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diff --git a/Documentation/vm/pagemap.rst b/Documentation/vm/pagemap.rst deleted file mode 100644 index 7ba8cbd57ad3..000000000000 --- a/Documentation/vm/pagemap.rst +++ /dev/null @@ -1,197 +0,0 @@ -.. _pagemap: - -============================= -Examining Process Page Tables -============================= - -pagemap is a new (as of 2.6.25) set of interfaces in the kernel that allow -userspace programs to examine the page tables and related information by -reading files in ``/proc``. - -There are four components to pagemap: - - * ``/proc/pid/pagemap``. This file lets a userspace process find out which - physical frame each virtual page is mapped to. It contains one 64-bit - value for each virtual page, containing the following data (from - ``fs/proc/task_mmu.c``, above pagemap_read): - - * Bits 0-54 page frame number (PFN) if present - * Bits 0-4 swap type if swapped - * Bits 5-54 swap offset if swapped - * Bit 55 pte is soft-dirty (see Documentation/vm/soft-dirty.rst) - * Bit 56 page exclusively mapped (since 4.2) - * Bits 57-60 zero - * Bit 61 page is file-page or shared-anon (since 3.5) - * Bit 62 page swapped - * Bit 63 page present - - Since Linux 4.0 only users with the CAP_SYS_ADMIN capability can get PFNs. - In 4.0 and 4.1 opens by unprivileged fail with -EPERM. Starting from - 4.2 the PFN field is zeroed if the user does not have CAP_SYS_ADMIN. - Reason: information about PFNs helps in exploiting Rowhammer vulnerability. - - If the page is not present but in swap, then the PFN contains an - encoding of the swap file number and the page's offset into the - swap. Unmapped pages return a null PFN. This allows determining - precisely which pages are mapped (or in swap) and comparing mapped - pages between processes. - - Efficient users of this interface will use ``/proc/pid/maps`` to - determine which areas of memory are actually mapped and llseek to - skip over unmapped regions. - - * ``/proc/kpagecount``. This file contains a 64-bit count of the number of - times each page is mapped, indexed by PFN. - - * ``/proc/kpageflags``. This file contains a 64-bit set of flags for each - page, indexed by PFN. - - The flags are (from ``fs/proc/page.c``, above kpageflags_read): - - 0. LOCKED - 1. ERROR - 2. REFERENCED - 3. UPTODATE - 4. DIRTY - 5. LRU - 6. ACTIVE - 7. SLAB - 8. WRITEBACK - 9. RECLAIM - 10. BUDDY - 11. MMAP - 12. ANON - 13. SWAPCACHE - 14. SWAPBACKED - 15. COMPOUND_HEAD - 16. COMPOUND_TAIL - 17. HUGE - 18. UNEVICTABLE - 19. HWPOISON - 20. NOPAGE - 21. KSM - 22. THP - 23. BALLOON - 24. ZERO_PAGE - 25. IDLE - - * ``/proc/kpagecgroup``. This file contains a 64-bit inode number of the - memory cgroup each page is charged to, indexed by PFN. Only available when - CONFIG_MEMCG is set. - -Short descriptions to the page flags -==================================== - -0 - LOCKED - page is being locked for exclusive access, e.g. by undergoing read/write IO -7 - SLAB - page is managed by the SLAB/SLOB/SLUB/SLQB kernel memory allocator - When compound page is used, SLUB/SLQB will only set this flag on the head - page; SLOB will not flag it at all. -10 - BUDDY - a free memory block managed by the buddy system allocator - The buddy system organizes free memory in blocks of various orders. - An order N block has 2^N physically contiguous pages, with the BUDDY flag - set for and _only_ for the first page. -15 - COMPOUND_HEAD - A compound page with order N consists of 2^N physically contiguous pages. - A compound page with order 2 takes the form of "HTTT", where H donates its - head page and T donates its tail page(s). The major consumers of compound - pages are hugeTLB pages (Documentation/vm/hugetlbpage.rst), the SLUB etc. - memory allocators and various device drivers. However in this interface, - only huge/giga pages are made visible to end users. -16 - COMPOUND_TAIL - A compound page tail (see description above). -17 - HUGE - this is an integral part of a HugeTLB page -19 - HWPOISON - hardware detected memory corruption on this page: don't touch the data! -20 - NOPAGE - no page frame exists at the requested address -21 - KSM - identical memory pages dynamically shared between one or more processes -22 - THP - contiguous pages which construct transparent hugepages -23 - BALLOON - balloon compaction page -24 - ZERO_PAGE - zero page for pfn_zero or huge_zero page -25 - IDLE - page has not been accessed since it was marked idle (see - Documentation/vm/idle_page_tracking.rst). Note that this flag may be - stale in case the page was accessed via a PTE. To make sure the flag - is up-to-date one has to read ``/sys/kernel/mm/page_idle/bitmap`` first. - -IO related page flags ---------------------- - -1 - ERROR - IO error occurred -3 - UPTODATE - page has up-to-date data - ie. for file backed page: (in-memory data revision >= on-disk one) -4 - DIRTY - page has been written to, hence contains new data - i.e. for file backed page: (in-memory data revision > on-disk one) -8 - WRITEBACK - page is being synced to disk - -LRU related page flags ----------------------- - -5 - LRU - page is in one of the LRU lists -6 - ACTIVE - page is in the active LRU list -18 - UNEVICTABLE - page is in the unevictable (non-)LRU list It is somehow pinned and - not a candidate for LRU page reclaims, e.g. ramfs pages, - shmctl(SHM_LOCK) and mlock() memory segments -2 - REFERENCED - page has been referenced since last LRU list enqueue/requeue -9 - RECLAIM - page will be reclaimed soon after its pageout IO completed -11 - MMAP - a memory mapped page -12 - ANON - a memory mapped page that is not part of a file -13 - SWAPCACHE - page is mapped to swap space, i.e. has an associated swap entry -14 - SWAPBACKED - page is backed by swap/RAM - -The page-types tool in the tools/vm directory can be used to query the -above flags. - -Using pagemap to do something useful -==================================== - -The general procedure for using pagemap to find out about a process' memory -usage goes like this: - - 1. Read ``/proc/pid/maps`` to determine which parts of the memory space are - mapped to what. - 2. Select the maps you are interested in -- all of them, or a particular - library, or the stack or the heap, etc. - 3. Open ``/proc/pid/pagemap`` and seek to the pages you would like to examine. - 4. Read a u64 for each page from pagemap. - 5. Open ``/proc/kpagecount`` and/or ``/proc/kpageflags``. For each PFN you - just read, seek to that entry in the file, and read the data you want. - -For example, to find the "unique set size" (USS), which is the amount of -memory that a process is using that is not shared with any other process, -you can go through every map in the process, find the PFNs, look those up -in kpagecount, and tally up the number of pages that are only referenced -once. - -Other notes -=========== - -Reading from any of the files will return -EINVAL if you are not starting -the read on an 8-byte boundary (e.g., if you sought an odd number of bytes -into the file), or if the size of the read is not a multiple of 8 bytes. - -Before Linux 3.11 pagemap bits 55-60 were used for "page-shift" (which is -always 12 at most architectures). Since Linux 3.11 their meaning changes -after first clear of soft-dirty bits. Since Linux 4.2 they are used for -flags unconditionally. |