.. _admin_guide_memory_hotplug:
-==============
-Memory Hotplug
-==============
+==================
+Memory Hot(Un)Plug
+==================
-: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.
+This document describes generic Linux support for memory hot(un)plug with
+a focus on System RAM, including ZONE_MOVABLE support.
.. contents:: :local:
-.. note::
+Introduction
+============
- (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``.
+Memory hot(un)plug allows for increasing and decreasing the size of physical
+memory available to a machine at runtime. In the simplest case, it consists of
+physically plugging or unplugging a DIMM at runtime, coordinated with the
+operating system.
+Memory hot(un)plug is used for various purposes:
-Introduction
-============
+- The physical memory available to a machine can be adjusted at runtime, up- or
+ downgrading the memory capacity. This dynamic memory resizing, sometimes
+ referred to as "capacity on demand", is frequently used with virtual machines
+ and logical partitions.
+
+- Replacing hardware, such as DIMMs or whole NUMA nodes, without downtime. One
+ example is replacing failing memory modules.
-Purpose of memory hotplug
--------------------------
+- Reducing energy consumption either by physically unplugging memory modules or
+ by logically unplugging (parts of) memory modules from Linux.
-Memory Hotplug allows users to increase/decrease the amount of memory.
-Generally, there are two purposes.
+Further, the basic memory hot(un)plug infrastructure in Linux is nowadays also
+used to expose persistent memory, other performance-differentiated memory and
+reserved memory regions as ordinary system RAM to Linux.
-(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.
+Linux only supports memory hot(un)plug on selected 64 bit architectures, such as
+x86_64, arm64, ppc64, s390x and ia64.
-(A) is required by highly virtualized environments and (B) is required by
-hardware which supports memory power management.
+Memory Hot(Un)Plug Granularity
+------------------------------
-Linux memory hotplug is designed for both purpose.
+Memory hot(un)plug in Linux uses the SPARSEMEM memory model, which divides the
+physical memory address space into chunks of the same size: memory sections. The
+size of a memory section is architecture dependent. For example, x86_64 uses
+128 MiB and ppc64 uses 16 MiB.
-Phases of memory hotplug
+Memory sections are combined into chunks referred to as "memory blocks". The
+size of a memory block is architecture dependent and corresponds to the smallest
+granularity that can be hot(un)plugged. The default size of a memory block is
+the same as memory section size, unless an architecture specifies otherwise.
+
+All memory blocks have the same size.
+
+Phases of Memory Hotplug
------------------------
-There are 2 phases in Memory Hotplug:
+Memory hotplug consists of two phases:
- 1) Physical Memory Hotplug phase
- 2) Logical Memory Hotplug phase.
+(1) Adding the memory to Linux
+(2) Onlining memory blocks
-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.
+In the first phase, metadata, such as the memory map ("memmap") and page tables
+for the direct mapping, is allocated and initialized, and memory blocks are
+created; the latter also creates sysfs files for managing newly created memory
+blocks.
-When memory is hotplugged, the kernel recognizes new memory, makes new memory
-management tables, and makes sysfs files for new memory's operation.
+In the second phase, added memory is exposed to the page allocator. After this
+phase, the memory is visible in memory statistics, such as free and total
+memory, of the system.
-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`).
+Phases of Memory Hotunplug
+--------------------------
-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.
+Memory hotunplug consists of two phases:
-In this document, this phase is described as online/offline.
+(1) Offlining memory blocks
+(2) Removing the memory from Linux
-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.)
+In the fist phase, memory is "hidden" from the page allocator again, for
+example, by migrating busy memory to other memory locations and removing all
+relevant free pages from the page allocator After this phase, the memory is no
+longer visible in memory statistics of the system.
-Unit of Memory online/offline operation
----------------------------------------
+In the second phase, the memory blocks are removed and metadata is freed.
-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 Hotplug Notifications
+============================
-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`.)
+There are various ways how Linux is notified about memory hotplug events such
+that it can start adding hotplugged memory. This description is limited to
+systems that support ACPI; mechanisms specific to other firmware interfaces or
+virtual machines are not described.
-To determine the size (in bytes) of a memory block please read this file::
+ACPI Notifications
+------------------
- /sys/devices/system/memory/block_size_bytes
+Platforms that support ACPI, such as x86_64, can support memory hotplug
+notifications via ACPI.
