Reservation Map Modifications <resv_map_modifications>` +for details on how this is accomplished. + +The mapping may be associated with a subpool. If so, the subpool is consulted +to ensure there is sufficient space for the mapping. It is possible that the +subpool has set aside reservations that can be used for the mapping. See the +section :ref:`Subpool Reservations <sub_pool_resv>` for more details. + +After consulting the reservation map and subpool, the number of needed new +reservations is known. The routine hugetlb_acct_memory() is called to check +for and take the requested number of reservations. hugetlb_acct_memory() +calls into routines that potentially allocate and adjust surplus page counts. +However, within those routines the code is simply checking to ensure there +are enough free huge pages to accommodate the reservation. If there are, +the global reservation count resv_huge_pages is adjusted something like the +following:: + + if (resv_needed <= (resv_huge_pages - free_huge_pages)) + resv_huge_pages += resv_needed; + +Note that the global lock hugetlb_lock is held when checking and adjusting +these counters. + +If there were enough free huge pages and the global count resv_huge_pages +was adjusted, then the reservation map associated with the mapping is +modified to reflect the reservations. In the case of a shared mapping, a +file_region will exist that includes the range 'from' - 'to'. For private +mappings, no modifications are made to the reservation map as lack of an +entry indicates a reservation exists. + +If hugetlb_reserve_pages() was successful, the global reservation count and +reservation map associated with the mapping will be modified as required to +ensure reservations exist for the range 'from' - 'to'. + +.. _consume_resv: + +Consuming Reservations/Allocating a Huge Page +============================================= + +Reservations are consumed when huge pages associated with the reservations +are allocated and instantiated in the corresponding mapping. The allocation +is performed within the routine alloc_huge_page():: + + struct page *alloc_huge_page(struct vm_area_struct *vma, + unsigned long addr, int avoid_reserve) + +alloc_huge_page is passed a VMA pointer and a virtual address, so it can +consult the reservation map to determine if a reservation exists. In addition, +alloc_huge_page takes the argument avoid_reserve which indicates reserves +should not be used even if it appears they have been set aside for the +specified address. The avoid_reserve argument is most often used in the case +of Copy on Write and Page Migration where additional copies of an existing +page are being allocated. + +The helper routine vma_needs_reservation() is called to determine if a +reservation exists for the address within the mapping(vma). See the section +:ref:`Reservation Map Helper Routines <resv_map_helpers>` for detailed +information on what this routine does. +The value returned from vma_needs_reservation() is generally +0 or 1. 0 if a reservation exists for the address, 1 if no reservation exists. +If a reservation does not exist, and there is a subpool associated with the +mapping the subpool is consulted to determine if it contains reservations. +If the subpool contains reservations, one can be used for this allocation. +However, in every case the avoid_reserve argument overrides the use of +a reservation for the allocation. After determining whether a reservation +exists and can be used for the allocation, the routine dequeue_huge_page_vma() +is called. This routine takes two arguments related to reservations: + +- avoid_reserve, this is the same value/argument passed to alloc_huge_page() +- chg, even though this argument is of type long only the values 0 or 1 are + passed to dequeue_huge_page_vma. If the value is 0, it indicates a + reservation exists (see the section "Memory Policy and Reservations" for + possible issues). If the value is 1, it indicates a reservation does not + exist and the page must be taken from the global free pool if possible. + +The free lists associated with the memory policy of the VMA are searched for +a free page. If a page is found, the value free_huge_pages is decremented +when the page is removed from the free list. If there was a reservation +associated with the page, the following adjustments are made:: + + SetPagePrivate(page); /* Indicates allocating this page consumed + * a reservation, and if an error is + * encountered such that the page must be + * freed, the reservation will be restored. */ + resv_huge_pages--; /* Decrement the global reservation count */ + +Note, if no huge page can be found that satisfies the VMA's memory policy +an attempt will be made to allocate one using the buddy allocator. This +brings up the issue of surplus huge pages and overcommit which is beyond +the scope reservations. Even if a surplus page is allocated, the same +reservation based adjustments as above will be made: SetPagePrivate(page) and +resv_huge_pages--. + +After obtaining a new huge page, (page)->private is set to the value of +the subpool associated with the page if it exists. This will be used for +subpool accounting when the page is freed. + +The routine vma_commit_reservation() is then called to adjust the reserve +map based on the consumption of the reservation. In general, this involves +ensuring the page is represented within a file_region structure of the region +map. For shared mappings where the reservation was present, an entry +in the reserve map already existed so no change is made. However, if there +was no reservation in a shared mapping or this was a private mapping a new +entry must be created. + +It is possible that the reserve map could have been changed between the call +to vma_needs_reservation() at the beginning of alloc_huge_page() and the +call to vma_commit_reservation() after the page was allocated. This would +be possible if hugetlb_reserve_pages was called for the same page in a shared +mapping. In such cases, the reservation count and subpool free page count +will be off by one. This rare condition can be identified by comparing the +return value from vma_needs_reservation and vma_commit_reservation. If such +a race is detected, the subpool and global reserve counts are adjusted to +compensate. See the section +:ref:`Reservation Map Helper Routines <resv_map_helpers>` for more +information on these routines. + + +Instantiate Huge Pages +====================== + +After huge page allocation, the page is typically added to the page tables +of the allocating task. Before this, pages in a shared mapping are added +to the page cache and pages in private mappings are added to an anonymous +reverse mapping. In both cases, the PagePrivate flag is cleared. Therefore, +when a huge page that has been instantiated is freed no adjustment is made +to the global reservation count (resv_huge_pages). + + +Freeing Huge Pages +================== + +Huge page freeing is performed by the routine free_huge_page(). This routine +is the destructor for hugetlbfs compound pages. As a result, it is only +passed a pointer to the page struct. When a huge page is freed, reservation +accounting may need to be performed. This would be the case if the page was +associated with a subpool that contained reserves, or the page is being freed +on an error path where a global reserve count must be restored. + +The page->private field points to any subpool associated with the page. +If the PagePrivate flag is set, it indicates the global reserve count should +be adjusted (see the section +:ref:`Consuming Reservations/Allocating a Huge Page <consume_resv>` +for information on how these are set). + +The routine first calls hugepage_subpool_put_pages() for the page. If this +routine returns a value of 0 (which does not equal the value passed 1) it +indicates reserves are associated with the subpool, and this newly free page +must be used to keep the number of subpool reserves above the minimum size. +Therefore, the global resv_huge_pages counter is incremented in this case. + +If the PagePrivate flag was set in the page, the global resv_huge_pages counter +will always be incremented. + +.. _sub_pool_resv: + +Subpool Reservations +==================== + +There is a struct hstate associated with each huge page size. The hstate +tracks all huge pages of the specified size. A subpool represents a subset +of pages within a hstate that is associated with a mounted hugetlbfs +filesystem. + +When a hugetlbfs filesystem is mounted a min_size option can be specified +which indicates the minimum number of huge pages required by the filesystem. +If this option is specified, the number of huge pages corresponding to +min_size are reserved for use by the filesystem. This number is tracked in +the min_hpages field of a struct hugepage_subpool. At mount time, +hugetlb_acct_memory(min_hpages) is called to reserve the specified number of +huge pages. If they can not be reserved, the mount fails. + +The routines hugepage_subpool_get/put_pages() are called when pages are +obtained from or released back to a subpool. They perform all subpool +accounting, and track any reservations associated with the subpool. +hugepage_subpool_get/put_pages are passed the number of huge pages by which +to adjust the subpool 'used page' count (down for get, up for put). Normally, +they return the same value that was passed or an error if not enough pages +exist in the subpool. + +However, if reserves are associated with the subpool a return value less +than the passed value may be returned. This return value indicates the +number of additional global pool adjustments which must be made. For example, +suppose a subpool contains 3 reserved huge pages and someone asks for 5. +The 3 reserved pages associated with the subpool can be used to satisfy part +of the request. But, 2 pages must be obtained from the global pools. To +relay this information to the caller, the value 2 is returned. The caller +is then responsible for attempting to obtain the additional two pages from +the global pools. + + +COW and Reservations +==================== + +Since shared mappings all point to and use the same underlying pages, the +biggest reservation concern for COW is private mappings. In this case, +two tasks can be pointing at the same previously allocated page. One task +attempts to write to the page, so a new page must be allocated so that each +task points to its own page. + +When the page was originally allocated, the reservation for that page was +consumed. When an attempt to allocate a new page is made as a result of +COW, it is possible that no free huge pages are free and the allocation +will fail. + +When the private mapping was originally created, the owner of the mapping +was noted by setting the HPAGE_RESV_OWNER bit in the pointer to the reservation +map of the owner. Since the owner created the mapping, the owner owns all +the reservations associated with the mapping. Therefore, when a write fault +occurs and there is no page available, different action is taken for the owner +and non-owner of the reservation. + +In the case where the faulting task is not the owner, the fault will fail and +the task will typically receive a SIGBUS. + +If the owner is the faulting task, we want it to succeed since it owned the +original reservation. To accomplish this, the page is unmapped from the +non-owning task. In this way, the only reference is from the owning task. +In addition, the HPAGE_RESV_UNMAPPED bit is set in the reservation map pointer +of the non-owning task. The non-owning task may receive a SIGBUS if it later +faults on a non-present page. But, the original owner of the +mapping/reservation will behave as expected. + + +.. _resv_map_modifications: + +Reservation Map Modifications +============================= + +The following low level routines are used to make modifications to a +reservation map. Typically, these routines are not called directly. Rather, +a reservation map helper routine is called which calls one of these low level +routines. These low level routines are fairly well documented in the source +code (mm/hugetlb.c). These routines are:: + + long region_chg(struct resv_map *resv, long f, long t); + long region_add(struct resv_map *resv, long f, long t); + void region_abort(struct resv_map *resv, long f, long t); + long region_count(struct resv_map *resv, long f, long t); + +Operations on the reservation map typically involve two operations: + +1) region_chg() is called to examine the reserve map and determine how + many pages in the specified range [f, t) are NOT currently represented. + + The calling code performs global checks and allocations to determine if + there are enough huge pages for the operation to succeed. + +2) + a) If the operation can succeed, region_add() is called to actually modify + the reservation map for the same range [f, t) previously passed to + region_chg(). + b) If the operation can not succeed, region_abort is called for the same + range [f, t) to abort the operation. + +Note that this is a two step process where region_add() and region_abort() +are guaranteed to succeed after a prior call to region_chg() for the same +range. region_chg() is responsible for pre-allocating any data structures +necessary to ensure the subsequent operations (specifically region_add())) +will succeed. + +As mentioned above, region_chg() determines the number of pages in the range +which are NOT currently represented in the map. This number is returned to +the caller. region_add() returns the number of pages in the range added to +the map. In most cases, the return value of region_add() is the same as the +return value of region_chg(). However, in the case of shared mappings it is +possible for changes to the reservation map to be made between the calls to +region_chg() and region_add(). In this case, the return value of region_add() +will not match the return value of region_chg(). It is likely that in such +cases global counts and subpool accounting will be incorrect and in need of +adjustment. It is the responsibility of the caller to check for this condition +and make the appropriate adjustments. + +The routine region_del() is called to remove regions from a reservation map. +It is typically called in the following situations: + +- When a file in the hugetlbfs filesystem is being removed, the inode will + be released and the reservation map freed. Before freeing the reservation + map, all the individual file_region structures must be freed. In this case + region_del is passed the range [0, LONG_MAX). +- When a hugetlbfs file is being truncated. In this case, all allocated pages + after the new file size must be freed. In addition, any file_region entries + in the reservation map past the new end of file must be deleted. In this + case, region_del is passed the range [new_end_of_file, LONG_MAX). +- When a hole is being punched in a hugetlbfs file. In this case, huge pages + are removed from the middle of the file one at a time. As the pages are + removed, region_del() is called to remove the corresponding entry from the + reservation map. In this case, region_del is passed the range + [page_idx, page_idx + 1). + +In every case, region_del() will return the number of pages removed from the +reservation map. In VERY rare cases, region_del() can fail. This can only +happen in the hole punch case where it has to split an existing file_region +entry and can not allocate a new structure. In this error case, region_del() +will return -ENOMEM. The problem here is that the reservation map will +indicate that there is a reservation for the page. However, the subpool and +global reservation counts will not reflect the reservation. To handle this +situation, the routine hugetlb_fix_reserve_counts() is called to adjust the +counters so that they correspond with the reservation map entry that could +not be deleted. + +region_count() is called when unmapping a private huge page mapping. In +private mappings, the lack of a entry in the reservation map indicates that +a reservation exists. Therefore, by counting the number of entries in the +reservation map we know how many reservations were consumed and how many are +outstanding (outstanding = (end - start) - region_count(resv, start, end)). +Since the mapping is going away, the subpool and global reservation counts +are decremented by the number of outstanding reservations. + +.. _resv_map_helpers: + +Reservation Map Helper Routines +=============================== + +Several helper routines exist to query and modify the reservation maps. +These routines are only interested with reservations for a specific huge +page, so they just pass in an address instead of a range. In addition, +they pass in the associated VMA. From the VMA, the type of mapping (private +or shared) and the location of the reservation map (inode or VMA) can be +determined. These routines simply call the underlying routines described +in the section "Reservation Map Modifications". However, they do take into +account the 'opposite' meaning of reservation map entries for private and +shared mappings and hide this detail from the caller:: + + long vma_needs_reservation(struct hstate *h, + struct vm_area_struct *vma, + unsigned long addr) + +This routine calls region_chg() for the specified page. If no reservation +exists, 1 is returned. If a reservation exists, 0 is returned:: + + long vma_commit_reservation(struct hstate *h, + struct vm_area_struct *vma, + unsigned long addr) + +This calls region_add() for the specified page. As in the case of region_chg +and region_add, this routine is to be called after a previous call to +vma_needs_reservation. It will add a reservation entry for the page. It +returns 1 if the reservation was added and 0 if not. The return value should +be compared with the return value of the previous call to +vma_needs_reservation. An unexpected difference indicates the reservation +map was modified between calls:: + + void vma_end_reservation(struct hstate *h, + struct vm_area_struct *vma, + unsigned long addr) + +This calls region_abort() for the specified page. As in the case of region_chg +and region_abort, this routine is to be called after a previous call to +vma_needs_reservation. It will abort/end the in progress reservation add +operation:: + + long vma_add_reservation(struct hstate *h, + struct vm_area_struct *vma, + unsigned long addr) + +This is a special wrapper routine to help facilitate reservation cleanup +on error paths. It is only called from the routine restore_reserve_on_error(). +This routine is used in conjunction with vma_needs_reservation in an attempt +to add a reservation to the reservation map. It takes into account the +different reservation map semantics for private and shared mappings. Hence, +region_add is called for shared mappings (as an entry present in the map +indicates a reservation), and region_del is called for private mappings (as +the absence of an entry in the map indicates a reservation). See the section +"Reservation cleanup in error paths" for more information on what needs to +be done on error paths. + + +Reservation Cleanup in Error Paths +================================== + +As mentioned in the section +:ref:`Reservation Map Helper Routines <resv_map_helpers>`, reservation +map modifications are performed in two steps. First vma_needs_reservation +is called before a page is allocated. If the allocation is successful, +then vma_commit_reservation is called. If not, vma_end_reservation is called. +Global and subpool reservation counts are adjusted based on success or failure +of the operation and all is well. + +Additionally, after a huge page is instantiated the PagePrivate flag is +cleared so that accounting when the page is ultimately freed is correct. + +However, there are several instances where errors are encountered after a huge +page is allocated but before it is instantiated. In this case, the page +allocation has consumed the reservation and made the appropriate subpool, +reservation map and global count adjustments. If the page is freed at this +time (before instantiation and clearing of PagePrivate), then free_huge_page +will increment the global reservation count. However, the reservation map +indicates the reservation was consumed. This resulting inconsistent state +will cause the 'leak' of a reserved huge page. The global reserve count will +be higher than it should and prevent allocation of a pre-allocated page. + +The routine restore_reserve_on_error() attempts to handle this situation. It +is fairly well documented. The intention of this routine is to restore +the reservation map to the way it was before the page allocation. In this +way, the state of the reservation map will correspond to the global reservation +count after the page is freed. + +The routine restore_reserve_on_error itself may encounter errors while +attempting to restore the reservation map entry. In this case, it will +simply clear the PagePrivate flag of the page. In this way, the global +reserve count will not be incremented