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diff --git a/Documentation/admin-guide/mm/memory-hotplug.rst b/Documentation/admin-guide/mm/memory-hotplug.rst new file mode 100644 index 000000000000..5c4432c96c4b --- /dev/null +++ b/Documentation/admin-guide/mm/memory-hotplug.rst @@ -0,0 +1,444 @@ +.. _admin_guide_memory_hotplug: + +============== +Memory Hotplug +============== + +:Created: Jul 28 2007 +:Updated: Add some details about locking internals: Aug 20 2018 + +This document is about memory hotplug including how-to-use and current status. +Because Memory Hotplug is still under development, contents of this text will +be changed often. + +.. contents:: :local: + +.. note:: + + (1) x86_64's has special implementation for memory hotplug. + This text does not describe it. + (2) This text assumes that sysfs is mounted at ``/sys``. + + +Introduction +============ + +Purpose of memory hotplug +------------------------- + +Memory Hotplug allows users to increase/decrease the amount of memory. +Generally, there are two purposes. + +(A) For changing the amount of memory. + This is to allow a feature like capacity on demand. +(B) For installing/removing DIMMs or NUMA-nodes physically. + This is to exchange DIMMs/NUMA-nodes, reduce power consumption, etc. + +(A) is required by highly virtualized environments and (B) is required by +hardware which supports memory power management. + +Linux memory hotplug is designed for both purpose. + +Phases of memory hotplug +------------------------ + +There are 2 phases in Memory Hotplug: + + 1) Physical Memory Hotplug phase + 2) Logical Memory Hotplug phase. + +The First phase is to communicate hardware/firmware and make/erase +environment for hotplugged memory. Basically, this phase is necessary +for the purpose (B), but this is good phase for communication between +highly virtualized environments too. + +When memory is hotplugged, the kernel recognizes new memory, makes new memory +management tables, and makes sysfs files for new memory's operation. + +If firmware supports notification of connection of new memory to OS, +this phase is triggered automatically. ACPI can notify this event. If not, +"probe" operation by system administration is used instead. +(see :ref:`memory_hotplug_physical_mem`). + +Logical Memory Hotplug phase is to change memory state into +available/unavailable for users. Amount of memory from user's view is +changed by this phase. The kernel makes all memory in it as free pages +when a memory range is available. + +In this document, this phase is described as online/offline. + +Logical Memory Hotplug phase is triggered by write of sysfs file by system +administrator. For the hot-add case, it must be executed after Physical Hotplug +phase by hand. +(However, if you writes udev's hotplug scripts for memory hotplug, these +phases can be execute in seamless way.) + +Unit of Memory online/offline operation +--------------------------------------- + +Memory hotplug uses SPARSEMEM memory model which allows memory to be divided +into chunks of the same size. These chunks are called "sections". The size of +a memory section is architecture dependent. For example, power uses 16MiB, ia64 +uses 1GiB. + +Memory sections are combined into chunks referred to as "memory blocks". The +size of a memory block is architecture dependent and represents the logical +unit upon which memory online/offline operations are to be performed. The +default size of a memory block is the same as memory section size unless an +architecture specifies otherwise. (see :ref:`memory_hotplug_sysfs_files`.) + +To determine the size (in bytes) of a memory block please read this file:: + + /sys/devices/system/memory/block_size_bytes + +Kernel Configuration +==================== + +To use memory hotplug feature, kernel must be compiled with following +config options. + +- For all memory hotplug: + - Memory model -> Sparse Memory (``CONFIG_SPARSEMEM``) + - Allow for memory hot-add (``CONFIG_MEMORY_HOTPLUG``) + +- To enable memory removal, the following are also necessary: + - Allow for memory hot remove (``CONFIG_MEMORY_HOTREMOVE``) + - Page Migration (``CONFIG_MIGRATION``) + +- For ACPI memory hotplug, the following are also necessary: + - Memory hotplug (under ACPI Support menu) (``CONFIG_ACPI_HOTPLUG_MEMORY``) + - This option can be kernel module. + +- As a related configuration, if your box