-Kernel Configuration
-====================
+In general, a firmware supporting memory hotplug defines a memory class object
+HID "PNP0C80". When notified about hotplug of a new memory device, the ACPI
+driver will hotplug the memory to Linux.
-To use memory hotplug feature, kernel must be compiled with following
-config options.
+If the firmware supports hotplug of NUMA nodes, it defines an object _HID
+"ACPI0004", "PNP0A05", or "PNP0A06". When notified about an hotplug event, all
+assigned memory devices are added to Linux by the ACPI driver.
-- For all memory hotplug:
- - Memory model -> Sparse Memory (``CONFIG_SPARSEMEM``)
- - Allow for memory hot-add (``CONFIG_MEMORY_HOTPLUG``)
+Similarly, Linux can be notified about requests to hotunplug a memory device or
+a NUMA node via ACPI. The ACPI driver will try offlining all relevant memory
+blocks, and, if successful, hotunplug the memory from Linux.
-- To enable memory removal, the following are also necessary:
- - Allow for memory hot remove (``CONFIG_MEMORY_HOTREMOVE``)
- - Page Migration (``CONFIG_MIGRATION``)
+Manual Probing
+--------------
-- 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.
+On some architectures, the firmware may not be able to notify the operating
+system about a memory hotplug event. Instead, the memory has to be manually
+probed from user space.
-- As a related configuration, if your box has a feature of NUMA-node hotplug
- via ACPI, then this option is necessary too.
+The probe interface is located at::
- - ACPI0004,PNP0A05 and PNP0A06 Container Driver (under ACPI Support menu)
- (``CONFIG_ACPI_CONTAINER``).
+ /sys/devices/system/memory/probe
- This option can be kernel module too.
+Only complete memory blocks can be probed. Individual memory blocks are probed
+by providing the physical start address of the memory block::
+ % echo addr > /sys/devices/system/memory/probe
-.. _memory_hotplug_sysfs_files:
+Which results in a memory block for the range [addr, addr + memory_block_size)
+being created.
-sysfs files for memory hotplug
-==============================
+.. note::
-All memory blocks have their device information in sysfs. Each memory block
-is described under ``/sys/devices/system/memory`` as::
+ Using the probe interface is discouraged as it is easy to crash the kernel,
+ because Linux cannot validate user input; this interface might be removed in
+ the future.
- /sys/devices/system/memory/memoryXXX
+Onlining and Offlining Memory Blocks
+====================================
-where XXX is the memory block id.
+After a memory block has been created, Linux has to be instructed to actually
+make use of that memory: the memory block has to be "online".
-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.
+Before a memory block can be removed, Linux has to stop using any memory part of
+the memory block: the memory block has to be "offlined".
-For example, assume 1GiB memory block size. A device for a memory starting at
-0x100000000 is ``/sys/device/system/memory/memory4``::
+The Linux kernel can be configured to automatically online added memory blocks
+and drivers automatically trigger offlining of memory blocks when trying
+hotunplug of memory. Memory blocks can only be removed once offlining succeeded
+and drivers may trigger offlining of memory blocks when attempting hotunplug of
+memory.
- (0x100000000 / 1Gib = 4)
+Onlining Memory Blocks Manually
+-------------------------------
-This device covers address range [0x100000000 ... 0x140000000)
+If auto-onlining of memory blocks isn't enabled, user-space has to manually
+trigger onlining of memory blocks. Often, udev rules are used to automate this
+task in user space.
-Under each memory block, you can see 5 files:
+Onlining of a memory block can be triggered via::
-- ``/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``
+ % echo online > /sys/devices/system/memory/memoryXXX/state
-=================== ============================================================
-``phys_index`` read-only and contains memory block id, same as XXX.
-``state`` read-write
+Or alternatively::
- - at read: contains online/offline state of memory.
- - at write: user can specify "online_kernel",
+ % echo 1 > /sys/devices/system/memory/memoryXXX/online
- "online_movable", "online", "offline" command
- which will be performed on all sections in the block.
-``phys_device`` read-only: legacy interface only ever used on s390x to
- expose the covered storage increment.