when the page is freed. However, the +reservation map will continue to look as though the reservation was consumed. +A page can still be allocated for the address, but it will not use a reserved +page as originally intended. + +There is some code (most notably userfaultfd) which can not call +restore_reserve_on_error. In this case, it simply modifies the PagePrivate +so that a reservation will not be leaked when the huge page is freed. + + +Reservations and Memory Policy +============================== +Per-node huge page lists existed in struct hstate when git was first used +to manage Linux code. The concept of reservations was added some time later. +When reservations were added, no attempt was made to take memory policy +into account. While cpusets are not exactly the same as memory policy, this +comment in hugetlb_acct_memory sums up the interaction between reservations +and cpusets/memory policy:: + + /* + * When cpuset is configured, it breaks the strict hugetlb page + * reservation as the accounting is done on a global variable. Such + * reservation is completely rubbish in the presence of cpuset because + * the reservation is not checked against page availability for the + * current cpuset. Application can still potentially OOM'ed by kernel + * with lack of free htlb page in cpuset that the task is in. + * Attempt to enforce strict accounting with cpuset is almost + * impossible (or too ugly) because cpuset is too fluid that + * task or memory node can be dynamically moved between cpusets. + * + * The change of semantics for shared hugetlb mapping with cpuset is + * undesirable. However, in order to preserve some of the semantics, + * we fall back to check against current free page availability as + * a best attempt and hopefully to minimize the impact of changing + * semantics that cpuset has. + */ + +Huge page reservations were added to prevent unexpected page allocation +failures (OOM) at page fault time. However, if an application makes use +of cpusets or memory policy there is no guarantee that huge pages will be +available on the required nodes. This is true even if there are a sufficient +number of global reservations. + +Hugetlbfs regression testing +============================ + +The most complete set of hugetlb tests are in the libhugetlbfs repository. +If you modify any hugetlb related code, use the libhugetlbfs test suite +to check for regressions. In addition, if you add any new hugetlb +functionality, please add appropriate tests to libhugetlbfs. + +-- +Mike Kravetz, 7 April 2017 diff --git a/Documentation/mm/hwpoison.rst b/Documentation/mm/hwpoison.rst new file mode 100644 index 000000000000..b9d5253c1305 --- /dev/null +++ b/Documentation/mm/hwpoison.rst @@ -0,0 +1,184 @@ +.. hwpoison: + +======== +hwpoison +======== + +What is hwpoison? +================= + +Upcoming Intel CPUs have support for recovering from some memory errors +(``MCA recovery``). This requires the OS to declare a page "poisoned", +kill the processes associated with it and avoid using it in the future. + +This patchkit implements the necessary infrastructure in the VM. + +To quote the overview comment:: + + High level machine check handler. Handles pages reported by the + hardware as being corrupted usually due to a 2bit ECC memory or cache + failure. + + This focusses on pages detected as corrupted in the background. + When the current CPU tries to consume corruption the currently + running process can just be killed directly instead. This implies + that if the error cannot be handled for some reason it's safe to + just ignore it because no corruption has been consumed yet. Instead + when that happens another machine check will happen. + + Handles page cache pages in various states. The tricky part + here is that we can access any page asynchronous to other VM + users, because memory failures could happen anytime and anywhere, + possibly violating some of their assumptions. This is why this code + has to be extremely careful. Generally it tries to use normal locking + rules, as in get the standard locks, even if that means the + error handling takes potentially a long time. + + Some of the operations here are somewhat inefficient and have non + linear algorithmic complexity, because the data structures have not + been optimized for this case. This is in particular the case + for the mapping from a vma to a process. Since this case is expected + to be rare we hope we can get away with this. + +The code consists of a the high level handler in mm/memory-failure.c, +a new page poison bit and various checks in the VM to handle poisoned +pages. + +The main target right now is KVM guests, but it works for all kinds +of applications. KVM support requires a recent qemu-kvm release. + +For the KVM use there was need for a new signal type so that +KVM can inject the machine check into the guest with the proper +address. This in theory allows other applications to handle +memory failures too. The expection is that near all applications +won't do that, but some very specialized ones might. + +Failure recovery modes +====================== + +There are two (actually three) modes memory failure recovery can be in: + +vm.memory_failure_recovery sysctl set to zero: + All memory failures cause a panic. Do not attempt recovery. + +early kill + (can be controlled globally and per process) + Send SIGBUS to the application as soon as the error is detected + This allows applications who can process memory errors in a gentle + way (e.g. drop affected object) + This is the mode used by KVM qemu. + +late kill + Send SIGBUS when the application runs into the corrupted page. + This is best for memory error unaware applications and default + Note some pages are always handled as late kill. + +User control +============ + +vm.memory_failure_recovery + See sysctl.txt + +vm.memory_failure_early_kill + Enable early kill mode globally + +PR_MCE_KILL + Set early/late kill mode/revert to system default + + arg1: PR_MCE_KILL_CLEAR: + Revert to system default + arg1: PR_MCE_KILL_SET: + arg2 defines thread specific mode + + PR_MCE_KILL_EARLY: + Early kill + PR_MCE_KILL_LATE: + Late kill + PR_MCE_KILL_DEFAULT + Use system global default + + Note that if you want to have a dedicated thread which handles + the SIGBUS(BUS_MCEERR_AO) on behalf of the process, you should + call prctl(PR_MCE_KILL_EARLY) on the designated thread. Otherwise, + the SIGBUS is sent to the main thread. + +PR_MCE_KILL_GET + return current mode + +Testing +======= + +* madvise(MADV_HWPOISON, ....) (as root) - Poison a page in the + process for testing + +* hwpoison-inject module through debugfs ``/sys/kernel/debug/hwpoison/`` + + corrupt-pfn + Inject hwpoison fault at PFN echoed into this file. This does + some early filtering to avoid corrupted unintended pages in test suites. + + unpoison-pfn + Software-unpoison page at PFN echoed into this file. This way + a page can be reused again. This only works for Linux + injected failures, not for real memory failures. Once any hardware + memory failure happens, this feature is disabled. + + Note these injection interfaces are not stable and might change between + kernel versions + + corrupt-filter-dev-major, corrupt-filter-dev-minor + Only handle memory failures to pages associated with the file + system defined by block device major/minor. -1U is the + wildcard value. This should be only used for testing with + artificial injection. + + corrupt-filter-memcg + Limit injection to pages owned by memgroup. Specified by inode + number of the memcg. + + Example:: + + mkdir /sys/fs/cgroup/mem/hwpoison + + usemem -m 100 -s 1000 & + echo `jobs -p` > /sys/fs/cgroup/mem/hwpoison/tasks + + memcg_ino=$(ls -id /sys/fs/cgroup/mem/hwpoison | cut -f1 -d' ') + echo $memcg_ino > /debug/hwpoison/corrupt-filter-memcg + + page-types -p `pidof init` --hwpoison # shall do nothing + page-types -p `pidof usemem` --hwpoison # poison its pages + + corrupt-filter-flags-mask, corrupt-filter-flags-value + When specified, only poison pages if ((page_flags & mask) == + value). This allows stress testing of many kinds of + pages. The page_flags are the same as in /proc/kpageflags. The + flag bits are defined in include/linux/kernel-page-flags.h and + documented in Documentation/admin-guide/mm/pagemap.rst + +* Architecture specific MCE injector + + x86 has mce-inject, mce-test + + Some portable hwpoison test programs in mce-test, see below. + +References +========== + +http://halobates.de/mce-lc09-2.pdf + Overview presentation from LinuxCon 09 + +git://git.kernel.org/pub/scm/utils/cpu/mce/mce-test.git + Test suite (hwpoison specific portable tests in tsrc) + +git://git.kernel.org/pub/scm/utils/cpu/mce/mce-inject.git + x86 specific injector + + +Limitations +=========== +- Not all page types are supported and never will. Most kernel internal + objects cannot be recovered, only LRU pages for now. + +--- +Andi Kleen, Oct 2009 diff --git a/Documentation/mm/index.rst b/Documentation/mm/index.rst new file mode 100644 index 000000000000..575ccd40e30c --- /dev/null +++ b/Documentation/mm/index.rst @@ -0,0 +1,68 @@ +===================================== +Linux Memory Management Documentation +===================================== + +Memory Management Guide +======================= + +This is a guide to understanding the memory management subsystem +of Linux. If you are looking for advice on simply allocating memory, +see the :ref:`memory_allocation`. For controlling and tuning guides, +see the :doc:`admin guide <../admin-guide/mm/index>`. + +.. toctree:: + :maxdepth: 1 + + physical_memory + page_tables + process_addrs + bootmem + page_allocation + vmalloc + slab + highmem + page_reclaim + swap + page_cache + shmfs + oom + +Legacy Documentation +==================== + +This is a collection of older documents about the Linux memory management +(MM) subsystem internals with different level of details ranging from +notes and mailing list responses for elaborating descriptions of data +structures and algorithms. It should all be integrated nicely into the +above structured documentation, or deleted if it has served its purpose. + +.. toctree:: + :maxdepth: 1 + + active_mm + arch_pgtable_helpers + balance + damon/index + free_page_reporting + frontswap + hmm + hwpoison + hugetlbfs_reserv + ksm + memory-model + mmu_notifier + numa + overcommit-accounting + page_migration + page_frags + page_owner + page_table_check + remap_file_pages + slub + split_page_table_lock + transhuge + unevictable-lru + vmalloced-kernel-stacks + vmemmap_dedup + z3fold + zsmalloc diff --git a/Documentation/mm/ksm.rst b/Documentation/mm/ksm.rst new file mode 100644 index 000000000000..9e37add068e6 --- /dev/null +++ b/Documentation/mm/ksm.rst @@ -0,0 +1,87 @@ +.. _ksm: + +======================= +Kernel Samepage Merging +======================= + +KSM is a memory-saving de-duplication feature, enabled by CONFIG_KSM=y, +added to the Linux kernel in 2.6.32. See ``mm/ksm.c`` for its implementation, +and http://lwn.net/Articles/306704/ and https://lwn.net/Articles/330589/ + +The userspace interface of KSM is described in :ref:`Documentation/admin-guide/mm/ksm.rst <admin_guide_ksm>` + +Design +====== + +Overview +-------- + +.. kernel-doc:: mm/ksm.c + :DOC: Overview + +Reverse mapping +--------------- +KSM maintains reverse mapping information for KSM pages in the stable +tree. + +If a KSM page is shared between less than ``max_page_sharing`` VMAs, +the node of the stable tree that represents such KSM page points to a +list of struct rmap_item and the ``page->mapping`` of the +KSM page points to the stable tree node. + +When the sharing passes this threshold, KSM adds a second dimension to +the stable tree. The tree node becomes a "chain" that links one or +more "dups". Each "dup" keeps reverse mapping information for a KSM +page with ``page->mapping`` pointing to that "dup". + +Every "chain" and all "dups" linked into a "chain" enforce the +invariant that they represent the same write protected memory content, +even if each "dup" will be pointed by a different KSM page copy of +that content. + +This way the stable tree lookup computational complexity is unaffected +if compared to an unlimited list of reverse mappings. It is still +enforced that there cannot be KSM page content duplicates in the +stable tree itself. + +The deduplication limit enforced by ``max_page_sharing`` is required +to avoid the virtual memory rmap lists to grow too large. The rmap +walk has O(N) complexity where N is the number of rmap_items +(i.e. virtual mappings) that are sharing the page, which is in turn +capped by ``max_page_sharing``. So this effectively spreads the linear +O(N) computational complexity from rmap walk context over different +KSM pages. The ksmd walk over the stable_node "chains" is also O(N), +but N is the number of stable_node "dups", not the number of +rmap_items, so it has not a significant impact on ksmd performance. In +practice the best stable_node "dup" candidate will be kept and found +at the head of the "dups" list. + +High values of ``max_page_sharing`` result in faster memory merging +(because there will be fewer stable_node dups queued into the +stable_node chain->hlist to check for pruning) and higher +deduplication factor at the expense of slower worst case for rmap +walks for any KSM page which can happen during swapping, compaction, +NUMA balancing and page migration. + +The ``stable_node_dups/stable_node_chains`` ratio is also affected by the +``max_page_sharing`` tunable, and an high ratio may indicate fragmentation +in the stable_node dups, which could be solved by introducing +fragmentation algorithms in ksmd which would refile rmap_items from +one stable_node dup to another stable_node dup, in order to free up +stable_node "dups" with few rmap_items in them, but that may increase +the ksmd CPU usage and possibly slowdown the readonly computations on +the KSM pages of the applications. + +The whole list of stable_node "dups" linked in the stable_node +"chains" is scanned periodically in order to prune stale stable_nodes. +The frequency of such scans is defined by +``stable_node_chains_prune_millisecs`` sysfs tunable. + +Reference +--------- +.. kernel-doc:: mm/ksm.c + :functions: mm_slot ksm_scan stable_node rmap_item + +-- +Izik Eidus, +Hugh Dickins, 17 Nov 2009 diff --git a/Documentation/mm/memory-model.rst b/Documentation/mm/memory-model.rst new file mode 100644 index 000000000000..3779e562dc76 --- /dev/null +++ b/Documentation/mm/memory-model.rst @@ -0,0 +1,177 @@ +.. SPDX-License-Identifier: GPL-2.0 + +.. _physical_memory_model: + +===================== +Physical Memory Model +===================== + +Physical memory in a system may be addressed in different ways. The +simplest case is when the physical memory starts at address 0 and +spans a contiguous range up to the maximal address. It could be, +however, that this range contains small holes that are not accessible +for the CPU. Then there could be several contiguous ranges at +completely distinct addresses. And, don't forget about NUMA, where +different memory banks are attached to different CPUs. + +Linux abstracts this diversity using one of the two memory models: +FLATMEM and SPARSEMEM. Each architecture defines what +memory models it supports, what the default memory model is and +whether it is possible to manually override that default. + +All the memory models track the status of physical page frames using +struct page arranged in one or more arrays. + +Regardless of the selected memory model, there exists one-to-one +mapping between the physical page frame number (PFN) and the +corresponding `struct page`. + +Each memory model defines :c:func:`pfn_to_page` and :c:func:`page_to_pfn` +helpers that allow the conversion from PFN to `struct page` and vice +versa. + +FLATMEM +======= + +The simplest memory model is FLATMEM. This model is suitable for +non-NUMA systems with contiguous, or mostly contiguous, physical +memory. + +In the FLATMEM memory model, there is a global `mem_map` array that +maps the entire physical memory. For most architectures, the holes +have entries in the `mem_map` array. The `struct page` objects +corresponding to the holes are never fully initialized. + +To allocate the `mem_map` array, architecture specific setup code should +call :c:func:`free_area_init` function. Yet, the mappings array is not +usable until the call to :c:func:`memblock_free_all` that hands all the +memory to the page allocator. + +An architecture may free parts of the `mem_map` array that do not cover the +actual physical pages. In such case, the architecture specific +:c:func:`pfn_valid` implementation should take the holes in the +`mem_map` into account. + +With FLATMEM, the conversion between a PFN and the `struct page` is +straightforward: `PFN - ARCH_PFN_OFFSET` is an index to the +`mem_map` array. + +The `ARCH_PFN_OFFSET` defines the first page frame number for +systems with physical memory starting at address different from 0. + +SPARSEMEM +========= + +SPARSEMEM is the most versatile memory model available in Linux and it +is the only memory model that supports several advanced features such +as hot-plug and hot-remove of the physical memory, alternative memory +maps for non-volatile memory devices and deferred initialization of +the memory map for larger systems. + +The SPARSEMEM model presents the physical memory as a collection of +sections. A section is represented with struct mem_section +that contains `section_mem_map` that is, logically, a pointer to an +array of struct pages. However, it is stored with some other magic +that aids the sections management. The section size and maximal number +of section is specified using `SECTION_SIZE_BITS` and +`MAX_PHYSMEM_BITS` constants defined by each architecture that +supports SPARSEMEM. While `MAX_PHYSMEM_BITS` is an actual width of a +physical address that an architecture supports, the +`SECTION_SIZE_BITS` is an arbitrary value. + +The maximal number of sections is denoted `NR_MEM_SECTIONS` and +defined as + +.. math:: + + NR\_MEM\_SECTIONS = 2 ^ {(MAX\_PHYSMEM\_BITS - SECTION\_SIZE\_BITS)} + +The `mem_section` objects are arranged in a two-dimensional array +called `mem_sections`. The size and placement of this array depend +on `CONFIG_SPARSEMEM_EXTREME` and the maximal possible number of +sections: + +* When `CONFIG_SPARSEMEM_EXTREME` is disabled, the `mem_sections` + array is static and has `NR_MEM_SECTIONS` rows. Each row holds a + single `mem_section` object. +* When `CONFIG_SPARSEMEM_EXTREME` is enabled, the `mem_sections` + array is dynamically allocated. Each row contains PAGE_SIZE worth of + `mem_section` objects and the number of rows is calculated to fit + all the memory sections. + +The architecture setup code should call sparse_init() to +initialize the memory sections and the memory maps. + +With SPARSEMEM there are two possible ways to convert a PFN to the +corresponding `struct page` - a "classic sparse" and "sparse +vmemmap". The selection is made at build time and it is determined by +the value of `CONFIG_SPARSEMEM_VMEMMAP`. + +The classic sparse encodes the section number of a page in page->flags +and uses high bits of a PFN to access the section that maps that page +frame. Inside a section, the PFN is the index to the array of pages. + +The sparse vmemmap uses a virtually mapped memory map to optimize +pfn_to_page and page_to_pfn operations. There is a global `struct +page *vmemmap` pointer that points to a virtually contiguous array of +`struct page` objects. A PFN is an index to that array and the +offset of the `struct page` from `vmemmap` is the PFN of that +page. + +To use vmemmap, an architecture has to reserve a range of virtual +addresses that will map the physical pages containing the memory +map and make sure that `vmemmap` points to that range. In addition, +the architecture should implement :c:func:`vmemmap_populate` method +that will allocate the physical memory and create page tables for the +virtual memory map. If an architecture does not have any special +requirements for the vmemmap mappings, it can use default +:c:func:`vmemmap_populate_basepages` provided by the generic memory +management. + +The virtually mapped memory map allows storing `struct page` objects +for persistent memory devices in pre-allocated storage on those +devices. This storage is represented with struct vmem_altmap +that is eventually passed to vmemmap_populate() through a long chain +of function calls. The vmemmap_populate() implementation may use the +`vmem_altmap` along with :c:func:`vmemmap_alloc_block_buf` helper to +allocate memory map on the persistent memory device. + +ZONE_DEVICE +=========== +The `ZONE_DEVICE` facility builds upon `SPARSEMEM_VMEMMAP` to offer +`struct page` `mem_map` services for device driver identified physical +address ranges. The "device" aspect of `ZONE_DEVICE` relates to the fact +that the page objects for these address ranges are never marked online, +and that a reference must be taken against the device, not just the page +to keep the memory