has a feature of NUMA-node hotplug + via ACPI, then this option is necessary too. + + - ACPI0004,PNP0A05 and PNP0A06 Container Driver (under ACPI Support menu) + (``CONFIG_ACPI_CONTAINER``). + + This option can be kernel module too. + + +.. _memory_hotplug_sysfs_files: + +sysfs files for memory hotplug +============================== + +All memory blocks have their device information in sysfs. Each memory block +is described under ``/sys/devices/system/memory`` as:: + + /sys/devices/system/memory/memoryXXX + +where XXX is the memory block id. + +For the memory block covered by the sysfs directory. It is expected that all +memory sections in this range are present and no memory holes exist in the +range. Currently there is no way to determine if there is a memory hole, but +the existence of one should not affect the hotplug capabilities of the memory +block. + +For example, assume 1GiB memory block size. A device for a memory starting at +0x100000000 is ``/sys/device/system/memory/memory4``:: + + (0x100000000 / 1Gib = 4) + +This device covers address range [0x100000000 ... 0x140000000) + +Under each memory block, you can see 5 files: + +- ``/sys/devices/system/memory/memoryXXX/phys_index`` +- ``/sys/devices/system/memory/memoryXXX/phys_device`` +- ``/sys/devices/system/memory/memoryXXX/state`` +- ``/sys/devices/system/memory/memoryXXX/removable`` +- ``/sys/devices/system/memory/memoryXXX/valid_zones`` + +=================== ============================================================ +``phys_index`` read-only and contains memory block id, same as XXX. +``state`` read-write + + - at read: contains online/offline state of memory. + - at write: user can specify "online_kernel", + + "online_movable", "online", "offline" command + which will be performed on all sections in the block. +``phys_device`` read-only: designed to show the name of physical memory + device. This is not well implemented now. +``removable`` read-only: contains an integer value indicating + whether the memory block is removable or not + removable. A value of 1 indicates that the memory + block is removable and a value of 0 indicates that + it is not removable. A memory block is removable only if + every section in the block is removable. +``valid_zones`` read-only: designed to show which zones this memory block + can be onlined to. + + The first column shows it`s default zone. + + "memory6/valid_zones: Normal Movable" shows this memoryblock + can be onlined to ZONE_NORMAL by default and to ZONE_MOVABLE + by online_movable. + + "memory7/valid_zones: Movable Normal" shows this memoryblock + can be onlined to ZONE_MOVABLE by default and to ZONE_NORMAL + by online_kernel. +=================== ============================================================ + +.. note:: + + These directories/files appear after physical memory hotplug phase. + +If CONFIG_NUMA is enabled the memoryXXX/ directories can also be accessed +via symbolic links located in the ``/sys/devices/system/node/node*`` directories. + +For example:: + + /sys/devices/system/node/node0/memory9 -> ../../memory/memory9 + +A backlink will also be created:: + + /sys/devices/system/memory/memory9/node0 -> ../../node/node0 + +.. _memory_hotplug_physical_mem: + +Physical memory hot-add phase +============================= + +Hardware(Firmware) Support +-------------------------- + +On x86_64/ia64 platform, memory hotplug by ACPI is supported. + +In general, the firmware (ACPI) which supports memory hotplug defines +memory class object of _HID "PNP0C80". When a notify is asserted to PNP0C80, +Linux's ACPI handler does hot-add memory to the system and calls a hotplug udev +script. This will be done automatically. + +But scripts for memory hotplug are not contained in generic udev package(now). +You may have to write it by yourself or online/offline memory by hand. +Please see :ref:`memory_hotplug_how_to_online_memory` and +:ref:`memory_hotplug_how_to_offline_memory`. + +If firmware supports NUMA-node hotplug, and defines an object _HID "ACPI0004", +"PNP0A05", or "PNP0A06", notification is asserted to it, and ACPI handler +calls hotplug code for all of objects which are defined in it. +If memory device is found, memory hotplug code will be called. + +Notify memory hot-add event by hand +----------------------------------- + +On some architectures, the firmware may not notify the kernel of a memory +hotplug