-``removable`` read-only: legacy interface that indicated whether a memory
- block was likely to be offlineable or not. Newer kernel
- versions return "1" if and only if the kernel supports
- memory offlining.
-``valid_zones`` read-only: designed to show by which zone memory provided by
- a memory block is managed, and to show by which zone memory
- provided by an offline memory block could be managed when
- onlining.
-
- 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.
-=================== ============================================================
+The kernel will select the target zone automatically, usually defaulting to
+``ZONE_NORMAL`` unless ``movablecore=1`` has been specified on the kernel
+command line or if the memory block would intersect the ZONE_MOVABLE already.
-.. note::
+One can explicitly request to associate an offline memory block with
+ZONE_MOVABLE by::
- These directories/files appear after physical memory hotplug phase.
+ % echo online_movable > /sys/devices/system/memory/memoryXXX/state
-If CONFIG_NUMA is enabled the memoryXXX/ directories can also be accessed
-via symbolic links located in the ``/sys/devices/system/node/node*`` directories.
+Or one can explicitly request a kernel zone (usually ZONE_NORMAL) by::
-For example::
+ % echo online_kernel > /sys/devices/system/memory/memoryXXX/state
- /sys/devices/system/node/node0/memory9 -> ../../memory/memory9
+In any case, if onlining succeeds, the state of the memory block is changed to
+be "online". If it fails, the state of the memory block will remain unchanged
+and the above commands will fail.
-A backlink will also be created::
+Onlining Memory Blocks Automatically
+------------------------------------
- /sys/devices/system/memory/memory9/node0 -> ../../node/node0
+The kernel can be configured to try auto-onlining of newly added memory blocks.
+If this feature is disabled, the memory blocks will stay offline until
+explicitly onlined from user space.
-.. _memory_hotplug_physical_mem:
+The configured auto-online behavior can be observed via::
-Physical memory hot-add phase
-=============================
+ % cat /sys/devices/system/memory/auto_online_blocks
-Hardware(Firmware) Support
---------------------------
+Auto-onlining can be enabled by writing ``online``, ``online_kernel`` or
+``online_movable`` to that file, like::
-On x86_64/ia64 platform, memory hotplug by ACPI is supported.
+ % echo online > /sys/devices/system/memory/auto_online_blocks
-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.
+Modifying the auto-online behavior will only affect all subsequently added
+memory blocks only.
-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`.
+.. note::
-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.
+ In corner cases, auto-onlining can fail. The kernel won't retry. Note that
+ auto-onlining is not expected to fail in default configurations.
-Notify memory hot-add event by hand
------------------------------------
+.. note::
-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.
+ DLPAR on ppc64 ignores the ``offline`` setting and will still online added
+ memory blocks; if onlining fails, memory blocks are removed again.
-Probe interface is located at::
+Offlining Memory Blocks
+-----------------------
- /sys/devices/system/memory/probe
+In the current implementation, Linux's memory offlining will try migrating all
+movable pages off the affected memory block. As most kernel allocations, such as
+page tables, are unmovable, page migration can fail and, therefore, inhibit
+memory offlining from succeeding.
-You can tell the physical address of new memory to the kernel by::
+Having the memory provided by memory block managed by ZONE_MOVABLE significantly
+increases memory offlining reliability; still, memory offlining can fail in
+some corner cases.
- % echo start_address_of_new_memory > /sys/devices/system/memory/probe
+Further, memory offlining might retry for a long time (or even forever), until
+aborted by the user.
-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`.
+Offlining of a memory block can be triggered via::
-Logical Memory hot-add phase
-============================
+ % echo offline > /sys/devices/system/memory/memoryXXX/state
-State of memory
----------------
+Or alternatively::
-To see (online/offline) state of a memory block, read 'state' file::
+ % echo 0 > /sys/devices/system/memory/memoryXXX/online
- % cat /sys/device/system/memory/memoryXXX/state
+If offlining succeeds, the state of the memory block is changed to be "offline".
+If it fails, the state of the memory block will remain unchanged and the above
+commands will fail, for example, via::
+ bash: echo: write error: Device or resource busy
-- If the memory block is online, you'll read "online".
-- If the memory block is offline, you'll read "offline".