pinned for active use. `ZONE_DEVICE`, via +:c:func:`devm_memremap_pages`, performs just enough memory hotplug to +turn on :c:func:`pfn_to_page`, :c:func:`page_to_pfn`, and +:c:func:`get_user_pages` service for the given range of pfns. Since the +page reference count never drops below 1 the page is never tracked as +free memory and the page's `struct list_head lru` space is repurposed +for back referencing to the host device / driver that mapped the memory. + +While `SPARSEMEM` presents memory as a collection of sections, +optionally collected into memory blocks, `ZONE_DEVICE` users have a need +for smaller granularity of populating the `mem_map`. Given that +`ZONE_DEVICE` memory is never marked online it is subsequently never +subject to its memory ranges being exposed through the sysfs memory +hotplug api on memory block boundaries. The implementation relies on +this lack of user-api constraint to allow sub-section sized memory +ranges to be specified to :c:func:`arch_add_memory`, the top-half of +memory hotplug. Sub-section support allows for 2MB as the cross-arch +common alignment granularity for :c:func:`devm_memremap_pages`. + +The users of `ZONE_DEVICE` are: + +* pmem: Map platform persistent memory to be used as a direct-I/O target + via DAX mappings. + +* hmm: Extend `ZONE_DEVICE` with `->page_fault()` and `->page_free()` + event callbacks to allow a device-driver to coordinate memory management + events related to device-memory, typically GPU memory. See + Documentation/mm/hmm.rst. + +* p2pdma: Create `struct page` objects to allow peer devices in a + PCI/-E topology to coordinate direct-DMA operations between themselves, + i.e. bypass host memory. diff --git a/Documentation/mm/mmu_notifier.rst b/Documentation/mm/mmu_notifier.rst new file mode 100644 index 000000000000..df5d7777fc6b --- /dev/null +++ b/Documentation/mm/mmu_notifier.rst @@ -0,0 +1,99 @@ +.. _mmu_notifier: + +When do you need to notify inside page table lock ? +=================================================== + +When clearing a pte/pmd we are given a choice to notify the event through +(notify version of \*_clear_flush call mmu_notifier_invalidate_range) under +the page table lock. But that notification is not necessary in all cases. + +For secondary TLB (non CPU TLB) like IOMMU TLB or device TLB (when device use +thing like ATS/PASID to get the IOMMU to walk the CPU page table to access a +process virtual address space). There is only 2 cases when you need to notify +those secondary TLB while holding page table lock when clearing a pte/pmd: + + A) page backing address is free before mmu_notifier_invalidate_range_end() + B) a page table entry is updated to point to a new page (COW, write fault + on zero page, __replace_page(), ...) + +Case A is obvious you do not want to take the risk for the device to write to +a page that might now be used by some completely different task. + +Case B is more subtle. For correctness it requires the following sequence to +happen: + + - take page table lock + - clear page table entry and notify ([pmd/pte]p_huge_clear_flush_notify()) + - set page table entry to point to new page + +If clearing the page table entry is not followed by a notify before setting +the new pte/pmd value then you can break memory model like C11 or C++11 for +the device. + +Consider the following scenario (device use a feature similar to ATS/PASID): + +Two address addrA and addrB such that \|addrA - addrB\| >= PAGE_SIZE we assume +they are write protected for COW (other case of B apply too). + +:: + + [Time N] -------------------------------------------------------------------- + CPU-thread-0 {try to write to addrA} + CPU-thread-1 {try to write to addrB} + CPU-thread-2 {} + CPU-thread-3 {} + DEV-thread-0 {read addrA and populate device TLB} + DEV-thread-2 {read addrB and populate device TLB} + [Time N+1] ------------------------------------------------------------------ + CPU-thread-0 {COW_step0: {mmu_notifier_invalidate_range_start(addrA)}} + CPU-thread-1 {COW_step0: {mmu_notifier_invalidate_range_start(addrB)}} + CPU-thread-2 {} + CPU-thread-3 {} + DEV-thread-0 {} + DEV-thread-2 {} + [Time N+2] ------------------------------------------------------------------ + CPU-thread-0 {COW_step1: {update page table to point to new page for addrA}} + CPU-thread-1 {COW_step1: {update page table to point to new page for addrB}} + CPU-thread-2 {} + CPU-thread-3 {} + DEV-thread-0 {} + DEV-thread-2 {} + [Time N+3] ------------------------------------------------------------------ + CPU-thread-0 {preempted} + CPU-thread-1 {preempted} + CPU-thread-2 {write to addrA which is a write to new page} + CPU-thread-3 {} + DEV-thread-0 {} + DEV-thread-2 {} + [Time N+3] ------------------------------------------------------------------ + CPU-thread-0 {preempted} + CPU-thread-1 {preempted} + CPU-thread-2 {} + CPU-thread-3 {write to addrB which is a write to new page} + DEV-thread-0 {} + DEV-thread-2 {} + [Time N+4] ------------------------------------------------------------------ + CPU-thread-0 {preempted} + CPU-thread-1 {COW_step3: {mmu_notifier_invalidate_range_end(addrB)}} + CPU-thread-2 {} + CPU-thread-3 {} + DEV-thread-0 {} + DEV-thread-2 {} + [Time N+5] ------------------------------------------------------------------ + CPU-thread-0 {preempted} + CPU-thread-1 {} + CPU-thread-2 {} + CPU-thread-3 {} + DEV-thread-0 {read addrA from old page} + DEV-thread-2 {read addrB from new page} + +So here because at time N+2 the clear page table entry was not pair with a +notification to invalidate the secondary TLB, the device see the new value for +addrB before seeing the new value for addrA. This break total memory ordering +for the device. + +When changing a pte to write protect or to point to a new write protected page +with same content (KSM) it is fine to delay the mmu_notifier_invalidate_range +call to mmu_notifier_invalidate_range_end() outside the page table lock. This +is true even if the thread doing the page table update is preempted right after +releasing page table lock but before call mmu_notifier_invalidate_range_end(). diff --git a/Documentation/mm/numa.rst b/Documentation/mm/numa.rst new file mode 100644 index 000000000000..99fdeca917ca --- /dev/null +++ b/Documentation/mm/numa.rst @@ -0,0 +1,150 @@ +.. _numa: + +Started Nov 1999 by Kanoj Sarcar <kanoj@sgi.com> + +============= +What is NUMA? +============= + +This question can be answered from a couple of perspectives: the +hardware view and the Linux software view. + +From the hardware perspective, a NUMA system is a computer platform that +comprises multiple components or assemblies each of which may contain 0 +or more CPUs, local memory, and/or IO buses. For brevity and to +disambiguate the hardware view of these physical components/assemblies +from the software abstraction thereof, we'll call the components/assemblies +'cells' in this document. + +Each of the 'cells' may be viewed as an SMP [symmetric multi-processor] subset +of the system--although some components necessary for a stand-alone SMP system +may not be populated on any given cell. The cells of the NUMA system are +connected together with some sort of system interconnect--e.g., a crossbar or +point-to-point link are common types of NUMA system interconnects. Both of +these types of interconnects can be aggregated to create NUMA platforms with +cells at multiple distances from other cells. + +For Linux, the NUMA platforms of interest are primarily what is known as Cache +Coherent NUMA or ccNUMA systems. With ccNUMA systems, all memory is visible +to and accessible from any CPU attached to any cell and cache coherency +is handled in hardware by the processor caches and/or the system interconnect. + +Memory access time and effective memory bandwidth varies depending on how far +away the cell containing the CPU or IO bus making the memory access is from the +cell containing the target memory. For example, access to memory by CPUs +attached to the same cell will experience faster access times and higher +bandwidths than accesses to memory on other, remote cells. NUMA platforms +can have cells at multiple remote distances from any given cell. + +Platform vendors don't build NUMA systems just to make software developers' +lives interesting. Rather, this architecture is a means to provide scalable +memory bandwidth. However, to achieve scalable memory bandwidth, system and +application software must arrange for a large majority of the memory references +[cache misses] to be to "local" memory--memory on the same cell, if any--or +to the closest cell with memory. + +This leads to the Linux software view of a NUMA system: + +Linux divides the system's hardware resources into multiple software +abstractions called "nodes". Linux maps the nodes onto the physical cells +of the hardware platform, abstracting away some of the details for some +architectures. As with physical cells, software nodes may contain 0 or more +CPUs, memory and/or IO buses. And, again, memory accesses to memory on +"closer" nodes--nodes that map to closer cells--will generally experience +faster access times and higher effective bandwidth than accesses to more +remote cells. + +For some architectures, such as x86, Linux will "hide" any node representing a +physical cell that has no memory attached, and reassign any CPUs attached to +that cell to a node representing a cell that does have memory. Thus, on +these architectures, one cannot assume that all CPUs that Linux associates with +a given node will see the same local memory access times and bandwidth. + +In addition, for some architectures, again x86 is an example, Linux supports +the emulation of additional nodes. For NUMA emulation, linux will carve up +the existing nodes--or the system memory for non-NUMA platforms--into multiple +nodes. Each emulated node will manage a fraction of the underlying cells' +physical memory. NUMA emluation is useful for testing NUMA kernel and +application features on non-NUMA platforms, and as a sort of memory resource +management mechanism when used together with cpusets. +[see Documentation/admin-guide/cgroup-v1/cpusets.rst] + +For each node with memory, Linux constructs an independent memory management +subsystem, complete with its own free page lists, in-use page lists, usage +statistics and locks to mediate access. In addition, Linux constructs for +each memory zone [one or more of DMA, DMA32, NORMAL, HIGH_MEMORY, MOVABLE], +an ordered "zonelist". A zonelist specifies the zones/nodes to visit when a +selected zone/node cannot satisfy the allocation request. This situation, +when a zone has no available memory to satisfy a request, is called +"overflow" or "fallback". + +Because some nodes contain multiple zones containing different types of +memory, Linux must decide whether to order the zonelists such that allocations +fall back to the same zone type on a different node, or to a different zone +type on the same node. This is an important consideration because some zones, +such as DMA or DMA32, represent relatively scarce resources. Linux chooses +a default Node ordered zonelist. This means it tries to fallback to other zones +from the same node before using remote nodes which are ordered by NUMA distance. + +By default, Linux will attempt to satisfy memory allocation requests from the +node to which the CPU that executes the request is assigned. Specifically, +Linux will attempt to allocate from the first node in the appropriate zonelist +for the node where the request originates. This is called "local allocation." +If the "local" node cannot satisfy the request, the kernel will examine other +nodes' zones in the selected zonelist looking for the first zone in the list +that can satisfy the request. + +Local allocation will tend to keep subsequent access to the allocated memory +"local" to the underlying physical resources and off the system interconnect-- +as long as the task on whose behalf the kernel allocated some memory does not +later migrate away from that memory. The Linux scheduler is aware of the +NUMA topology of the platform--embodied in the "scheduling domains" data +structures [see Documentation/scheduler/sched-domains.rst]--and the scheduler +attempts to minimize task migration to distant scheduling domains. However, +the scheduler does not take a task's NUMA footprint into account directly. +Thus, under sufficient imbalance, tasks can migrate between nodes, remote +from their initial node and kernel data structures. + +System administrators and application designers can restrict a task's migration +to improve NUMA locality using various CPU affinity command line interfaces, +such as taskset(1) and numactl(1), and program interfaces such as +sched_setaffinity(2). Further, one can modify the kernel's default local +allocation behavior using Linux NUMA memory policy. [see +:ref:`Documentation/admin-guide/mm/numa_memory_policy.rst <numa_memory_policy>`]. + +System administrators can restrict the CPUs and nodes' memories that a non- +privileged user can specify in the scheduling or NUMA commands and functions +using control groups and CPUsets. [see Documentation/admin-guide/cgroup-v1/cpusets.rst] + +On architectures that do not hide memoryless nodes, Linux will include only +zones [nodes] with memory in the zonelists. This means that for a memoryless +node the "local memory node"--the node of the first zone in CPU's node's +zonelist--will not be the node itself. Rather, it will be the node that the +kernel selected as the nearest node with memory when it built the zonelists. +So, default, local allocations will succeed with the kernel supplying the +closest available memory. This is a consequence of the same mechanism that +allows such allocations to fallback to other nearby nodes when a node that +does contain memory overflows. + +Some kernel allocations do not want or cannot tolerate this allocation fallback +behavior. Rather they want to be sure they get memory from the specified node +or get notified that the node has no free memory. This is usually the case when +a subsystem allocates per CPU memory resources, for example. + +A typical model for making such an allocation is to obtain the node id of the +node to which the "current CPU" is attached using one of the kernel's +numa_node_id() or CPU_to_node() functions and then request memory from only +the node id returned. When such an allocation fails, the requesting subsystem +may revert to its own fallback path. The slab kernel memory allocator is an +example of this. Or, the subsystem may choose to disable or not to enable +itself on allocation failure. The kernel profiling subsystem is an example of +this. + +If the architecture supports--does not hide--memoryless nodes, then CPUs +attached to memoryless nodes would always incur the fallback path overhead +or some subsystems would fail to initialize if they attempted to allocated +memory exclusively from a node without memory. To support such +architectures transparently, kernel subsystems can use the numa_mem_id() +or cpu_to_mem() function to locate the "local memory node" for the calling or +specified CPU. Again, this is the same node from which default, local page +allocations will be attempted. diff --git a/Documentation/mm/oom.rst b/Documentation/mm/oom.rst new file mode 100644 index 000000000000..18e9e40c1ec1 --- /dev/null +++ b/Documentation/mm/oom.rst @@ -0,0 +1,5 @@ +.. SPDX-License-Identifier: GPL-2.0 + +====================== +Out Of Memory Handling +====================== diff --git a/Documentation/mm/overcommit-accounting.rst b/Documentation/mm/overcommit-accounting.rst new file mode 100644 index 000000000000..1addb0c374a4 --- /dev/null +++ b/Documentation/mm/overcommit-accounting.rst @@ -0,0 +1,88 @@ +.. _overcommit_accounting: + +===================== +Overcommit Accounting +===================== + +The Linux kernel supports the following overcommit handling modes + +0 + Heuristic overcommit handling. Obvious overcommits of address + space are refused. Used for a typical system. It ensures a + seriously wild allocation fails while allowing overcommit to + reduce swap usage. root is allowed to allocate slightly more + memory in this mode. This is the default. + +1 + Always overcommit. Appropriate for some scientific + applications. Classic example is code using sparse arrays and + just relying on the virtual memory consisting almost entirely + of zero pages. + +2 + Don't overcommit. The total address space commit for the + system is not permitted to exceed swap + a configurable amount + (default is 50%) of physical RAM. Depending on the amount you + use, in most situations this means a process will not be + killed while accessing pages but will receive errors on memory + allocation as appropriate. + + Useful for applications that want to guarantee their memory + allocations will be available in the future without having to + initialize every page. + +The overcommit policy is set via the sysctl ``vm.overcommit_memory``. + +The overcommit amount can be set via ``vm.overcommit_ratio`` (percentage) +or ``vm.overcommit_kbytes`` (absolute value). These only have an effect +when ``vm.overcommit_memory`` is set to 2. + +The current overcommit limit and amount committed are viewable in +``/proc/meminfo`` as CommitLimit and Committed_AS respectively. + +Gotchas +======= + +The C language stack growth does an implicit mremap. If you want absolute +guarantees and run close to the edge you MUST mmap your stack for the +largest size you think you will need. For typical stack usage this does +not matter much but it's a corner case if you really really care + +In mode 2 the MAP_NORESERVE flag is ignored. + + +How It Works +============ + +The overcommit is based on the following rules + +For a file backed map + | SHARED or READ-only - 0 cost (the file is the map not swap) + | PRIVATE WRITABLE - size of mapping per instance + +For an anonymous or ``/dev/zero`` map + | SHARED - size of mapping + | PRIVATE READ-only - 0 cost (but of little use) + | PRIVATE WRITABLE - size of mapping per instance + +Additional accounting + | Pages made writable copies by mmap + | shmfs memory drawn from the same pool + +Status +====== + +* We account mmap memory mappings +* We account mprotect changes in commit +* We account mremap changes in size +* We account brk +* We account munmap +* We report the commit status in /proc +* Account and check on fork +* Review stack handling/building on exec +* SHMfs accounting +* Implement actual limit enforcement + +To Do +===== +* Account ptrace pages (this is hard) diff --git a/Documentation/mm/page_allocation.rst b/Documentation/mm/page_allocation.rst new file mode 100644 index 000000000000..d9b4495561f1 --- /dev/null +++ b/Documentation/mm/page_allocation.rst @@ -0,0 +1,5 @@ +.. SPDX-License-Identifier: GPL-2.0 + +=============== +Page Allocation +=============== diff --git a/Documentation/mm/page_cache.rst b/Documentation/mm/page_cache.rst new file mode 100644 index 000000000000..75eba7c431b2 --- /dev/null +++ b/Documentation/mm/page_cache.rst @@ -0,0 +1,5 @@ +.. SPDX-License-Identifier: GPL-2.0 + +========== +Page Cache +========== diff --git a/Documentation/mm/page_frags.rst b/Documentation/mm/page_frags.rst new file mode 100644 index 000000000000..7d6f9385d129 --- /dev/null +++ b/Documentation/mm/page_frags.rst @@ -0,0 +1,45 @@ +.. _page_frags: + +============== +Page fragments +============== + +A page fragment is an arbitrary-length arbitrary-offset area of memory +which resides within a 0 or higher order compound page. Multiple +fragments within that page are individually refcounted, in the page's +reference counter. + +The page_frag functions, page_frag_alloc and page_frag_free, provide a +simple allocation framework for page fragments. This is used by the +network stack and network device drivers to provide a backing region of +memory for use as either an sk_buff->head, or to be used in the "frags" +portion of skb_shared_info. + +In order to make use of the page fragment APIs a backing page fragment +cache is needed. This provides a central point for the fragment allocation +and tracks allows multiple calls to make use of a cached page. The +advantage to doing this is that multiple calls to get_page can be avoided +which can be expensive at allocation time. However due to the nature of +this caching it is required that any calls to the cache be protected by +either a per-cpu limitation, or a per-cpu limitation and forcing interrupts +to be disabled when executing the fragment allocation. + +The network stack uses two separate caches per CPU to handle fragment +allocation. The netdev_alloc_cache is used by callers making use of the +netdev_alloc_frag and __netdev_alloc_skb calls. The napi_alloc_cache is +used by callers of the __napi_alloc_frag and __napi_alloc_skb calls. The +main difference between these two calls is the context in which they may be +called. The "netdev" prefixed functions are usable in any context as these +functions will disable interrupts, while the "napi" prefixed functions are +only usable within the