event. Therefore, the memory "probe" interface is supported to +explicitly notify the kernel. This interface depends on +CONFIG_ARCH_MEMORY_PROBE and can be configured on powerpc, sh, and x86 +if hotplug is supported, although for x86 this should be handled by ACPI +notification. + +Probe interface is located at:: + + /sys/devices/system/memory/probe + +You can tell the physical address of new memory to the kernel by:: + + % echo start_address_of_new_memory > /sys/devices/system/memory/probe + +Then, [start_address_of_new_memory, start_address_of_new_memory + +memory_block_size] memory range is hot-added. In this case, hotplug script is +not called (in current implementation). You'll have to online memory by +yourself. Please see :ref:`memory_hotplug_how_to_online_memory`. + +Logical Memory hot-add phase +============================ + +State of memory +--------------- + +To see (online/offline) state of a memory block, read 'state' file:: + + % cat /sys/device/system/memory/memoryXXX/state + + +- If the memory block is online, you'll read "online". +- If the memory block is offline, you'll read "offline". + + +.. _memory_hotplug_how_to_online_memory: + +How to online memory +-------------------- + +When the memory is hot-added, the kernel decides whether or not to "online" +it according to the policy which can be read from "auto_online_blocks" file:: + + % cat /sys/devices/system/memory/auto_online_blocks + +The default depends on the CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE kernel config +option. If it is disabled the default is "offline" which means the newly added +memory is not in a ready-to-use state and you have to "online" the newly added +memory blocks manually. Automatic onlining can be requested by writing "online" +to "auto_online_blocks" file:: + + % echo online > /sys/devices/system/memory/auto_online_blocks + +This sets a global policy and impacts all memory blocks that will subsequently +be hotplugged. Currently offline blocks keep their state. It is possible, under +certain circumstances, that some memory blocks will be added but will fail to +online. User space tools can check their "state" files +(``/sys/devices/system/memory/memoryXXX/state``) and try to online them manually. + +If the automatic onlining wasn't requested, failed, or some memory block was +offlined it is possible to change the individual block's state by writing to the +"state" file:: + + % echo online > /sys/devices/system/memory/memoryXXX/state + +This onlining will not change the ZONE type of the target memory block, +If the memory block doesn't belong to any zone an appropriate kernel zone +(usually ZONE_NORMAL) will be used unless movable_node kernel command line +option is specified when ZONE_MOVABLE will be used. + +You can explicitly request to associate it with ZONE_MOVABLE by:: + + % echo online_movable > /sys/devices/system/memory/memoryXXX/state + +.. note:: current limit: this memory block must be adjacent to ZONE_MOVABLE + +Or you can explicitly request a kernel zone (usually ZONE_NORMAL) by:: + + % echo online_kernel > /sys/devices/system/memory/memoryXXX/state + +.. note:: current limit: this memory block must be adjacent to ZONE_NORMAL + +An explicit zone onlining can fail (e.g. when the range is already within +and existing and incompatible zone already). + +After this, memory block XXX's state will be 'online' and the amount of +available memory will be increased. + +This may be changed in future. + +Logical memory remove +===================== + +Memory offline and ZONE_MOVABLE +------------------------------- + +Memory offlining is more complicated than memory online. Because memory offline +has to make the whole memory block be unused, memory offline can fail if +the memory block includes memory which cannot be freed. + +In general, memory offline can use 2 techniques. + +(1) reclaim and free all memory in the memory block. +(2) migrate all pages in the memory block. + +In the current implementation, Linux's memory offline uses method (2), freeing +all pages in the memory block by page migration. But not all pages are +migratable. Under current Linux, migratable pages are anonymous pages and +page caches. For offlining a memory block by migration, the kernel has to +guarantee that the memory block contains only migratable pages. + +Now, a boot option for making a memory block which consists