+or via::
+ bash: echo: write error: Invalid argument
-.. _memory_hotplug_how_to_online_memory:
+Observing the State of Memory Blocks
+------------------------------------
-How to online memory
---------------------
+The state (online/offline/going-offline) of a memory block can be observed
+either via::
-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/device/system/memory/memoryXXX/state
- % cat /sys/devices/system/memory/auto_online_blocks
+Or alternatively (1/0) via::
-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::
+ % cat /sys/device/system/memory/memoryXXX/online
- % echo online > /sys/devices/system/memory/auto_online_blocks
+For an online memory block, the managing zone can be observed via::
-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.
+ % cat /sys/device/system/memory/memoryXXX/valid_zones
-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::
+Configuring Memory Hot(Un)Plug
+==============================
- % echo online > /sys/devices/system/memory/memoryXXX/state
+There are various ways how system administrators can configure memory
+hot(un)plug and interact with memory blocks, especially, to online them.
-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.
+Memory Hot(Un)Plug Configuration via Sysfs
+------------------------------------------
-You can explicitly request to associate it with ZONE_MOVABLE by::
+Some memory hot(un)plug properties can be configured or inspected via sysfs in::
- % echo online_movable > /sys/devices/system/memory/memoryXXX/state
+ /sys/devices/system/memory/
-.. note:: current limit: this memory block must be adjacent to ZONE_MOVABLE
+The following files are currently defined:
-Or you can explicitly request a kernel zone (usually ZONE_NORMAL) by::
+====================== =========================================================
+``auto_online_blocks`` read-write: set or get the default state of new memory
+ blocks; configure auto-onlining.
- % echo online_kernel > /sys/devices/system/memory/memoryXXX/state
+ The default value depends on the
+ CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE kernel configuration
+ option.
-.. note:: current limit: this memory block must be adjacent to ZONE_NORMAL
+ See the ``state`` property of memory blocks for details.
+``block_size_bytes`` read-only: the size in bytes of a memory block.
+``probe`` write-only: add (probe) selected memory blocks manually
+ from user space by supplying the physical start address.
-An explicit zone onlining can fail (e.g. when the range is already within
-and existing and incompatible zone already).
+ Availability depends on the CONFIG_ARCH_MEMORY_PROBE
+ kernel configuration option.
+``uevent`` read-write: generic udev file for device subsystems.
+====================== =========================================================
-After this, memory block XXX's state will be 'online' and the amount of
-available memory will be increased.
+.. note::
-This may be changed in future.
+ When the CONFIG_MEMORY_FAILURE kernel configuration option is enabled, two
+ additional files ``hard_offline_page`` and ``soft_offline_page`` are available
+ to trigger hwpoisoning of pages, for example, for testing purposes. Note that
+ this functionality is not really related to memory hot(un)plug or actual
+ offlining of memory blocks.
-Logical memory remove
-=====================
+Memory Block Configuration via Sysfs
+------------------------------------
-Memory offline and ZONE_MOVABLE
--------------------------------
+Each memory block is represented as a memory block device that can be
+onlined or offlined. All memory blocks have their device information located in
+sysfs. Each present memory block is listed under
+``/sys/devices/system/memory`` as::
-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.
+ /sys/devices/system/memory/memoryXXX
-In general, memory offline can use 2 techniques.
+where XXX is the memory block id; the number of digits is variable.
-(1) reclaim and free all memory in the memory block.
-(2) migrate all pages in the memory block.
+A present memory block indicates that some memory in the range is present;
+however, a memory block might span memory holes. A memory block spanning memory
+holes cannot be offlined.
-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.
+For example, assume 1 GiB memory block size. A device for a memory starting at
+0x100000000 is ``/sys/device/system/memory/memory4``::
-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)
+ (0x100000000 / 1Gib = 4)
-Assume the system has "TOTAL" amount of memory at boot time, this boot option
-creates ZONE_MOVABLE as following.
+This device covers address range [0x100000000 ... 0x140000000)
-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.
+The following files are currently defined:
-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.
+=================== ============================================================
+``online`` read-write: simplified interface to trigger onlining /
+ offlining and to observe the state of a memory block.
+ When onlining, the zone is selected automatically.
+``phys_device`` read-only: legacy interface only ever used on s390x to
+ expose the covered storage increment.