softirq context. + +Many network device drivers use a similar methodology for allocating page +fragments, but the page fragments are cached at the ring or descriptor +level. In order to enable these cases it is necessary to provide a generic +way of tearing down a page cache. For this reason __page_frag_cache_drain +was implemented. It allows for freeing multiple references from a single +page via a single call. The advantage to doing this is that it allows for +cleaning up the multiple references that were added to a page in order to +avoid calling get_page per allocation. + +Alexander Duyck, Nov 29, 2016. diff --git a/Documentation/mm/page_migration.rst b/Documentation/mm/page_migration.rst new file mode 100644 index 000000000000..8c5cb8147e55 --- /dev/null +++ b/Documentation/mm/page_migration.rst @@ -0,0 +1,288 @@ +.. _page_migration: + +============== +Page migration +============== + +Page migration allows moving the physical location of pages between +nodes in a NUMA system while the process is running. This means that the +virtual addresses that the process sees do not change. However, the +system rearranges the physical location of those pages. + +Also see :ref:`Heterogeneous Memory Management (HMM) <hmm>` +for migrating pages to or from device private memory. + +The main intent of page migration is to reduce the latency of memory accesses +by moving pages near to the processor where the process accessing that memory +is running. + +Page migration allows a process to manually relocate the node on which its +pages are located through the MF_MOVE and MF_MOVE_ALL options while setting +a new memory policy via mbind(). The pages of a process can also be relocated +from another process using the sys_migrate_pages() function call. The +migrate_pages() function call takes two sets of nodes and moves pages of a +process that are located on the from nodes to the destination nodes. +Page migration functions are provided by the numactl package by Andi Kleen +(a version later than 0.9.3 is required. Get it from +https://github.com/numactl/numactl.git). numactl provides libnuma +which provides an interface similar to other NUMA functionality for page +migration. cat ``/proc/<pid>/numa_maps`` allows an easy review of where the +pages of a process are located. See also the numa_maps documentation in the +proc(5) man page. + +Manual migration is useful if for example the scheduler has relocated +a process to a processor on a distant node. A batch scheduler or an +administrator may detect the situation and move the pages of the process +nearer to the new processor. The kernel itself only provides +manual page migration support. Automatic page migration may be implemented +through user space processes that move pages. A special function call +"move_pages" allows the moving of individual pages within a process. +For example, A NUMA profiler may obtain a log showing frequent off-node +accesses and may use the result to move pages to more advantageous +locations. + +Larger installations usually partition the system using cpusets into +sections of nodes. Paul Jackson has equipped cpusets with the ability to +move pages when a task is moved to another cpuset (See +:ref:`CPUSETS <cpusets>`). +Cpusets allow the automation of process locality. If a task is moved to +a new cpuset then also all its pages are moved with it so that the +performance of the process does not sink dramatically. Also the pages +of processes in a cpuset are moved if the allowed memory nodes of a +cpuset are changed. + +Page migration allows the preservation of the relative location of pages +within a group of nodes for all migration techniques which will preserve a +particular memory allocation pattern generated even after migrating a +process. This is necessary in order to preserve the memory latencies. +Processes will run with similar performance after migration. + +Page migration occurs in several steps. First a high level +description for those trying to use migrate_pages() from the kernel +(for userspace usage see the Andi Kleen's numactl package mentioned above) +and then a low level description of how the low level details work. + +In kernel use of migrate_pages() +================================ + +1. Remove pages from the LRU. + + Lists of pages to be migrated are generated by scanning over + pages and moving them into lists. This is done by + calling isolate_lru_page(). + Calling isolate_lru_page() increases the references to the page + so that it cannot vanish while the page migration occurs. + It also prevents the swapper or other scans from encountering + the page. + +2. We need to have a function of type new_page_t that can be + passed to migrate_pages(). This function should figure out + how to allocate the correct new page given the old page. + +3. The migrate_pages() function is called which attempts + to do the migration. It will call the function to allocate + the new page for each page that is considered for + moving. + +How migrate_pages() works +========================= + +migrate_pages() does several passes over its list of pages. A page is moved +if all references to a page are removable at the time. The page has +already been removed from the LRU via isolate_lru_page() and the refcount +is increased so that the page cannot be freed while page migration occurs. + +Steps: + +1. Lock the page to be migrated. + +2. Ensure that writeback is complete. + +3. Lock the new page that we want to move to. It is locked so that accesses to + this (not yet up-to-date) page immediately block while the move is in progress. + +4. All the page table references to the page are converted to migration + entries. This decreases the mapcount of a page. If the resulting + mapcount is not zero then we do not migrate the page. All user space + processes that attempt to access the page will now wait on the page lock + or wait for the migration page table entry to be removed. + +5. The i_pages lock is taken. This will cause all processes trying + to access the page via the mapping to block on the spinlock. + +6. The refcount of the page is examined and we back out if references remain. + Otherwise, we know that we are the only one referencing this page. + +7. The radix tree is checked and if it does not contain the pointer to this + page then we back out because someone else modified the radix tree. + +8. The new page is prepped with some settings from the old page so that + accesses to the new page will discover a page with the correct settings. + +9. The radix tree is changed to point to the new page. + +10. The reference count of the old page is dropped because the address space + reference is gone. A reference to the new page is established because + the new page is referenced by the address space. + +11. The i_pages lock is dropped. With that lookups in the mapping + become possible again. Processes will move from spinning on the lock + to sleeping on the locked new page. + +12. The page contents are copied to the new page. + +13. The remaining page flags are copied to the new page. + +14. The old page flags are cleared to indicate that the page does + not provide any information anymore. + +15. Queued up writeback on the new page is triggered. + +16. If migration entries were inserted into the page table, then replace them + with real ptes. Doing so will enable access for user space processes not + already waiting for the page lock. + +17. The page locks are dropped from the old and new page. + Processes waiting on the page lock will redo their page faults + and will reach the new page. + +18. The new page is moved to the LRU and can be scanned by the swapper, + etc. again. + +Non-LRU page migration +====================== + +Although migration originally aimed for reducing the latency of memory accesses +for NUMA, compaction also uses migration to create high-order pages. + +Current problem of the implementation is that it is designed to migrate only +*LRU* pages. However, there are potential non-LRU pages which can be migrated +in drivers, for example, zsmalloc, virtio-balloon pages. + +For virtio-balloon pages, some parts of migration code path have been hooked +up and added virtio-balloon specific functions to intercept migration logics. +It's too specific to a driver so other drivers who want to make their pages +movable would have to add their own specific hooks in the migration path. + +To overcome the problem, VM supports non-LRU page migration which provides +generic functions for non-LRU movable pages without driver specific hooks +in the migration path. + +If a driver wants to make its pages movable, it should define three functions +which are function pointers of struct address_space_operations. + +1. ``bool (*isolate_page) (struct page *page, isolate_mode_t mode);`` + + What VM expects from isolate_page() function of driver is to return *true* + if driver isolates the page successfully. On returning true, VM marks the page + as PG_isolated so concurrent isolation in several CPUs skip the page + for isolation. If a driver cannot isolate the page, it should return *false*. + + Once page is successfully isolated, VM uses page.lru fields so driver + shouldn't expect to preserve values in those fields. + +2. ``int (*migratepage) (struct address_space *mapping,`` +| ``struct page *newpage, struct page *oldpage, enum migrate_mode);`` + + After isolation, VM calls migratepage() of driver with the isolated page. + The function of migratepage() is to move the contents of the old page to the + new page + and set up fields of struct page newpage. Keep in mind that you should + indicate to the VM the oldpage is no longer movable via __ClearPageMovable() + under page_lock if you migrated the oldpage successfully and returned + MIGRATEPAGE_SUCCESS. If driver cannot migrate the page at the moment, driver + can return -EAGAIN. On -EAGAIN, VM will retry page migration in a short time + because VM interprets -EAGAIN as "temporary migration failure". On returning + any error except -EAGAIN, VM will give up the page migration without + retrying. + + Driver shouldn't touch the page.lru field while in the migratepage() function. + +3. ``void (*putback_page)(struct page *);`` + + If migration fails on the isolated page, VM should return the isolated page + to the driver so VM calls the driver's putback_page() with the isolated page. + In this function, the driver should put the isolated page back into its own data + structure. + +Non-LRU movable page flags + + There are two page flags for supporting non-LRU movable page. + + * PG_movable + + Driver should use the function below to make page movable under page_lock:: + + void __SetPageMovable(struct page *page, struct address_space *mapping) + + It needs argument of address_space for registering migration + family functions which will be called by VM. Exactly speaking, + PG_movable is not a real flag of struct page. Rather, VM + reuses the page->mapping's lower bits to represent it:: + + #define PAGE_MAPPING_MOVABLE 0x2 + page->mapping = page->mapping | PAGE_MAPPING_MOVABLE; + + so driver shouldn't access page->mapping directly. Instead, driver should + use page_mapping() which masks off the low two bits of page->mapping under + page lock so it can get the right struct address_space. + + For testing of non-LRU movable pages, VM supports __PageMovable() function. + However, it doesn't guarantee to identify non-LRU movable pages because + the page->mapping field is unified with other variables in struct page. + If the driver releases the page after isolation by VM, page->mapping + doesn't have a stable value although it has PAGE_MAPPING_MOVABLE set + (look at __ClearPageMovable). But __PageMovable() is cheap to call whether + page is LRU or non-LRU movable once the page has been isolated because LRU + pages can never have PAGE_MAPPING_MOVABLE set in page->mapping. It is also + good for just peeking to test non-LRU movable pages before more expensive + checking with lock_page() in pfn scanning to select a victim. + + For guaranteeing non-LRU movable page, VM provides PageMovable() function. + Unlike __PageMovable(), PageMovable() validates page->mapping and + mapping->a_ops->isolate_page under lock_page(). The lock_page() prevents + sudden destroying of page->mapping. + + Drivers using __SetPageMovable() should clear the flag via + __ClearMovablePage() under page_lock() before the releasing the page. + + * PG_isolated + + To prevent concurrent isolation among several CPUs, VM marks isolated page + as PG_isolated under lock_page(). So if a CPU encounters PG_isolated + non-LRU movable page, it can skip it. Driver doesn't need to manipulate the + flag because VM will set/clear it automatically. Keep in mind that if the + driver sees a PG_isolated page, it means the page has been isolated by the + VM so it shouldn't touch the page.lru field. + The PG_isolated flag is aliased with the PG_reclaim flag so drivers + shouldn't use PG_isolated for its own purposes. + +Monitoring Migration +===================== + +The following events (counters) can be used to monitor page migration. + +1. PGMIGRATE_SUCCESS: Normal page migration success. Each count means that a + page was migrated. If the page was a non-THP and non-hugetlb page, then + this counter is increased by one. If the page was a THP or hugetlb, then + this counter is increased by the number of THP or hugetlb subpages. + For example, migration of a single 2MB THP that has 4KB-size base pages + (subpages) will cause this counter to increase by 512. + +2. PGMIGRATE_FAIL: Normal page migration failure. Same counting rules as for + PGMIGRATE_SUCCESS, above: this will be increased by the number of subpages, + if it was a THP or hugetlb. + +3. THP_MIGRATION_SUCCESS: A THP was migrated without being split. + +4. THP_MIGRATION_FAIL: A THP could not be migrated nor it could be split. + +5. THP_MIGRATION_SPLIT: A THP was migrated, but not as such: first, the THP had + to be split. After splitting, a migration retry was used for it's sub-pages. + +THP_MIGRATION_* events also update the appropriate PGMIGRATE_SUCCESS or +PGMIGRATE_FAIL events. For example, a THP migration failure will cause both +THP_MIGRATION_FAIL and PGMIGRATE_FAIL to increase. + +Christoph Lameter, May 8, 2006. +Minchan Kim, Mar 28, 2016. diff --git a/Documentation/mm/page_owner.rst b/Documentation/mm/page_owner.rst new file mode 100644 index 000000000000..f5c954afe97c --- /dev/null +++ b/Documentation/mm/page_owner.rst @@ -0,0 +1,196 @@ +.. _page_owner: + +================================================== +page owner: Tracking about who allocated each page +================================================== + +Introduction +============ + +page owner is for the tracking about who allocated each page. +It can be used to debug memory leak or to find a memory hogger. +When allocation happens, information about allocation such as call stack +and order of pages is stored into certain storage for each page. +When we need to know about status of all pages, we can get and analyze +this information. + +Although we already have tracepoint for tracing page allocation/free, +using it for analyzing who allocate each page is rather complex. We need +to enlarge the trace buffer for preventing overlapping until userspace +program launched. And, launched program continually dump out the trace +buffer for later analysis and it would change system behaviour with more +possibility rather than just keeping it in memory, so bad for debugging. + +page owner can also be used for various purposes. For example, accurate +fragmentation statistics can be obtained through gfp flag information of +each page. It is already implemented and activated if page owner is +enabled. Other usages are more than welcome. + +page owner is disabled by default. So, if you'd like to use it, you need +to add "page_owner=on" to your boot cmdline. If the kernel is built +with page owner and page owner is disabled in runtime due to not enabling +boot option, runtime overhead is marginal. If disabled in runtime, it +doesn't require memory to store owner information, so there is no runtime +memory overhead. And, page owner inserts just two unlikely branches into +the page allocator hotpath and if not enabled, then allocation is done +like as the kernel without page owner. These two unlikely branches should +not affect to allocation performance, especially if the static keys jump +label patching functionality is available. Following is the kernel's code +size change due to this facility. + +- Without page owner:: + + text data bss dec hex filename + 48392 2333 644 51369 c8a9 mm/page_alloc.o + +- With page owner:: + + text data bss dec hex filename + 48800 2445 644 51889 cab1 mm/page_alloc.o + 6662 108 29 6799 1a8f mm/page_owner.o + 1025 8 8 1041 411 mm/page_ext.o + +Although, roughly, 8 KB code is added in total, page_alloc.o increase by +520 bytes and less than half of it is in hotpath. Building the kernel with +page owner and turning it on if needed would be great option to debug +kernel memory problem. + +There is one notice that is caused by implementation detail. page owner +stores information into the memory from struct page extension. This memory +is initialized some time later than that page allocator starts in sparse +memory system, so, until initialization, many pages can be allocated and +they would have no owner information. To fix it up, these early allocated +pages are investigated and marked as allocated in initialization phase. +Although it doesn't mean that they have the right owner information, +at least, we can tell whether the page is allocated or not, +more accurately. On 2GB memory x86-64 VM box, 13343 early allocated pages +are catched and marked, although they are mostly allocated from struct +page extension feature. Anyway, after that, no page is left in +un-tracking state. + +Usage +===== + +1) Build user-space helper:: + + cd tools/vm + make page_owner_sort + +2) Enable page owner: add "page_owner=on" to boot cmdline. + +3) Do the job that you want to debug. + +4) Analyze information from page owner:: + + cat /sys/kernel/debug/page_owner > page_owner_full.txt + ./page_owner_sort page_owner_full.txt sorted_page_owner.txt + + The general output of ``page_owner_full.txt`` is as follows:: + + Page allocated via order XXX, ... + PFN XXX ... + // Detailed stack + + Page allocated via order XXX, ... + PFN XXX ... + // Detailed stack + + The ``page_owner_sort`` tool ignores ``PFN`` rows, puts the remaining rows + in buf, uses regexp to extract the page order value, counts the times + and pages of buf, and finally sorts them according to the parameter(s). + + See the result about who allocated each page + in the ``sorted_page_owner.txt``. General output:: + + XXX times, XXX pages: + Page allocated via order XXX, ... + // Detailed stack + + By default, ``page_owner_sort`` is sorted according to the times of buf. + If you want to sort by the page nums of buf, use the ``-m`` parameter. + The detailed parameters are: + + fundamental function:: + + Sort: + -a Sort by memory allocation time. + -m Sort by total memory. + -p Sort by pid. + -P Sort by tgid. + -n Sort by task command name. + -r Sort by memory release time. + -s Sort by stack trace. + -t Sort by times (default). + --sort <order> Specify sorting order. Sorting syntax is [+|-]key[,[+|-]key[,...]]