of migratable pages +is supported. By specifying "kernelcore=" or "movablecore=" boot option, you can +create ZONE_MOVABLE...a zone which is just used for movable pages. +(See also Documentation/admin-guide/kernel-parameters.rst) + +Assume the system has "TOTAL" amount of memory at boot time, this boot option +creates ZONE_MOVABLE as following. + +1) When kernelcore=YYYY boot option is used, + Size of memory not for movable pages (not for offline) is YYYY. + Size of memory for movable pages (for offline) is TOTAL-YYYY. + +2) When movablecore=ZZZZ boot option is used, + Size of memory not for movable pages (not for offline) is TOTAL - ZZZZ. + Size of memory for movable pages (for offline) is ZZZZ. + +.. note:: + + Unfortunately, there is no information to show which memory block belongs + to ZONE_MOVABLE. This is TBD. + +.. _memory_hotplug_how_to_offline_memory: + +How to offline memory +--------------------- + +You can offline a memory block by using the same sysfs interface that was used +in memory onlining:: + + % echo offline > /sys/devices/system/memory/memoryXXX/state + +If offline succeeds, the state of the memory block is changed to be "offline". +If it fails, some error core (like -EBUSY) will be returned by the kernel. +Even if a memory block does not belong to ZONE_MOVABLE, you can try to offline +it. If it doesn't contain 'unmovable' memory, you'll get success. + +A memory block under ZONE_MOVABLE is considered to be able to be offlined +easily. But under some busy state, it may return -EBUSY. Even if a memory +block cannot be offlined due to -EBUSY, you can retry offlining it and may be +able to offline it (or not). (For example, a page is referred to by some kernel +internal call and released soon.) + +Consideration: + Memory hotplug's design direction is to make the possibility of memory + offlining higher and to guarantee unplugging memory under any situation. But + it needs more work. Returning -EBUSY under some situation may be good because + the user can decide to retry more or not by himself. Currently, memory + offlining code does some amount of retry with 120 seconds timeout. + +Physical memory remove +====================== + +Need more implementation yet.... + - Notification completion of remove works by OS to firmware. + - Guard from remove if not yet. + + +Locking Internals +================= + +When adding/removing memory that uses memory block devices (i.e. ordinary RAM), +the device_hotplug_lock should be held to: + +- synchronize against online/offline requests (e.g. via sysfs). This way, memory + block devices can only be accessed (.online/.state attributes) by user + space once memory has been fully added. And when removing memory, we + know nobody is in critical sections. +- synchronize against CPU hotplug and similar (e.g. relevant for ACPI and PPC) + +Especially, there is a possible lock inversion that is avoided using +device_hotplug_lock when adding memory and user space tries to online that +memory faster than expected: + +- device_online() will first take the device_lock(), followed by + mem_hotplug_lock +- add_memory_resource() will first take the mem_hotplug_lock, followed by + the device_lock() (while creating the devices, during bus_add_device()). + +As the device is visible to user space before taking the device_lock(), this +can result in a lock inversion. + +onlining/offlining of memory should be done via device_online()/ +device_offline() - to make sure it is properly synchronized to actions +via sysfs. Holding device_hotplug_lock is advised (to e.g. protect online_type) + +When adding/removing/onlining/offlining memory or adding/removing +heterogeneous/device memory, we should always hold the mem_hotplug_lock in +write mode to serialise memory hotplug (e.g. access to global/zone +variables). + +In addition, mem_hotplug_lock (in contrast to device_hotplug_lock) in read +mode allows for a quite efficient get_online_mems/put_online_mems +implementation, so code accessing memory can protect from that memory +vanishing. + + +Future Work +=========== + + - allowing memory hot-add to ZONE_MOVABLE. maybe we need some switch like + sysctl or new control file. + - showing memory block and physical device relationship. + - test and make it better memory offlining. + - support HugeTLB page migration and offlining. + - memmap removing at memory offline. + - physical remove memory. |