+``phys_index`` read-only: the memory block id (XXX).
+``removable`` read-only: legacy interface that indicated whether a memory
+ block was likely to be offlineable or not. Nowadays, the
+ kernel return ``1`` if and only if it supports memory
+ offlining.
+``state`` read-write: advanced interface to trigger onlining /
+ offlining and to observe the state of a memory block.
+
+ When writing, ``online``, ``offline``, ``online_kernel`` and
+ ``online_movable`` are supported.
+
+ ``online_movable`` specifies onlining to ZONE_MOVABLE.
+ ``online_kernel`` specifies onlining to the default kernel
+ zone for the memory block, such as ZONE_NORMAL.
+ ``online`` let's the kernel select the zone automatically.
+
+ When reading, ``online``, ``offline`` and ``going-offline``
+ may be returned.
+``uevent`` read-write: generic uevent file for devices.
+``valid_zones`` read-only: when a block is online, shows the zone it
+ belongs to; when a block is offline, shows what zone will
+ manage it when the block will be onlined.
+
+ For online memory blocks, ``DMA``, ``DMA32``, ``Normal``,
+ ``Movable`` and ``none`` may be returned. ``none`` indicates
+ that memory provided by a memory block is managed by
+ multiple zones or spans multiple nodes; such memory blocks
+ cannot be offlined. ``Movable`` indicates ZONE_MOVABLE.
+ Other values indicate a kernel zone.
+
+ For offline memory blocks, the first column shows the
+ zone the kernel would select when onlining the memory block
+ right now without further specifying a zone.
+
+ Availability depends on the CONFIG_MEMORY_HOTREMOVE
+ kernel configuration option.
+=================== ============================================================
.. note::
- Unfortunately, there is no information to show which memory block belongs
- to ZONE_MOVABLE. This is TBD.
+ If the CONFIG_NUMA kernel configuration option 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
+
+Command Line Parameters
+-----------------------
+
+Some command line parameters affect memory hot(un)plug handling. The following
+command line parameters are relevant:
+
+======================== =======================================================
+``memhp_default_state`` configure auto-onlining by essentially setting
+ ``/sys/devices/system/memory/auto_online_blocks``.
+``movablecore`` configure automatic zone selection of the kernel. When
+ set, the kernel will default to ZONE_MOVABLE, unless
+ other zones can be kept contiguous.
+======================== =======================================================
+
+Module Parameters
+------------------
- Memory offlining can fail when dissolving a free huge page on ZONE_MOVABLE
- and the feature of freeing unused vmemmap pages associated with each hugetlb
- page is enabled.
+Instead of additional command line parameters or sysfs files, the
+``memory_hotplug`` subsystem now provides a dedicated namespace for module
+parameters. Module parameters can be set via the command line by predicating
+them with ``memory_hotplug.`` such as::
+
+ memory_hotplug.memmap_on_memory=1
+
+and they can be observed (and some even modified at runtime) via::
+
+ /sys/modules/memory_hotplug/parameters/
+
+The following module parameters are currently defined:
+
+======================== =======================================================
+``memmap_on_memory`` read-write: Allocate memory for the memmap from the
+ added memory block itself. Even if enabled, actual
+ support depends on various other system properties and
+ should only be regarded as a hint whether the behavior
+ would be desired.
+
+ While allocating the memmap from the memory block
+ itself makes memory hotplug less likely to fail and
+ keeps the memmap on the same NUMA node in any case, it
+ can fragment physical memory in a way that huge pages
+ in bigger granularity cannot be formed on hotplugged
+ memory.
+======================== =======================================================
+
+ZONE_MOVABLE
+============
+
+ZONE_MOVABLE is an important mechanism for more reliable memory offlining.
+Further, having system RAM managed by ZONE_MOVABLE instead of one of the
+kernel zones can increase the number of possible transparent huge pages and
+dynamically allocated huge pages.
+
+Most kernel allocations are unmovable. Important examples include the memory
+map (usually 1/64ths of memory), page tables, and kmalloc(). Such allocations
+can only be served from the kernel zones.
+
+Most user space pages, such as anonymous memory, and page cache pages are
+movable. Such allocations can be served from ZONE_MOVABLE and the kernel zones.