. + Choose a key from the **STANDARD FORMAT SPECIFIERS** section. The "+" is + optional since default direction is increasing numerical or lexicographic + order. Mixed use of abbreviated and complete-form of keys is allowed. + + Examples: + ./page_owner_sort <input> <output> --sort=n,+pid,-tgid + ./page_owner_sort <input> <output> --sort=at + + additional function:: + + Cull: + --cull <rules> + Specify culling rules.Culling syntax is key[,key[,...]].Choose a + multi-letter key from the **STANDARD FORMAT SPECIFIERS** section. + + <rules> is a single argument in the form of a comma-separated list, + which offers a way to specify individual culling rules. The recognized + keywords are described in the **STANDARD FORMAT SPECIFIERS** section below. + <rules> can be specified by the sequence of keys k1,k2, ..., as described in + the STANDARD SORT KEYS section below. Mixed use of abbreviated and + complete-form of keys is allowed. + + Examples: + ./page_owner_sort <input> <output> --cull=stacktrace + ./page_owner_sort <input> <output> --cull=st,pid,name + ./page_owner_sort <input> <output> --cull=n,f + + Filter: + -f Filter out the information of blocks whose memory has been released. + + Select: + --pid <pidlist> Select by pid. This selects the blocks whose process ID + numbers appear in <pidlist>. + --tgid <tgidlist> Select by tgid. This selects the blocks whose thread + group ID numbers appear in <tgidlist>. + --name <cmdlist> Select by task command name. This selects the blocks whose + task command name appear in <cmdlist>. + + <pidlist>, <tgidlist>, <cmdlist> are single arguments in the form of a comma-separated list, + which offers a way to specify individual selecting rules. + + + Examples: + ./page_owner_sort <input> <output> --pid=1 + ./page_owner_sort <input> <output> --tgid=1,2,3 + ./page_owner_sort <input> <output> --name name1,name2 + +STANDARD FORMAT SPECIFIERS +========================== +:: + + For --sort option: + + KEY LONG DESCRIPTION + p pid process ID + tg tgid thread group ID + n name task command name + st stacktrace stack trace of the page allocation + T txt full text of block + ft free_ts timestamp of the page when it was released + at alloc_ts timestamp of the page when it was allocated + ator allocator memory allocator for pages + + For --curl option: + + KEY LONG DESCRIPTION + p pid process ID + tg tgid thread group ID + n name task command name + f free whether the page has been released or not + st stacktrace stack trace of the page allocation + ator allocator memory allocator for pages diff --git a/Documentation/mm/page_reclaim.rst b/Documentation/mm/page_reclaim.rst new file mode 100644 index 000000000000..50a30b7f8ac3 --- /dev/null +++ b/Documentation/mm/page_reclaim.rst @@ -0,0 +1,5 @@ +.. SPDX-License-Identifier: GPL-2.0 + +============ +Page Reclaim +============ diff --git a/Documentation/mm/page_table_check.rst b/Documentation/mm/page_table_check.rst new file mode 100644 index 000000000000..1a09472f10a3 --- /dev/null +++ b/Documentation/mm/page_table_check.rst @@ -0,0 +1,56 @@ +.. SPDX-License-Identifier: GPL-2.0 + +.. _page_table_check: + +================ +Page Table Check +================ + +Introduction +============ + +Page table check allows to harden the kernel by ensuring that some types of +the memory corruptions are prevented. + +Page table check performs extra verifications at the time when new pages become +accessible from the userspace by getting their page table entries (PTEs PMDs +etc.) added into the table. + +In case of detected corruption, the kernel is crashed. There is a small +performance and memory overhead associated with the page table check. Therefore, +it is disabled by default, but can be optionally enabled on systems where the +extra hardening outweighs the performance costs. Also, because page table check +is synchronous, it can help with debugging double map memory corruption issues, +by crashing kernel at the time wrong mapping occurs instead of later which is +often the case with memory corruptions bugs. + +Double mapping detection logic +============================== + ++-------------------+-------------------+-------------------+------------------+ +| Current Mapping | New mapping | Permissions | Rule | ++===================+===================+===================+==================+ +| Anonymous | Anonymous | Read | Allow | ++-------------------+-------------------+-------------------+------------------+ +| Anonymous | Anonymous | Read / Write | Prohibit | ++-------------------+-------------------+-------------------+------------------+ +| Anonymous | Named | Any | Prohibit | ++-------------------+-------------------+-------------------+------------------+ +| Named | Anonymous | Any | Prohibit | ++-------------------+-------------------+-------------------+------------------+ +| Named | Named | Any | Allow | ++-------------------+-------------------+-------------------+------------------+ + +Enabling Page Table Check +========================= + +Build kernel with: + +- PAGE_TABLE_CHECK=y + Note, it can only be enabled on platforms where ARCH_SUPPORTS_PAGE_TABLE_CHECK + is available. + +- Boot with 'page_table_check=on' kernel parameter. + +Optionally, build kernel with PAGE_TABLE_CHECK_ENFORCED in order to have page +table support without extra kernel parameter. diff --git a/Documentation/mm/page_tables.rst b/Documentation/mm/page_tables.rst new file mode 100644 index 000000000000..96939571d7bc --- /dev/null +++ b/Documentation/mm/page_tables.rst @@ -0,0 +1,5 @@ +.. SPDX-License-Identifier: GPL-2.0 + +=========== +Page Tables +=========== diff --git a/Documentation/mm/physical_memory.rst b/Documentation/mm/physical_memory.rst new file mode 100644 index 000000000000..2ab7b8c1c863 --- /dev/null +++ b/Documentation/mm/physical_memory.rst @@ -0,0 +1,5 @@ +.. SPDX-License-Identifier: GPL-2.0 + +=============== +Physical Memory +=============== diff --git a/Documentation/mm/process_addrs.rst b/Documentation/mm/process_addrs.rst new file mode 100644 index 000000000000..e8618fbc62c9 --- /dev/null +++ b/Documentation/mm/process_addrs.rst @@ -0,0 +1,5 @@ +.. SPDX-License-Identifier: GPL-2.0 + +================= +Process Addresses +================= diff --git a/Documentation/mm/remap_file_pages.rst b/Documentation/mm/remap_file_pages.rst new file mode 100644 index 000000000000..7bef6718e3a9 --- /dev/null +++ b/Documentation/mm/remap_file_pages.rst @@ -0,0 +1,33 @@ +.. _remap_file_pages: + +============================== +remap_file_pages() system call +============================== + +The remap_file_pages() system call is used to create a nonlinear mapping, +that is, a mapping in which the pages of the file are mapped into a +nonsequential order in memory. The advantage of using remap_file_pages() +over using repeated calls to mmap(2) is that the former approach does not +require the kernel to create additional VMA (Virtual Memory Area) data +structures. + +Supporting of nonlinear mapping requires significant amount of non-trivial +code in kernel virtual memory subsystem including hot paths. Also to get +nonlinear mapping work kernel need a way to distinguish normal page table +entries from entries with file offset (pte_file). Kernel reserves flag in +PTE for this purpose. PTE flags are scarce resource especially on some CPU +architectures. It would be nice to free up the flag for other usage. + +Fortunately, there are not many users of remap_file_pages() in the wild. +It's only known that one enterprise RDBMS implementation uses the syscall +on 32-bit systems to map files bigger than can linearly fit into 32-bit +virtual address space. This use-case is not critical anymore since 64-bit +systems are widely available. + +The syscall is deprecated and replaced it with an emulation now. The +emulation creates new VMAs instead of nonlinear mappings. It's going to +work slower for rare users of remap_file_pages() but ABI is preserved. + +One side effect of emulation (apart from performance) is that user can hit +vm.max_map_count limit more easily due to additional VMAs. See comment for +DEFAULT_MAX_MAP_COUNT for more details on the limit. diff --git a/Documentation/mm/shmfs.rst b/Documentation/mm/shmfs.rst new file mode 100644 index 000000000000..8b01ebb4c30e --- /dev/null +++ b/Documentation/mm/shmfs.rst @@ -0,0 +1,5 @@ +.. SPDX-License-Identifier: GPL-2.0 + +======================== +Shared Memory Filesystem +======================== diff --git a/Documentation/mm/slab.rst b/Documentation/mm/slab.rst new file mode 100644 index 000000000000..87d5a5bb172f --- /dev/null +++ b/Documentation/mm/slab.rst @@ -0,0 +1,5 @@ +.. SPDX-License-Identifier: GPL-2.0 + +=============== +Slab Allocation +=============== diff --git a/Documentation/mm/slub.rst b/Documentation/mm/slub.rst new file mode 100644 index 000000000000..43063ade737a --- /dev/null +++ b/Documentation/mm/slub.rst @@ -0,0 +1,452 @@ +.. _slub: + +========================== +Short users guide for SLUB +========================== + +The basic philosophy of SLUB is very different from SLAB. SLAB +requires rebuilding the kernel to activate debug options for all +slab caches. SLUB always includes full debugging but it is off by default. +SLUB can enable debugging only for selected slabs in order to avoid +an impact on overall system performance which may make a bug more +difficult to find. + +In order to switch debugging on one can add an option ``slub_debug`` +to the kernel command line. That will enable full debugging for +all slabs. + +Typically one would then use the ``slabinfo`` command to get statistical +data and perform operation on the slabs. By default ``slabinfo`` only lists +slabs that have data in them. See "slabinfo -h" for more options when +running the command. ``slabinfo`` can be compiled with +:: + + gcc -o slabinfo tools/vm/slabinfo.c + +Some of the modes of operation of ``slabinfo`` require that slub debugging +be enabled on the command line. F.e. no tracking information will be +available without debugging on and validation can only partially +be performed if debugging was not switched on. + +Some more sophisticated uses of slub_debug: +------------------------------------------- + +Parameters may be given to ``slub_debug``. If none is specified then full +debugging is enabled. Format: + +slub_debug=<Debug-Options> + Enable options for all slabs + +slub_debug=<Debug-Options>,<slab name1>,<slab name2>,... + Enable options only for select slabs (no spaces + after a comma) + +Multiple blocks of options for all slabs or selected slabs can be given, with +blocks of options delimited by ';'. The last of "all slabs" blocks is applied +to all slabs except those that match one of the "select slabs" block. Options +of the first "select slabs" blocks that matches the slab's name are applied. + +Possible debug options are:: + + F Sanity checks on (enables SLAB_DEBUG_CONSISTENCY_CHECKS + Sorry SLAB legacy issues) + Z Red zoning + P Poisoning (object and padding) + U User tracking (free and alloc) + T Trace (please only use on single slabs) + A Enable failslab filter mark for the cache + O Switch debugging off for caches that would have + caused higher minimum slab orders + - Switch all debugging off (useful if the kernel is + configured with CONFIG_SLUB_DEBUG_ON) + +F.e. in order to boot just with sanity checks and red zoning one would specify:: + + slub_debug=FZ + +Trying to find an issue in the dentry cache? Try:: + + slub_debug=,dentry + +to only enable debugging on the dentry cache. You may use an asterisk at the +end of the slab name, in order to cover all slabs with the same prefix. For +example, here's how you can poison the dentry cache as well as all kmalloc +slabs:: + + slub_debug=P,kmalloc-*,dentry + +Red zoning and tracking may realign the slab. We can just apply sanity checks +to the dentry cache with:: + + slub_debug=F,dentry + +Debugging options may require the minimum possible slab order to increase as +a result of storing the metadata (for example, caches with PAGE_SIZE object +sizes). This has a higher liklihood of resulting in slab allocation errors +in low memory situations or if there's high fragmentation of memory. To +switch off debugging for such caches by default, use:: + + slub_debug=O + +You can apply different options to different list of slab names, using blocks +of options. This will enable red zoning for dentry and user tracking for +kmalloc. All other slabs will not get any debugging enabled:: + + slub_debug=Z,dentry;U,kmalloc-* + +You can also enable options (e.g. sanity checks and poisoning) for all caches +except some that are deemed too performance critical and don't need to be +debugged by specifying global debug options followed by a list of slab names +with "-" as options:: + + slub_debug=FZ;-,zs_handle,zspage + +The state of each debug option for a slab can be found in the respective files +under:: + + /sys/kernel/slab/<slab name>/ + +If the file contains 1, the option is enabled, 0 means disabled. The debug +options from the ``slub_debug`` parameter translate to the following files:: + + F sanity_checks + Z red_zone + P poison + U store_user + T trace + A failslab + +Careful with tracing: It may spew out lots of information and never stop if +used on the wrong slab. + +Slab merging +============ + +If no debug options are specified then SLUB may merge similar slabs together +in order to reduce overhead and increase cache hotness of objects. +``slabinfo -a`` displays which slabs were merged together. + +Slab validation +=============== + +SLUB can validate all object if the kernel was booted with slub_debug. In +order to do so you must have the ``slabinfo`` tool. Then you can do +:: + + slabinfo -v + +which will test all objects. Output will be generated to the syslog. + +This also works in a more limited way if boot was without slab debug. +In that case ``slabinfo -v`` simply tests all reachable objects. Usually +these are in the cpu slabs and the partial slabs. Full slabs are not +tracked by SLUB in a non debug situation. + +Getting more performance +======================== + +To some degree SLUB's performance is limited by the need to take the +list_lock once in a while to deal with partial slabs. That overhead is +governed by the order of the allocation for each slab. The allocations +can be influenced by kernel parameters: + +.. slub_min_objects=x (default 4) +.. slub_min_order=x (default 0) +.. slub_max_order=x (default 3 (PAGE_ALLOC_COSTLY_ORDER)) + +``slub_min_objects`` + allows to specify how many objects must at least fit into one + slab in order for the allocation order to be acceptable. In + general slub will be able to perform this number of + allocations on a slab without consulting centralized resources + (list_lock) where contention may occur. + +``slub_min_order`` + specifies a minimum order of slabs. A similar effect like + ``slub_min_objects``. + +``slub_max_order`` + specified the order at which ``slub_min_objects`` should no + longer be checked. This is useful to avoid SLUB trying to + generate super large order pages to fit ``slub_min_objects`` + of a slab cache with large object sizes into one high order + page. Setting command line parameter + ``debug_guardpage_minorder=N`` (N > 0), forces setting + ``slub_max_order`` to 0, what cause minimum possible order of + slabs allocation. + +SLUB Debug output +================= + +Here is a sample of slub debug output:: + + ==================================================================== + BUG kmalloc-8: Right Redzone overwritten + -------------------------------------------------------------------- + + INFO: 0xc90f6d28-0xc90f6d2b. First byte 0x00 instead of 0xcc + INFO: Slab 0xc528c530 flags=0x400000c3 inuse=61 fp=0xc90f6d58 + INFO: Object 0xc90f6d20 @offset=3360 fp=0xc90f6d58 + INFO: Allocated in get_modalias+0x61/0xf5 age=53 cpu=1 pid=554 + + Bytes b4 (0xc90f6d10): 00 00 00 00 00 00 00 00 5a 5a 5a 5a 5a 5a 5a 5a ........ZZZZZZZZ + Object (0xc90f6d20): 31 30 31 39 2e 30 30 35 1019.005 + Redzone (0xc90f6d28): 00 cc cc cc . + Padding (0xc90f6d50): 5a 5a 5a 5a 5a 5a 5a 5a ZZZZZZZZ + + [<c010523d>] dump_trace+0x63/0x1eb + [<c01053df>] show_trace_log_lvl+0x1a/0x2f + [<c010601d>] show_trace+0x12/0x14 + [<c0106035>] dump_stack+0x16/0x18 + [<c017e0fa>] object_err+0x143/0x14b + [<c017e2cc>] check_object+0x66/0x234 + [<c017eb43>] __slab_free+0x239/0x384 + [<c017f446>] kfree+0xa6/0xc6 + [<c02e2335>] get_modalias+0xb9/0xf5 + [<c02e23b7>] dmi_dev_uevent+0x27/0x3c + [<c027866a>] dev_uevent+0x1ad/0x1da + [<c0205024>] kobject_uevent_env+0x20a/0x45b + [<c020527f>] kobject_uevent+0xa/0xf + [<c02779f1>] store_uevent+0x4f/0x58 + [<c027758e>] dev_attr_store+0x29/0x2f + [<c01bec4f>] sysfs_write_file+0x16e/0x19c + [<c0183ba7>] vfs_write+0xd1/0x15a + [<c01841d7>] sys_write+0x3d/0x72 + [<c0104112>] sysenter_past_esp+0x5f/0x99 + [<b7f7b410>] 0xb7f7b410 + ======================= + + FIX kmalloc-8: Restoring Redzone 0xc90f6d28-0xc90f6d2b=0xcc + +If SLUB encounters a corrupted object (full detection requires the kernel +to be booted with slub_debug) then the following output will be dumped +into the syslog: + +1. Description of the problem encountered + + This will be a message in the system log starting with:: + + =============================================== + BUG <slab cache affected>: <What went wrong> + ----------------------------------------------- + + INFO: <corruption start>-<corruption_end> <more info> + INFO: Slab <address> <slab information> + INFO: Object <address> <object information> + INFO: Allocated in <kernel function> age=<jiffies since alloc> cpu=<allocated by + cpu> pid=<pid of the process> + INFO: Freed in <kernel function> age=<jiffies since free> cpu=<freed by cpu> + pid=<pid of the process> + + (Object allocation / free information is only available if SLAB_STORE_USER is + set for the slab. slub_debug sets that option) + +2. The object contents if an object was involved. + + Various types of lines can follow the BUG SLUB line: + + Bytes b4 <address> : <bytes> + Shows a few bytes before the object where the problem was detected. + Can be useful if the corruption does not stop with the start of the + object. + + Object <address> : <bytes> + The bytes of the object. If the object is inactive then the bytes + typically contain poison values. Any non-poison value shows a + corruption by a write after free. + + Redzone <address> : <bytes> + The Redzone following the object. The Redzone is used to detect + writes after the object. All bytes should always have the same + value. If there is any deviation then it is due to a write after + the object boundary. + + (Redzone information is only available if SLAB_RED_ZONE is set. + slub_debug sets that option) + + Padding <address> : <bytes> + Unused data to fill up the space in order to get the next object + properly aligned. In the debug case we make sure that there are + at least 4 bytes of padding. This allows the detection of writes + before the object. + +3. A stackdump + + The stackdump describes the location where the error was detected. The cause + of the corruption is may be more likely found by looking at the function that + allocated or freed the object. + +4. Report on how the problem was dealt with in order to ensure the continued + operation of the system. + + These are messages in the system log beginning with:: + + FIX <slab cache affected>: <corrective action taken> + + In the above sample SLUB found that the Redzone of an active object has + been overwritten. Here a string of 8 characters was written into a slab that + has the length of 8 characters. However, a 8 character string needs a + terminating 0. That zero has overwritten the first byte of the Redzone field. + After reporting the details of the issue encountered the FIX SLUB message + tells us that SLUB has restored the Redzone to its proper value and then + system operations continue. + +Emergency operations +==================== + +Minimal debugging (sanity checks alone) can be enabled by booting with:: + + slub_debug=F + +This will be generally be enough to enable the resiliency features of slub +which will keep the system running even if a bad kernel component will +keep corrupting objects. This may be important for production systems. +Performance will be impacted by the sanity checks and there will be a +continual stream of error messages to the syslog but no additional memory +will be used (unlike full debugging). + +No guarantees. The kernel component still needs to be fixed. Performance +may be optimized further by locating the slab that experiences corruption +and enabling debugging only for that cache + +I.e.:: + + slub_debug=F,dentry + +If the corruption occurs by writing after the end of the object then it +may be advisable to enable a Redzone to avoid corrupting the beginning +of other objects:: + + slub_debug=FZ,dentry + +Extended slabinfo mode and plotting +=================================== + +The ``slabinfo`` tool has a special 'extended' ('-X') mode that includes: + - Slabcache Totals + - Slabs sorted by size (up to -N <num> slabs, default 1) + - Slabs sorted by loss (up to -N <num> slabs, default 1) + +Additionally, in this mode ``slabinfo`` does not dynamically scale +sizes (G/M/K) and reports everything in bytes (this functionality is +also available to other slabinfo modes via '-B' option) which makes +reporting more precise and accurate. Moreover, in some sense the `-X' +mode also simplifies the analysis of slabs' behaviour, because its +output can be plotted using the ``slabinfo-gnuplot.sh`` script. So it +pushes the analysis from looking through the numbers (tons of numbers) +to something easier -- visual analysis. + +To generate plots: + +a) collect slabinfo extended records, for example:: + + while [ 1 ]; do slabinfo -X >> FOO_STATS; sleep 1; done + +b) pass stats file(-s) to ``slabinfo-gnuplot.sh`` script:: + + slabinfo-gnuplot.sh FOO_STATS [FOO_STATS2 .. FOO_STATSN] + + The ``slabinfo-gnuplot.sh`` script will pre-processes the collected records + and generates 3 png files (and 3 pre-processing cache files) per STATS + file: + - Slabcache Totals: FOO_STATS-totals.png + - Slabs sorted by size: FOO_STATS-slabs-by-size.png + - Slabs sorted by loss: FOO_STATS-slabs-by-loss.png + +Another use case, when ``slabinfo-gnuplot.sh`` can be useful, is when you +need to compare slabs' behaviour "prior to" and "after" some code +modification. To help you out there, ``slabinfo-gnuplot.sh`` script +can 'merge' the `Slabcache Totals` sections from different +measurements. To visually compare N plots: + +a) Collect as many STATS1, STATS2, .. STATSN files as you need:: + + while [ 1 ]; do slabinfo -X >> STATS<X>; sleep 1; done + +b) Pre-process those STATS files:: + + slabinfo-gnuplot.sh STATS1 STATS2 .. STATSN + +c) Execute ``slabinfo-gnuplot.sh`` in '-t' mode, passing all of the + generated pre-processed \*-totals:: + + slabinfo-gnuplot.sh -t STATS1-totals STATS2-totals .. STATSN-totals + + This will produce a single plot (png file). + + Plots, expectedly, can be large so some fluctuations or small spikes + can go unnoticed. To deal with that, ``slabinfo-gnuplot.sh`` has two + options to 'zoom-in'/'zoom-out': + + a) ``-s %d,%d`` -- overwrites the default image width and height + b) ``-r %d,%d`` -- specifies a range of samples to use (for example, + in ``slabinfo -X >> FOO_STATS; sleep 1;`` case, using a ``-r + 40,60`` range will plot only samples collected between 40th and + 60th seconds). + + +DebugFS files for SLUB +====================== + +For more information about current state of SLUB caches with the user tracking +debug option enabled, debugfs files are available, typically under +/sys/kernel/debug/slab/<cache>/ (created only for caches with enabled user +tracking). There are 2 types of these files with the following debug +information: + +1. alloc_traces:: + + Prints information about unique allocation traces of the currently + allocated objects. The output is sorted by frequency of each trace. + + Information in the output: + Number of objects, allocating function, minimal/average/maximal jiffies since alloc, + pid range of the allocating processes, cpu mask of allocating cpus, and stack trace. + + Example::: + + 1085 populate_error_injection_list+0x97/0x110 age=166678/166680/166682 pid=1 cpus=1:: + __slab_alloc+0x6d/0x90 + kmem_cache_alloc_trace+0x2eb/0x300 + populate_error_injection_list+0x97/0x110 + init_error_injection+0x1b/0x71 + do_one_initcall+0x5f/0x2d0 + kernel_init_freeable+0x26f/0x2d7 + kernel_init+0xe/0x118 + ret_from_fork+0x22/0x30 + + +2. free_traces:: + + Prints information about unique freeing traces of the currently allocated + objects. The freeing traces thus come from the previous life-cycle of the + objects and are reported as not available for objects allocated for the first + time. The output is sorted by frequency of each trace. + + Information in the output: + Number of objects, freeing function, minimal/average/maximal jiffies since free, + pid range of the freeing processes, cpu mask of freeing cpus, and stack trace. + + Example::: + + 1980 <not-available> age=4294912290 pid=0 cpus=0 + 51 acpi_ut_update_ref_count+0x6a6/0x782 age=236886/237027/237772 pid=1 cpus=1 + kfree+0x2db/0x420 + acpi_ut_update_ref_count+0x6a6/0x782 + acpi_ut_update_object_reference+0x1ad/0x234 + acpi_ut_remove_reference+0x7d/0x84 + acpi_rs_get_prt_method_data+0x97/0xd6 + acpi_get_irq_routing_table+0x82/0xc4 + acpi_pci_irq_find_prt_entry+0x8e/0x2e0 + acpi_pci_irq_lookup+0x3a/0x1e0 + acpi_pci_irq_enable+0x77/0x240 + pcibios_enable_device+0x39/0x40 + do_pci_enable_device.part.0+0x5d/0xe0 + pci_enable_device_flags+0xfc/0x120 + pci_enable_device+0x13/0x20 + virtio_pci_probe+0x9e/0x170 + local_pci_probe+0x48/0x80 + pci_device_probe+0x105/0x1c0 + +Christoph Lameter, May 30, 2007 +Sergey Senozhatsky, October 23, 2015 diff --git a/Documentation/mm/split_page_table_lock.rst b/Documentation/mm/split_page_table_lock.rst new file mode 100644 index 000000000000..c08919662704 --- /dev/null +++ b/Documentation/mm/split_page_table_lock.rst @@ -0,0 +1,100 @@ +.. _split_page_table_lock: + +===================== +Split page table lock +===================== + +Originally, mm->page_table_lock spinlock protected all page tables of the +mm_struct. But this approach leads to poor page fault scalability of +multi-threaded applications due high contention on the lock. To improve +scalability, split page table lock was introduced. + +With split page table lock we have separate per-table lock to serialize +access to the table. At the moment we use split lock for PTE and PMD +tables. Access to higher level tables protected by mm->page_table_lock. + +There are helpers to lock/unlock a table and other accessor functions: + + - pte_offset_map_lock() + maps pte and takes PTE table lock, returns pointer to the taken + lock; + - pte_unmap_unlock() + unlocks and unmaps PTE table; + - pte_alloc_map_lock() + allocates PTE table if needed and take the lock, returns pointer + to taken lock or NULL if allocation failed; + - pte_lockptr() + returns pointer to PTE table lock; + - pmd_lock() + takes PMD table lock, returns pointer to taken lock; + - pmd_lockptr() + returns pointer to PMD table lock; + +Split page table lock for PTE tables is enabled compile-time if +CONFIG_SPLIT_PTLOCK_CPUS (usually 4) is less or equal to NR_CPUS. +If split lock is disabled, all tables are guarded by mm->page_table_lock. + +Split page table lock for PMD tables is enabled, if it's enabled for PTE +tables and the architecture supports it (see below). + +Hugetlb and split page table lock +================================= + +Hugetlb can support several page sizes. We use split lock only for PMD +level, but not for PUD. + +Hugetlb-specific helpers: + + - huge_pte_lock() + takes pmd split lock for PMD_SIZE page, mm->page_table_lock + otherwise; + - huge_pte_lockptr() + returns pointer to table lock; + +Support of split page table lock by an architecture +=================================================== + +There's no need in special enabling of PTE split page table lock: everything +required is done by pgtable_pte_page_ctor() and pgtable_pte_page_dtor(), which +must be called on PTE table allocation / freeing. + +Make sure the architecture doesn't use slab allocator for page table +allocation: slab uses page->slab_cache for its pages. +This field shares storage with page->ptl. + +PMD split lock only makes sense if you have more than two page table +levels. + +PMD split lock enabling requires pgtable_pmd_page_ctor() call on PMD table +allocation and pgtable_pmd_page_dtor() on freeing. + +Allocation usually happens in pmd_alloc_one(), freeing in pmd_free() and +pmd_free_tlb(), but make sure you cover all PMD table allocation / freeing +paths: i.e X86_PAE preallocate few PMDs on pgd_alloc(). + +With everything in place you can set CONFIG_ARCH_ENABLE_SPLIT_PMD_PTLOCK. + +NOTE: pgtable_pte_page_ctor() and pgtable_pmd_page_ctor() can fail -- it must +be handled properly. + +page->ptl +========= + +page->ptl is used to access split page table lock, where 'page' is struct +page of page containing the table. It shares storage with page->private +(and few other fields in union). + +To avoid increasing size of struct page and have best performance, we use a +trick: + + - if spinlock_t fits into long, we use page->ptr as spinlock, so we + can avoid indirect access and save a cache line. + - if size of spinlock_t is bigger then size of long, we use page->ptl as + pointer to spinlock_t and allocate it dynamically. This allows to use + split lock with enabled DEBUG_SPINLOCK or DEBUG_LOCK_ALLOC, but costs + one more cache line for indirect access; + +The spinlock_t allocated in pgtable_pte_page_ctor() for PTE table and in +pgtable_pmd_page_ctor() for PMD table. + +Please, never access page->ptl directly -- use appropriate helper. diff --git a/Documentation/mm/swap.rst b/Documentation/mm/swap.rst new file mode 100644 index 000000000000..78819bd4d745 --- /dev/null +++ b/Documentation/mm/swap.rst @@ -0,0 +1,5 @@ +.. SPDX-License-Identifier: GPL-2.0 + +==== +Swap +==== diff --git a/Documentation/mm/transhuge.rst b/Documentation/mm/transhuge.rst new file mode 100644 index 000000000000..216db1d67d04 --- /dev/null +++ b/Documentation/mm/transhuge.rst @@ -0,0 +1,187 @@ +.. _transhuge: + +============================ +Transparent Hugepage Support +============================ + +This document describes design principles for Transparent Hugepage (THP) +support and its interaction with other parts of the memory management +system. + +Design principles +================= + +- "graceful fallback": mm components which don't have transparent hugepage + knowledge fall back to breaking huge pmd mapping into table of ptes and, + if necessary, split a transparent hugepage. Therefore these components + can continue working on the regular pages or regular pte mappings. + +- if a hugepage allocation fails because of memory fragmentation, + regular pages should be gracefully allocated instead and mixed in + the same vma without any failure or significant delay and without + userland noticing + +- if some task quits and more hugepages become available (either + immediately in the buddy or through the VM), guest physical memory + backed by regular pages should be relocated on hugepages + automatically (with khugepaged) + +- it doesn't require memory reservation and in turn it uses hugepages + whenever possible (the only possible reservation here is kernelcore= + to avoid unmovable pages to fragment all the memory but such a tweak + is not specific to transparent hugepage support and it's a generic + feature that applies to all dynamic high order allocations in the + kernel) + +get_user_pages and follow_page +============================== + +get_user_pages and follow_page if run on a hugepage, will return the +head or tail pages as usual (exactly as they would do on +hugetlbfs). Most GUP users will only care about the actual physical +address of the page and its temporary pinning to release after the I/O +is complete, so they won't ever notice the fact the page is huge. But +if any driver is going to mangle over the page structure of the tail +page (like for checking page->mapping or other bits that are relevant +for the head page and not the tail page), it should be updated to jump +to check head page instead. Taking a reference on any head/tail page would +prevent the page from being split by anyone. + +.. note:: + these aren't new constraints to the GUP API, and they match the + same constraints that apply to hugetlbfs too, so any driver capable + of handling GUP on hugetlbfs will also work fine on transparent + hugepage backed mappings. + +Graceful fallback +================= + +Code walking pagetables but unaware about huge pmds can simply call +split_huge_pmd(vma, pmd, addr) where the pmd is the one returned by +pmd_offset. It's trivial to make the code transparent hugepage aware +by just grepping for "pmd_offset" and adding split_huge_pmd where +missing after pmd_offset returns the pmd. Thanks to the graceful +fallback design, with a one liner change, you can avoid to write +hundreds if not thousands of lines of complex code to make your code +hugepage aware. + +If you're not walking pagetables but you run into a physical hugepage +that you can't handle natively in your code, you can split it by +calling split_huge_page(page). This is what the Linux VM does before +it tries to swapout the hugepage for example. split_huge_page() can fail +if the page is pinned and you must handle this correctly. + +Example to make mremap.c transparent hugepage aware with a one liner +change:: + + diff --git a/mm/mremap.c b/mm/mremap.c + --- a/mm/mremap.c + +++ b/mm/mremap.c + @@ -41,6 +41,7 @@ static pmd_t *get_old_pmd(struct mm_stru + return NULL; + + pmd = pmd_offset(pud, addr); + + split_huge_pmd(vma, pmd, addr); + if (pmd_none_or_clear_bad(pmd)) + return NULL; + +Locking in hugepage aware code +============================== + +We want as much code as possible hugepage aware, as calling +split_huge_page() or split_huge_pmd() has a cost. + +To make pagetable walks huge pmd aware, all you need to do is to call +pmd_trans_huge() on the pmd returned by pmd_offset. You must hold the +mmap_lock in read (or write) mode to be sure a huge pmd cannot be +created from under you by khugepaged (khugepaged collapse_huge_page +takes the mmap_lock in write mode in addition to the anon_vma lock). If +pmd_trans_huge returns false, you just fallback in the old code +paths. If instead pmd_trans_huge returns true, you have to take the +page table lock (pmd_lock()) and re-run pmd_trans_huge. Taking the +page table lock will prevent the huge pmd being converted into a +regular pmd from under you (split_huge_pmd can run in parallel to the +pagetable walk). If the second pmd_trans_huge returns false, you +should just drop the page table lock and fallback to the old code as +before. Otherwise, you can proceed to process the huge pmd and the +hugepage natively. Once finished, you can drop the page table lock. + +Refcounts and transparent huge pages +==================================== + +Refcounting on THP is mostly consistent with refcounting on other compound +pages: + + - get_page()/put_page() and GUP operate on head page's ->_refcount. + + - ->_refcount in tail pages is always zero: get_page_unless_zero() never + succeeds on tail pages. + + - map/unmap of the pages with PTE entry increment/decrement ->_mapcount + on relevant sub-page of the compound page. + + - map/unmap of the whole compound page is accounted for in compound_mapcount + (stored in first tail page). For file huge pages, we also increment + ->_mapcount of all sub-pages in order to have race-free detection of + last unmap of subpages. + +PageDoubleMap() indicates that the page is *possibly* mapped with PTEs. + +For anonymous pages, PageDoubleMap() also indicates ->_mapcount in all +subpages is offset up by one. This additional reference is required to +get race-free detection of unmap of subpages when we have them mapped with +both PMDs and PTEs. + +This optimization is required to lower the overhead of per-subpage mapcount +tracking. The alternative is to alter ->_mapcount in all subpages on each +map/unmap of the whole compound page. + +For anonymous pages, we set PG_double_map when a PMD of the page is split +for the first time, but still have a PMD mapping. The additional references +go away with the last compound_mapcount. + +File pages get PG_double_map set on the first map of the page with PTE and +goes away when the page gets evicted from the page cache. + +split_huge_page internally has to distribute the refcounts in the head +page to the tail pages before clearing all PG_head/tail bits from the page +structures. It can be done easily for refcounts taken by page table +entries, but we don't have enough information on how to distribute any +additional pins (i.e. from get_user_pages). split_huge_page() fails any +requests to split pinned huge pages: it expects page count to be equal to +the sum of mapcount of all sub-pages plus one (split_huge_page caller must +have a reference to the head page). + +split_huge_page uses migration entries to stabilize page->_refcount and +page->_mapcount of anonymous pages. File pages just get unmapped. + +We are safe against physical memory scanners too: the only legitimate way +a scanner can get a reference to a page is get_page_unless_zero(). + +All tail pages have zero ->_refcount until atomic_add(). This prevents the +scanner from getting a reference to the tail page up to that point. After the +atomic_add() we don't care about the ->_refcount value. We already know how +many references should be uncharged from the head page. + +For head page get_page_unless_zero() will succeed and we don't mind. It's +clear where references should go after split: it will stay on the head page. + +Note that split_huge_pmd() doesn't have any limitations on refcounting: +pmd can be split at any point and never fails. + +Partial unmap and deferred_split_huge_page() +============================================ + +Unmapping part of THP (with munmap() or other way) is not going to free +memory immediately. Instead, we detect that a subpage of THP is not in use +in page_remove_rmap() and queue the THP for splitting if memory pressure +comes. Splitting will free up unused subpages. + +Splitting the page right away is not an option due to locking context in +the place where we can detect partial unmap. It also might be +counterproductive since in many cases partial unmap happens during exit(2) if +a THP crosses a VMA boundary. + +The function deferred_split_huge_page() is used to queue a page for splitting. +The splitting itself will happen when we get memory pressure via shrinker +interface. diff --git a/Documentation/mm/unevictable-lru.rst b/Documentation/mm/unevictable-lru.rst new file mode 100644 index 000000000000..b280367d6a44 --- /dev/null +++ b/Documentation/mm/unevictable-lru.rst @@ -0,0 +1,554 @@ +.. _unevictable_lru: + +============================== +Unevictable LRU Infrastructure +============================== + +.. contents:: :local: + + +Introduction +============ + +This document describes the Linux memory manager's "Unevictable LRU" +infrastructure and the use of this to manage several types of "unevictable" +pages. + +The document attempts to provide the overall rationale behind this mechanism +and the rationale for some of the design decisions that drove the +implementation. The latter design rationale is discussed in the context of an +implementation description. Admittedly, one can obtain the implementation +details - the "what does it do?" - by reading the code. One hopes that the +descriptions below add value by provide the answer to "why does it do that?". + + + +The Unevictable LRU +=================== + +The Unevictable LRU facility adds an additional LRU list to track unevictable +pages and to hide these pages from vmscan. This mechanism is based on a patch +by Larry Woodman of Red Hat to address several scalability problems with page +reclaim in Linux. The problems have been observed at customer sites on large +memory x86_64 systems. + +To illustrate this with an example, a non-NUMA x86_64 platform with 128GB of +main memory will have over 32 million 4k pages in a single node. When a large +fraction of these pages are not evictable for any reason [see below], vmscan +will spend a lot of time scanning the LRU lists looking for the small fraction +of pages that are evictable. This can result in a situation where all CPUs are +spending 100% of their time in vmscan for hours or days on end, with the system +completely unresponsive. + +The unevictable list addresses the following classes of unevictable pages: + + * Those owned by ramfs. + + * Those mapped into SHM_LOCK'd shared memory regions. + + * Those mapped into VM_LOCKED [mlock()ed] VMAs. + +The infrastructure may also be able to handle other conditions that make pages +unevictable, either by definition or by circumstance, in the future. + + +The Unevictable LRU Page List +----------------------------- + +The Unevictable LRU page list is a lie. It was never an LRU-ordered list, but a +companion to the LRU-ordered anonymous and file, active and inactive page lists; +and now it is not even a page list. But following familiar convention, here in +this document and in the source, we often imagine it as a fifth LRU page list. + +The Unevictable LRU infrastructure consists of an additional, per-node, LRU list +called the "unevictable" list and an associated page flag, PG_unevictable, to +indicate that the page is being managed on the unevictable list. + +The PG_unevictable flag is analogous to, and mutually exclusive with, the +PG_active flag in that it indicates on which LRU list a page resides when +PG_lru is set. + +The Unevictable LRU infrastructure maintains unevictable pages as if they were +on an additional LRU list for a few reasons: + + (1) We get to "treat unevictable pages just like we treat other pages in the + system - which means we get to use the same code to manipulate them, the + same code to isolate them (for migrate, etc.), the same code to keep track + of the statistics, etc..." [Rik van Riel] + + (2) We want to be able to migrate unevictable pages between nodes for memory + defragmentation, workload management and memory hotplug. The Linux kernel + can only migrate pages that it can successfully isolate from the LRU + lists (or "Movable" pages: outside of consideration here). If we were to + maintain pages elsewhere than on an LRU-like list, where they can be + detected by isolate_lru_page(), we would prevent their migration. + +The unevictable list does not differentiate between file-backed and anonymous, +swap-backed pages. This differentiation is only important while the pages are, +in fact, evictable. + +The unevictable list benefits from the "arrayification" of the per-node LRU +lists and statistics originally proposed and posted by Christoph Lameter. + + +Memory Control Group Interaction +-------------------------------- + +The unevictable LRU facility interacts with the memory control group [aka +memory controller; see Documentation/admin-guide/cgroup-v1/memory.rst] by +extending the lru_list enum. + +The memory controller data structure automatically gets a per-node unevictable +list as a result of the "arrayification" of the per-node LRU lists (one per +lru_list enum element). The memory controller tracks the movement of pages to +and from the unevictable list. + +When a memory control group comes under memory pressure, the controller will +not attempt to reclaim pages on the unevictable list. This has a couple of +effects: + + (1) Because the pages are "hidden" from reclaim on the unevictable list, the + reclaim process can be more efficient, dealing only with pages that have a + chance of being reclaimed. + + (2) On the other hand, if too many of the pages charged to the control group + are unevictable, the evictable portion of the working set of the tasks in + the control group may not fit into the available memory. This can cause + the control group to thrash or to OOM-kill tasks. + + +.. _mark_addr_space_unevict: + +Marking Address Spaces Unevictable +---------------------------------- + +For facilities such as ramfs none of the pages attached to the address space +may be evicted. To prevent eviction of any such pages, the AS_UNEVICTABLE +address space flag is provided, and this can be manipulated by a filesystem +using a number of wrapper functions: + + * ``void mapping_set_unevictable(struct address_space *mapping);`` + + Mark the address space as being completely unevictable. + + * ``void mapping_clear_unevictable(struct address_space *mapping);`` + + Mark the address space as being evictable. + + * ``int mapping_unevictable(struct address_space *mapping);`` + + Query the address space, and return true if it is completely + unevictable. + +These are currently used in three places in the kernel: + + (1) By ramfs to mark the address spaces of its inodes when they are created, + and this mark remains for the life of the inode. + + (2) By SYSV SHM to mark SHM_LOCK'd address spaces until SHM_UNLOCK is called. + Note that SHM_LOCK is not required to page in the locked pages if they're + swapped out; the application must touch the pages manually if it wants to + ensure they're in memory. + + (3) By the i915 driver to mark pinned address space until it's unpinned. The + amount of unevictable memory marked by i915 driver is roughly the bounded + object size in debugfs/dri/0/i915_gem_objects. + + +Detecting Unevictable Pages +--------------------------- + +The function page_evictable() in mm/internal.h determines whether a page is +evictable or not using the query function outlined above [see section +:ref:`Marking address spaces unevictable <mark_addr_space_unevict>`] +to check the AS_UNEVICTABLE flag. + +For address spaces that are so marked after being populated (as SHM regions +might be), the lock action (e.g. SHM_LOCK) can be lazy, and need not populate +the page tables for the region as does, for example, mlock(), nor need it make +any special effort to push any pages in the SHM_LOCK'd area to the unevictable +list. Instead, vmscan will do this if and when it encounters the pages during +a reclamation scan. + +On an unlock action (such as SHM_UNLOCK), the unlocker (e.g. shmctl()) must scan +the pages in the region and "rescue" them from the unevictable list if no other +condition is keeping them unevictable. If an unevictable region is destroyed, +the pages are also "rescued" from the unevictable list in the process of +freeing them. + +page_evictable() also checks for mlocked pages by testing an additional page +flag, PG_mlocked (as wrapped by PageMlocked()), which is set when a page is +faulted into a VM_LOCKED VMA, or found in a VMA being VM_LOCKED. + + +Vmscan's Handling of Unevictable Pages +-------------------------------------- + +If unevictable pages are culled in the fault path, or moved to the unevictable +list at mlock() or mmap() time, vmscan will not encounter the pages until they +have become evictable again (via munlock() for example) and have been "rescued" +from the unevictable list. However, there may be situations where we decide, +for the sake of expediency, to leave an unevictable page on one of the regular +active/inactive LRU lists for vmscan to deal with. vmscan checks for such +pages in all of the shrink_{active|inactive|page}_list() functions and will +"cull" such pages that it encounters: that is, it diverts those pages to the +unevictable list for the memory cgroup and node being scanned. + +There may be situations where a page is mapped into a VM_LOCKED VMA, but the +page is not marked as PG_mlocked. Such pages will make it all the way to +shrink_active_list() or shrink_page_list() where they will be detected when +vmscan walks the reverse map in page_referenced() or try_to_unmap(). The page +is culled to the unevictable list when it is released by the shrinker. + +To "cull" an unevictable page, vmscan simply puts the page back on the LRU list +using putback_lru_page() - the inverse operation to isolate_lru_page() - after +dropping the page lock. Because the condition which makes the page unevictable +may change once the page is unlocked, __pagevec_lru_add_fn() will recheck the +unevictable state of a page before placing it on the unevictable list. + + +MLOCKED Pages +============= + +The unevictable page list is also useful for mlock(), in addition to ramfs and +SYSV SHM. Note that mlock() is only available in CONFIG_MMU=y situations; in +NOMMU situations, all mappings are effectively mlocked. + + +History +------- + +The "Unevictable mlocked Pages" infrastructure is based on work originally +posted by Nick Piggin in an RFC patch entitled "mm: mlocked pages off LRU". +Nick posted his patch as an alternative to a patch posted by Christoph Lameter +to achieve the same objective: hiding mlocked pages from vmscan. + +In Nick's patch, he used one of the struct page LRU list link fields as a count +of VM_LOCKED VMAs that map the page (Rik van Riel had the same idea three years +earlier). But this use of the link field for a count prevented the management +of the pages on an LRU list, and thus mlocked pages were not migratable as +isolate_lru_page() could not detect them, and the LRU list link field was not +available to the migration subsystem. + +Nick resolved this by putting mlocked pages back on the LRU list before +attempting to isolate them, thus abandoning the count of VM_LOCKED VMAs. When +Nick's patch was integrated with the Unevictable LRU work, the count was +replaced by walking the reverse map when munlocking, to determine whether any +other VM_LOCKED VMAs still mapped the page. + +However, walking the reverse map for each page when munlocking was ugly and +inefficient, and could lead to catastrophic contention on a file's rmap lock, +when many processes which had it mlocked were trying to exit. In 5.18, the +idea of keeping mlock_count in Unevictable LRU list link field was revived and +put to work, without preventing the migration of mlocked pages. This is why +the "Unevictable LRU list" cannot be a linked list of pages now; but there was +no use for that linked list anyway - though its size is maintained for meminfo. + + +Basic Management +---------------- + +mlocked pages - pages mapped into a VM_LOCKED VMA - are a class of unevictable +pages. When such a page has been "noticed" by the memory management subsystem, +the page is marked with the PG_mlocked flag. This can be manipulated using the +PageMlocked() functions. + +A PG_mlocked page will be placed on the unevictable list when it is added to +the LRU. Such pages can be "noticed" by memory management in several places: + + (1) in the mlock()/mlock2()/mlockall() system call handlers; + + (2) in the mmap() system call handler when mmapping a region with the + MAP_LOCKED flag; + + (3) mmapping a region in a task that has called mlockall() with the MCL_FUTURE + flag; + + (4) in the fault path and when a VM_LOCKED stack segment is expanded; or + + (5) as mentioned above, in vmscan:shrink_page_list() when attempting to + reclaim a page in a VM_LOCKED VMA by page_referenced() or try_to_unmap(). + +mlocked pages become unlocked and rescued from the unevictable list when: + + (1) mapped in a range unlocked via the munlock()/munlockall() system calls; + + (2) munmap()'d out of the last VM_LOCKED VMA that maps the page, including + unmapping at task exit; + + (3) when the page is truncated from the last VM_LOCKED VMA of an mmapped file; + or + + (4) before a page is COW'd in a VM_LOCKED VMA. + + +mlock()/mlock2()/mlockall() System Call Handling +------------------------------------------------ + +mlock(), mlock2() and mlockall() system call handlers proceed to mlock_fixup() +for each VMA in the range specified by the call. In the case of mlockall(), +this is the entire active address space of the task. Note that mlock_fixup() +is used for both mlocking and munlocking a range of memory. A call to mlock() +an already VM_LOCKED VMA, or to munlock() a VMA that is not VM_LOCKED, is +treated as a no-op and mlock_fixup() simply returns. + +If the VMA passes some filtering as described in "Filtering Special VMAs" +below, mlock_fixup() will attempt to merge the VMA with its neighbors or split +off a subset of the VMA if the range does not cover the entire VMA. Any pages +already present in the VMA are then marked as mlocked by mlock_page() via +mlock_pte_range() via walk_page_range() via mlock_vma_pages_range(). + +Before returning from the system call, do_mlock() or mlockall() will call +__mm_populate() to fault in the remaining pages via get_user_pages() and to +mark those pages as mlocked as they are faulted. + +Note that the VMA being mlocked might be mapped with PROT_NONE. In this case, +get_user_pages() will be unable to fault in the pages. That's okay. If pages +do end up getting faulted into this VM_LOCKED VMA, they will be handled in the +fault path - which is also how mlock2()'s MLOCK_ONFAULT areas are handled. + +For each PTE (or PMD) being faulted into a VMA, the page add rmap function +calls mlock_vma_page(), which calls mlock_page() when the VMA is VM_LOCKED +(unless it is a PTE mapping of a part of a transparent huge page). Or when +it is a newly allocated anonymous page, lru_cache_add_inactive_or_unevictable() +calls mlock_new_page() instead: similar to mlock_page(), but can make better +judgments, since this page is held exclusively and known not to be on LRU yet. + +mlock_page() sets PageMlocked immediately, then places the page on the CPU's +mlock pagevec, to batch up the rest of the work to be done under lru_lock by +__mlock_page(). __mlock_page() sets PageUnevictable, initializes mlock_count +and moves the page to unevictable state ("the unevictable LRU", but with +mlock_count in place of LRU threading). Or if the page was already PageLRU +and PageUnevictable and PageMlocked, it simply increments the mlock_count. + +But in practice that may not work ideally: the page may not yet be on an LRU, or +it may have been temporarily isolated from LRU. In such cases the mlock_count +field cannot be touched, but will be set to 0 later when __pagevec_lru_add_fn() +returns the page to "LRU". Races prohibit mlock_count from being set to 1 then: +rather than risk stranding a page indefinitely as unevictable, always err with +mlock_count on the low side, so that when munlocked the page will be rescued to +an evictable LRU, then perhaps be mlocked again later if vmscan finds it in a +VM_LOCKED VMA. + + +Filtering Special VMAs +---------------------- + +mlock_fixup() filters several classes of "special" VMAs: + +1) VMAs with VM_IO or VM_PFNMAP set are skipped entirely. The pages behind + these mappings are inherently pinned, so we don't need to mark them as + mlocked. In any case, most of the pages have no struct page in which to so + mark the page. Because of this, get_user_pages() will fail for these VMAs, + so there is no sense in attempting to visit them. + +2) VMAs mapping hugetlbfs page are already effectively pinned into memory. We + neither need nor want to mlock() these pages. But __mm_populate() includes + hugetlbfs ranges, allocating the huge pages and populating the PTEs. + +3) VMAs with VM_DONTEXPAND are generally userspace mappings of kernel pages, + such as the VDSO page, relay channel pages, etc. These pages are inherently + unevictable and are not managed on the LRU lists. __mm_populate() includes + these ranges, populating the PTEs if not already populated. + +4) VMAs with VM_MIXEDMAP set are not marked VM_LOCKED, but __mm_populate() + includes these ranges, populating the PTEs if not already populated. + +Note that for all of these special VMAs, mlock_fixup() does not set the +VM_LOCKED flag. Therefore, we won't have to deal with them later during +munlock(), munmap() or task exit. Neither does mlock_fixup() account these +VMAs against the task's "locked_vm". + + +munlock()/munlockall() System Call Handling +------------------------------------------- + +The munlock() and munlockall() system calls are handled by the same +mlock_fixup() function as mlock(), mlock2() and mlockall() system calls are. +If called to munlock an already munlocked VMA, mlock_fixup() simply returns. +Because of the VMA filtering discussed above, VM_LOCKED will not be set in +any "special" VMAs. So, those VMAs will be ignored for munlock. + +If the VMA is VM_LOCKED, mlock_fixup() again attempts to merge or split off the +specified range. All pages in the VMA are then munlocked by munlock_page() via +mlock_pte_range() via walk_page_range() via mlock_vma_pages_range() - the same +function used when mlocking a VMA range, with new flags for the VMA indicating +that it is munlock() being performed. + +munlock_page() uses the mlock pagevec to batch up work to be done under +lru_lock by __munlock_page(). __munlock_page() decrements the page's +mlock_count, and when that reaches 0 it clears PageMlocked and clears +PageUnevictable, moving the page from unevictable state to inactive LRU. + +But in practice that may not work ideally: the page may not yet have reached +"the unevictable LRU", or it may have been temporarily isolated from it. In +those cases its mlock_count field is unusable and must be assumed to be 0: so +that the page will be rescued to an evictable LRU, then perhaps be mlocked +again later if vmscan finds it in a VM_LOCKED VMA. + + +Migrating MLOCKED Pages +----------------------- + +A page that is being migrated has been isolated from the LRU lists and is held +locked across unmapping of the page, updating the page's address space entry +and copying the contents and state, until the page table entry has been +replaced with an entry that refers to the new page. Linux supports migration +of mlocked pages and other unevictable pages. PG_mlocked is cleared from the +the old page when it is unmapped from the last VM_LOCKED VMA, and set when the +new page is mapped in place of migration entry in a VM_LOCKED VMA. If the page +was unevictable because mlocked, PG_unevictable follows PG_mlocked; but if the +page was unevictable for other reasons, PG_unevictable is copied explicitly. + +Note that page migration can race with mlocking or munlocking of the same page. +There is mostly no problem since page migration requires unmapping all PTEs of +the old page (including munlock where VM_LOCKED), then mapping in the new page +(including mlock where VM_LOCKED). The page table locks provide sufficient +synchronization. + +However, since mlock_vma_pages_range() starts by setting VM_LOCKED on a VMA, +before mlocking any pages already present, if one of those pages were migrated +before mlock_pte_range() reached it, it would get counted twice in mlock_count. +To prevent that, mlock_vma_pages_range() temporarily marks the VMA as VM_IO, +so that mlock_vma_page() will skip it. + +To complete page migration, we place the old and new pages back onto the LRU +afterwards. The "unneeded" page - old page on success, new page on failure - +is freed when the reference count held by the migration process is released. + + +Compacting MLOCKED Pages +------------------------ + +The memory map can be scanned for compactable regions and the default behavior +is to let unevictable pages be moved. /proc/sys/vm/compact_unevictable_allowed +controls this behavior (see Documentation/admin-guide/sysctl/vm.rst). The work +of compaction is mostly handled by the page migration code and the same work +flow as described in Migrating MLOCKED Pages will apply. + + +MLOCKING Transparent Huge Pages +------------------------------- + +A transparent huge page is represented by a single entry on an LRU list. +Therefore, we can only make unevictable an entire compound page, not +individual subpages. + +If a user tries to mlock() part of a huge page, and no user mlock()s the +whole of the huge page, we want the rest of the page to be reclaimable. + +We cannot just split the page on partial mlock() as split_huge_page() can +fail and a new intermittent failure mode for the syscall is undesirable. + +We handle this by keeping PTE-mlocked huge pages on evictable LRU lists: +the PMD on the border of a VM_LOCKED VMA will be split into a PTE table. + +This way the huge page is accessible for vmscan. Under memory pressure the +page will be split, subpages which belong to VM_LOCKED VMAs will be moved +to the unevictable LRU and the rest can be reclaimed. + +/proc/meminfo's Unevictable and Mlocked amounts do not include those parts +of a transparent huge page which are mapped only by PTEs in VM_LOCKED VMAs. + + +mmap(MAP_LOCKED) System Call Handling +------------------------------------- + +In addition to the mlock(), mlock2() and mlockall() system calls, an application +can request that a region of memory be mlocked by supplying the MAP_LOCKED flag +to the mmap() call. There is one important and subtle difference here, though. +mmap() + mlock() will fail if the range cannot be faulted in (e.g. because +mm_populate fails) and returns with ENOMEM while mmap(MAP_LOCKED) will not fail. +The mmaped area will still have properties of the locked area - pages will not +get swapped out - but major page faults to fault memory in might still happen. + +Furthermore, any mmap() call or brk() call that expands the heap by a task +that has previously called mlockall() with the MCL_FUTURE flag will result +in the newly mapped memory being mlocked. Before the unevictable/mlock +changes, the kernel simply called make_pages_present() to allocate pages +and populate the page table. + +To mlock a range of memory under the unevictable/mlock infrastructure, +the mmap() handler and task address space expansion functions call +populate_vma_page_range() specifying the vma and the address range to mlock. + + +munmap()/exit()/exec() System Call Handling +------------------------------------------- + +When unmapping an mlocked region of memory, whether by an explicit call to +munmap() or via an internal unmap from exit() or exec() processing, we must +munlock the pages if we're removing the last VM_LOCKED VMA that maps the pages. +Before the unevictable/mlock changes, mlocking did not mark the pages in any +way, so unmapping them required no processing. + +For each PTE (or PMD) being unmapped from a VMA, page_remove_rmap() calls +munlock_vma_page(), which calls munlock_page() when the VMA is VM_LOCKED +(unless it was a PTE mapping of a part of a transparent huge page). + +munlock_page() uses the mlock pagevec to batch up work to be done under +lru_lock by __munlock_page(). __munlock_page() decrements the page's +mlock_count, and when that reaches 0 it clears PageMlocked and clears +PageUnevictable, moving the page from unevictable state to inactive LRU. + +But in practice that may not work ideally: the page may not yet have reached +"the unevictable LRU", or it may have been temporarily isolated from it. In +those cases its mlock_count field is unusable and must be assumed to be 0: so +that the page will be rescued to an evictable LRU, then perhaps be mlocked +again later if vmscan finds it in a VM_LOCKED VMA. + + +Truncating MLOCKED Pages +------------------------ + +File truncation or hole punching forcibly unmaps the deleted pages from +userspace; truncation even unmaps and deletes any private anonymous pages +which had been Copied-On-Write from the file pages now being truncated. + +Mlocked pages can be munlocked and deleted in this way: like with munmap(), +for each PTE (or PMD) being unmapped from a VMA, page_remove_rmap() calls +munlock_vma_page(), which calls munlock_page() when the VMA is VM_LOCKED +(unless it was a PTE mapping of a part of a transparent huge page). + +However, if there is a racing munlock(), since mlock_vma_pages_range() starts +munlocking by clearing VM_LOCKED from a VMA, before munlocking all the pages +present, if one of those pages were unmapped by truncation or hole punch before +mlock_pte_range() reached it, it would not be recognized as mlocked by this VMA, +and would not be counted out of mlock_count. In this rare case, a page may +still appear as PageMlocked after it has been fully unmapped: and it is left to +release_pages() (or __page_cache_release()) to clear it and update statistics +before freeing (this event is counted in /proc/vmstat unevictable_pgs_cleared, +which is usually 0). + + +Page Reclaim in shrink_*_list() +------------------------------- + +vmscan's shrink_active_list() culls any obviously unevictable pages - +i.e. !page_evictable(page) pages - diverting those to the unevictable list. +However, shrink_active_list() only sees unevictable pages that made it onto the +active/inactive LRU lists. Note that these pages do not have PageUnevictable +set - otherwise they would be on the unevictable list and shrink_active_list() +would never see them. + +Some examples of these unevictable pages on the LRU lists are: + + (1) ramfs pages that have been placed on the LRU lists when first allocated. + + (2) SHM_LOCK'd shared memory pages. shmctl(SHM_LOCK) does not attempt to + allocate or fault in the pages in the shared memory region. This happens + when an application accesses the page the first time after SHM_LOCK'ing + the segment. + + (3) pages still mapped into VM_LOCKED VMAs, which should be marked mlocked, + but events left mlock_count too low, so they were munlocked too early. + +vmscan's shrink_inactive_list() and shrink_page_list() also divert obviously +unevictable pages found on the inactive lists to the appropriate memory cgroup +and node unevictable list. + +rmap's page_referenced_one(), called via vmscan's shrink_active_list() or +shrink_page_list(), and rmap's try_to_unmap_one() called via shrink_page_list(), +check for (3) pages still mapped into VM_LOCKED VMAs, and call mlock_vma_page() +to correct them. Such pages are culled to the unevictable list when released +by the shrinker. diff --git a/Documentation/mm/vmalloc.rst b/Documentation/mm/vmalloc.rst new file mode 100644 index 000000000000..363fe20d6b9f --- /dev/null +++ b/Documentation/mm/vmalloc.rst @@ -0,0 +1,5 @@ +.. SPDX-License-Identifier: GPL-2.0 + +====================================== +Virtually Contiguous Memory Allocation +====================================== diff --git a/Documentation/mm/vmalloced-kernel-stacks.rst b/Documentation/mm/vmalloced-kernel-stacks.rst new file mode 100644 index 000000000000..fc8c67833af6 --- /dev/null +++ b/Documentation/mm/vmalloced-kernel-stacks.rst @@ -0,0 +1,153 @@ +.. SPDX-License-Identifier: GPL-2.0 + +===================================== +Virtually Mapped Kernel Stack Support +===================================== + +:Author: Shuah Khan <skhan@linuxfoundation.org> + +.. contents:: :local: + +Overview +-------- + +This is a compilation of information from the code and original patch +series that introduced the `Virtually Mapped Kernel Stacks feature +<https://lwn.net/Articles/694348/>` + +Introduction +------------ + +Kernel stack overflows are often hard to debug and make the kernel +susceptible to exploits. Problems could show up at a later time making +it difficult to isolate and root-cause. + +Virtually-mapped kernel stacks with guard pages causes kernel stack +overflows to be caught immediately rather than causing difficult to +diagnose corruptions. + +HAVE_ARCH_VMAP_STACK and VMAP_STACK configuration options enable +support for virtually mapped stacks with guard pages. This feature +causes reliable faults when the stack overflows. The usability of +the stack trace after overflow and response to the overflow itself +is architecture dependent. + +.. note:: + As of this writing, arm64, powerpc, riscv, s390, um, and x86 have + support for VMAP_STACK. + +HAVE_ARCH_VMAP_STACK +-------------------- + +Architectures that can support Virtually Mapped Kernel Stacks should +enable this bool configuration option. The requirements are: + +- vmalloc space must be large enough to hold many kernel stacks. This + may rule out many 32-bit architectures. +- Stacks in vmalloc space need to work reliably. For example, if + vmap page tables are created on demand, either this mechanism + needs to work while the stack points to a virtual address with + unpopulated page tables or arch code (switch_to() and switch_mm(), + most likely) needs to ensure that the stack's page table entries + are populated before running on a possibly unpopulated stack. +- If the stack overflows into a guard page, something reasonable + should happen. The definition of "reasonable" is flexible, but + instantly rebooting without logging anything would be unfriendly. + +VMAP_STACK +---------- + +VMAP_STACK bool configuration option when enabled allocates virtually +mapped task stacks. This option depends on HAVE_ARCH_VMAP_STACK. + +- Enable this if you want the use virtually-mapped kernel stacks + with guard pages. This causes kernel stack overflows to be caught + immediately rather than causing difficult-to-diagnose corruption. + +.. note:: + + Using this feature with KASAN requires architecture support + for backing virtual mappings with real shadow memory, and + KASAN_VMALLOC must be enabled. + +.. note:: + + VMAP_STACK is enabled, it is not possible to run DMA on stack + allocated data. + +Kernel configuration options and dependencies keep changing. Refer to +the latest code base: + +`Kconfig <https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/Kconfig>` + +Allocation +----------- + +When a new kernel thread is created, thread stack is allocated from +virtually contiguous memory pages from the page level allocator. These +pages are mapped into contiguous kernel virtual space with PAGE_KERNEL +protections. + +alloc_thread_stack_node() calls __vmalloc_node_range() to allocate stack +with PAGE_KERNEL protections. + +- Allocated stacks are cached and later reused by new threads, so memcg + accounting is performed manually on assigning/releasing stacks to tasks. + Hence, __vmalloc_node_range is called without __GFP_ACCOUNT. +- vm_struct is cached to be able to find when thread free is initiated + in interrupt context. free_thread_stack() can be called in interrupt + context. +- On arm64, all VMAP's stacks need to have the same alignment to ensure + that VMAP'd stack overflow detection works correctly. Arch specific + vmap stack allocator takes care of this detail. +- This does not address interrupt stacks - according to the original patch + +Thread stack allocation is initiated from clone(), fork(), vfork(), +kernel_thread() via kernel_clone(). Leaving a few hints for searching +the code base to understand when and how thread stack is allocated. + +Bulk of the code is in: +`kernel/fork.c <https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/fork.c>`. + +stack_vm_area pointer in task_struct keeps track of the virtually allocated +stack and a non-null stack_vm_area pointer serves as a indication that the +virtually mapped kernel stacks are enabled. + +:: + + struct vm_struct *stack_vm_area; + +Stack overflow handling +----------------------- + +Leading and trailing guard pages help detect stack overflows. When stack +overflows into the guard pages, handlers have to be careful not overflow +the stack again. When handlers are called, it is likely that very little +stack space is left. + +On x86, this is done by handling the page fault indicating the kernel +stack overflow on the double-fault stack. + +Testing VMAP allocation with guard pages +---------------------------------------- + +How do we ensure that VMAP_STACK is actually allocating with a leading +and trailing guard page? The following lkdtm tests can help detect any +regressions. + +:: + + void lkdtm_STACK_GUARD_PAGE_LEADING() + void lkdtm_STACK_GUARD_PAGE_TRAILING() + +Conclusions +----------- + +- A percpu cache of vmalloced stacks appears to be a bit faster than a + high-order stack allocation, at least when the cache hits. +- THREAD_INFO_IN_TASK gets rid of arch-specific thread_info entirely and + simply embed the thread_info (containing only flags) and 'int cpu' into + task_struct. +- The thread stack can be free'ed as soon as the task is dead (without + waiting for RCU) and then, if vmapped stacks are in use, cache the + entire stack for reuse on the same cpu. diff --git a/Documentation/mm/vmemmap_dedup.rst b/Documentation/mm/vmemmap_dedup.rst new file mode 100644 index 000000000000..c9c495f62d12 --- /dev/null +++ b/Documentation/mm/vmemmap_dedup.rst @@ -0,0 +1,223 @@ +.. SPDX-License-Identifier: GPL-2.0 + +========================================= +A vmemmap diet for HugeTLB and Device DAX +========================================= + +HugeTLB +======= + +The struct page structures (page structs) are used to describe a physical +page frame. By default, there is a one-to-one mapping from a page frame to +it's corresponding page struct. + +HugeTLB pages consist of multiple base page size pages and is supported by many +architectures. See Documentation/admin-guide/mm/hugetlbpage.rst for more +details. On the x86-64 architecture, HugeTLB pages of size 2MB and 1GB are +currently supported. Since the base page size on x86 is 4KB, a 2MB HugeTLB page +consists of 512 base pages and a 1GB HugeTLB page consists of 4096 base pages. +For each base page, there is a corresponding page struct. + +Within the HugeTLB subsystem, only the first 4 page structs are used to +contain unique information about a HugeTLB page. __NR_USED_SUBPAGE provides +this upper limit. The only 'useful' information in the remaining page structs +is the compound_head field, and this field is the same for all tail pages. + +By removing redundant page structs for HugeTLB pages, memory can be returned +to the buddy allocator for other uses. + +Different architectures support different HugeTLB pages. For example, the +following table is the HugeTLB page size supported by x86 and arm64 +architectures. Because arm64 supports 4k, 16k, and 64k base pages and +supports contiguous entries, so it supports many kinds of sizes of HugeTLB +page. + ++--------------+-----------+-----------------------------------------------+ +| Architecture | Page Size | HugeTLB Page Size | ++--------------+-----------+-----------+-----------+-----------+-----------+ +| x86-64 | 4KB | 2MB | 1GB | | | ++--------------+-----------+-----------+-----------+-----------+-----------+ +| | 4KB | 64KB | 2MB | 32MB | 1GB | +| +-----------+-----------+-----------+-----------+-----------+ +| arm64 | 16KB | 2MB | 32MB | 1GB | | +| +-----------+-----------+-----------+-----------+-----------+ +| | 64KB | 2MB | 512MB | 16GB | | ++--------------+-----------+-----------+-----------+-----------+-----------+ + +When the system boot up, every HugeTLB page has more than one struct page +structs which size is (unit: pages):: + + struct_size = HugeTLB_Size / PAGE_SIZE * sizeof(struct page) / PAGE_SIZE + +Where HugeTLB_Size is the size of the HugeTLB page. We know that the size +of the HugeTLB page is always n times PAGE_SIZE. So we can get the following +relationship:: + + HugeTLB_Size = n * PAGE_SIZE + +Then:: + + struct_size = n * PAGE_SIZE / PAGE_SIZE * sizeof(struct page) / PAGE_SIZE + = n * sizeof(struct page) / PAGE_SIZE + +We can use huge mapping at the pud/pmd level for the HugeTLB page. + +For the HugeTLB page of the pmd level mapping, then:: + + struct_size = n * sizeof(struct page) / PAGE_SIZE + = PAGE_SIZE / sizeof(pte_t) * sizeof(struct page) / PAGE_SIZE + = sizeof(struct page) / sizeof(pte_t) + = 64 / 8 + = 8 (pages) + +Where n is how many pte entries which one page can contains. So the value of +n is (PAGE_SIZE / sizeof(pte_t)). + +This optimization only supports 64-bit system, so the value of sizeof(pte_t) +is 8. And this optimization also applicable only when the size of struct page +is a power of two. In most cases, the size of struct page is 64 bytes (e.g. +x86-64 and arm64). So if we use pmd level mapping for a HugeTLB page, the +size of struct page structs of it is 8 page frames which size depends on the +size of the base page. + +For the HugeTLB page of the pud level mapping, then:: + + struct_size = PAGE_SIZE / sizeof(pmd_t) * struct_size(pmd) + = PAGE_SIZE / 8 * 8 (pages) + = PAGE_SIZE (pages) + +Where the struct_size(pmd) is the size of the struct page structs of a +HugeTLB page of the pmd level mapping. + +E.g.: A 2MB HugeTLB page on x86_64 consists in 8 page frames while 1GB +HugeTLB page consists in 4096. + +Next, we take the pmd level mapping of the HugeTLB page as an example to +show the internal implementation of this optimization. There are 8 pages +struct page structs associated with a HugeTLB page which is pmd mapped. + +Here is how things look before optimization:: + + HugeTLB struct pages(8 pages) page frame(8 pages) + +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+ + | | | 0 | -------------> | 0 | + | | +-----------+ +-----------+ + | | | 1 | -------------> | 1 | + | | +-----------+ +-----------+ + | | | 2 | -------------> | 2 | + | | +-----------+ +-----------+ + | | | 3 | -------------> | 3 | + | | +-----------+ +-----------+ + | | | 4 | -------------> | 4 | + | PMD | +-----------+ +-----------+ + | level | | 5 | -------------> | 5 | + | mapping | +-----------+ +-----------+ + | | | 6 | -------------> | 6 | + | | +-----------+ +-----------+ + | | | 7 | -------------> | 7 | + | | +-----------+ +-----------+ + | | + | | + | | + +-----------+ + +The value of page->compound_head is the same for all tail pages. The first +page of page structs (page 0) associated with the HugeTLB page contains the 4 +page structs necessary to describe the HugeTLB. The only use of the remaining +pages of page structs (page 1 to page 7) is to point to page->compound_head. +Therefore, we can remap pages 1 to 7 to page 0. Only 1 page of page structs +will be used for each HugeTLB page. This will allow us to free the remaining +7 pages to the buddy allocator. + +Here is how things look after remapping:: + + HugeTLB struct pages(8 pages) page frame(8 pages) + +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+ + | | | 0 | -------------> | 0 | + | | +-----------+ +-----------+ + | | | 1 | ---------------^ ^ ^ ^ ^ ^ ^ + | | +-----------+ | | | | | | + | | | 2 | -----------------+ | | | | | + | | +-----------+ | | | | | + | | | 3 | -------------------+ | | | | + | | +-----------+ | | | | + | | | 4 | ---------------------+ | | | + | PMD | +-----------+ | | | + | level | | 5 | -----------------------+ | | + | mapping | +-----------+ | | + | | | 6 | -------------------------+ | + | | +-----------+ | + | | | 7 | ---------------------------+ + | | +-----------+ + | | + | | + | | + +-----------+ + +When a HugeTLB is freed to the buddy system, we should allocate 7 pages for +vmemmap pages and restore the previous mapping relationship. + +For the HugeTLB page of the pud level mapping. It is similar to the former. +We also can use this approach to free (PAGE_SIZE - 1) vmemmap pages. + +Apart from the HugeTLB page of the pmd/pud level mapping, some architectures +(e.g. aarch64) provides a contiguous bit in the translation table entries +that hints to the MMU to indicate that it is one of a contiguous set of +entries that can be cached in a single TLB entry. + +The contiguous bit is used to increase the mapping size at the pmd and pte +(last) level. So this type of HugeTLB page can be optimized only when its +size of the struct page structs is greater than 1 page. + +Notice: The head vmemmap page is not freed to the buddy allocator and all +tail vmemmap pages are mapped to the head vmemmap page frame. So we can see +more than one struct page struct with PG_head (e.g. 8 per 2 MB HugeTLB page) +associated with each HugeTLB page. The compound_head() can handle this +correctly (more details refer to the comment above compound_head()). + +Device DAX +========== + +The device-dax interface uses the same tail deduplication technique explained +in the previous chapter, except when used with the vmemmap in +the device (altmap). + +The following page sizes are supported in DAX: PAGE_SIZE (4K on x86_64), +PMD_SIZE (2M on x86_64) and PUD_SIZE (1G on x86_64). + +The differences with HugeTLB are relatively minor. + +It only use 3 page structs for storing all information as opposed +to 4 on HugeTLB pages. + +There's no remapping of vmemmap given that device-dax memory is not part of +System RAM ranges initialized at boot. Thus the tail page deduplication +happens at a later stage when we populate the sections. HugeTLB reuses the +the head vmemmap page representing, whereas device-dax reuses the tail +vmemmap page. This results in only half of the savings compared to HugeTLB. + +Deduplicated tail pages are not mapped read-only. + +Here's how things look like on device-dax after the sections are populated:: + + +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+ + | | | 0 | -------------> | 0 | + | | +-----------+ +-----------+ + | | | 1 | -------------> | 1 | + | | +-----------+ +-----------+ + | | | 2 | ----------------^ ^ ^ ^ ^ ^ + | | +-----------+ | | | | | + | | | 3 | ------------------+ | | | | + | | +-----------+ | | | | + | | | 4 | --------------------+ | | | + | PMD | +-----------+ | | | + | level | | 5 | ----------------------+ | | + | mapping | +-----------+ | | + | | | 6 | ------------------------+ | + | | +-----------+ | + | | | 7 | --------------------------+ + | | +-----------+ + | | + | | + | | + +-----------+ diff --git a/Documentation/mm/z3fold.rst b/Documentation/mm/z3fold.rst new file mode 100644 index 000000000000..224e3c61d686 --- /dev/null +++ b/Documentation/mm/z3fold.rst @@ -0,0 +1,30 @@ +.. _z3fold: + +====== +z3fold +====== + +z3fold is a special purpose allocator for storing compressed pages. +It is designed to store up to three compressed pages per physical page. +It is a zbud derivative which allows for higher compression +ratio keeping the simplicity and determinism of its predecessor. + +The main differences between z3fold and zbud are: + +* unlike zbud, z3fold allows for up to PAGE_SIZE allocations +* z3fold can hold up to 3 compressed pages in its page +* z3fold doesn't export any API itself and is thus intended to be used + via the zpool API. + +To keep the determinism and simplicity, z3fold, just like zbud, always +stores an integral number of compressed pages per page, but it can store +up to 3 pages unlike zbud which can store at most 2. Therefore the +compression ratio goes to around 2.7x while zbud's one is around 1.7x. + +Unlike zbud (but like zsmalloc for that matter) z3fold_alloc() does not +return a dereferenceable pointer. Instead, it returns an unsigned long +handle which encodes actual location of the allocated object. + +Keeping effective compression ratio close to zsmalloc's, z3fold doesn't +depend on MMU enabled and provides more predictable reclaim behavior +which makes it a better fit for small and response-critical systems. diff --git a/Documentation/mm/zsmalloc.rst b/Documentation/mm/zsmalloc.rst new file mode 100644 index 000000000000..6e79893d6132 --- /dev/null +++ b/Documentation/mm/zsmalloc.rst @@ -0,0 +1,82 @@ +.. _zsmalloc: + +======== +zsmalloc +======== + +This allocator is designed for use with zram. Thus, the allocator is +supposed to work well under low memory conditions. In particular, it +never attempts higher order page allocation which is very likely to +fail under memory pressure. On the other hand, if we just use single +(0-order) pages, it would suffer from very high fragmentation -- +any object of size PAGE_SIZE/2 or larger would occupy an entire page. +This was one of the major issues with its predecessor (xvmalloc). + +To overcome these issues, zsmalloc allocates a bunch of 0-order pages +and links them together using various 'struct page' fields. These linked +pages act as a single higher-order page i.e. an object can span 0-order +page boundaries. The code refers to these linked pages as a single entity +called zspage. + +For simplicity, zsmalloc can only allocate objects of size up to PAGE_SIZE +since this satisfies the requirements of all its current users (in the +worst case, page is incompressible and is thus stored "as-is" i.e. in +uncompressed form). For allocation requests larger than this size, failure +is returned (see zs_malloc). + +Additionally, zs_malloc() does not return a dereferenceable pointer. +Instead, it returns an opaque handle (unsigned long) which encodes actual +location of the allocated object. The reason for this indirection is that +zsmalloc does not keep zspages permanently mapped since that would cause +issues on 32-bit systems where the VA region for kernel space mappings +is very small. So, before using the allocating memory, the object has to +be mapped using zs_map_object() to get a usable pointer and subsequently +unmapped using zs_unmap_object(). + +stat +==== + +With CONFIG_ZSMALLOC_STAT, we could see zsmalloc internal information via +``/sys/kernel/debug/zsmalloc/<user name>``. Here is a sample of stat output:: + + # cat /sys/kernel/debug/zsmalloc/zram0/classes + + class size almost_full almost_empty obj_allocated obj_used pages_used pages_per_zspage + ... + ... + 9 176 0 1 186 129 8 4 + 10 192 1 0 2880 2872 135 3 + 11 208 0 1 819 795 42 2 + 12 224 0 1 219 159 12 4 + ... + ... + + +class + index +size + object size zspage stores +almost_empty + the number of ZS_ALMOST_EMPTY zspages(see below) +almost_full + the number of ZS_ALMOST_FULL zspages(see below) +obj_allocated + the number of objects allocated +obj_used + the number of objects allocated to the user +pages_used + the number of pages allocated for the class +pages_per_zspage + the number of 0-order pages to make a zspage + +We assign a zspage to ZS_ALMOST_EMPTY fullness group when n <= N / f, where + +* n = number of allocated objects +* N = total number of objects zspage can store +* f = fullness_threshold_frac(ie, 4 at the moment) + +Similarly, we assign zspage to: + +* ZS_ALMOST_FULL when n > N / f +* ZS_EMPTY when n == 0 +* ZS_FULL when n == N |