+
+Only movable allocations are served from ZONE_MOVABLE, resulting in unmovable
+allocations being limited to the kernel zones. Without ZONE_MOVABLE, there is
+absolutely no guarantee whether a memory block can be offlined successfully.
+
+Zone Imbalances
+---------------
- This can happen when we have plenty of ZONE_MOVABLE memory, but not enough
- kernel memory to allocate vmemmmap pages. We may even be able to migrate
- huge page contents, but will not be able to dissolve the source huge page.
- This will prevent an offline operation and is unfortunate as memory offlining
- is expected to succeed on movable zones. Users that depend on memory hotplug
- to succeed for movable zones should carefully consider whether the memory
- savings gained from this feature are worth the risk of possibly not being
- able to offline memory in certain situations.
+Having too much system RAM managed by ZONE_MOVABLE is called a zone imbalance,
+which can harm the system or degrade performance. As one example, the kernel
+might crash because it runs out of free memory for unmovable allocations,
+although there is still plenty of free memory left in ZONE_MOVABLE.
+
+Usually, MOVABLE:KERNEL ratios of up to 3:1 or even 4:1 are fine. Ratios of 63:1
+are definitely impossible due to the overhead for the memory map.
+
+Actual safe zone ratios depend on the workload. Extreme cases, like excessive
+long-term pinning of pages, might not be able to deal with ZONE_MOVABLE at all.
.. note::
- Techniques that rely on long-term pinnings of memory (especially, RDMA and
- vfio) are fundamentally problematic with ZONE_MOVABLE and, therefore, memory
- hot remove. Pinned pages cannot reside on ZONE_MOVABLE, to guarantee that
- memory can still get hot removed - be aware that pinning can fail even if
- there is plenty of free memory in ZONE_MOVABLE. In addition, using
- ZONE_MOVABLE might make page pinning more expensive, because pages have to be
- migrated off that zone first.
-.. _memory_hotplug_how_to_offline_memory:
+ CMA memory part of a kernel zone essentially behaves like memory in
+ ZONE_MOVABLE and similar considerations apply, especially when combining
+ CMA with ZONE_MOVABLE.
-How to offline memory
----------------------
+ZONE_MOVABLE Sizing Considerations
+----------------------------------
-You can offline a memory block by using the same sysfs interface that was used
-in memory onlining::
+We usually expect that a large portion of available system RAM will actually
+be consumed by user space, either directly or indirectly via the page cache. In
+the normal case, ZONE_MOVABLE can be used when allocating such pages just fine.
- % echo offline > /sys/devices/system/memory/memoryXXX/state
+With that in mind, it makes sense that we can have a big portion of system RAM
+managed by ZONE_MOVABLE. However, there are some things to consider when using
+ZONE_MOVABLE, especially when fine-tuning zone ratios:
+
+- Having a lot of offline memory blocks. Even offline memory blocks consume
+ memory for metadata and page tables in the direct map; having a lot of offline
+ memory blocks is not a typical case, though.
+
+- Memory ballooning without balloon compaction is incompatible with
+ ZONE_MOVABLE. Only some implementations, such as virtio-balloon and
+ pseries CMM, fully support balloon compaction.
+
+ Further, the CONFIG_BALLOON_COMPACTION kernel configuration option might be
+ disabled. In that case, balloon inflation will only perform unmovable
+ allocations and silently create a zone imbalance, usually triggered by
+ inflation requests from the hypervisor.
+
+- Gigantic pages are unmovable, resulting in user space consuming a
+ lot of unmovable memory.
+
+- Huge pages are unmovable when an architectures does not support huge
+ page migration, resulting in a similar issue as with gigantic pages.
+
+- Page tables are unmovable. Excessive swapping, mapping extremely large
+ files or ZONE_DEVICE memory can be problematic, although only really relevant
+ in corner cases. When we manage a lot of user space memory that has been
+ swapped out or is served from a file/persistent memory/... we still need a lot
+ of page tables to manage that memory once user space accessed that memory.
+
+- In certain DAX configurations the memory map for the device memory will be
+ allocated from the kernel zones.
+
+- KASAN can have a significant memory overhead, for example, consuming 1/8th of
+ the total system memory size as (unmovable) tracking metadata.
+
+- Long-term pinning of pages. Techniques that rely on long-term pinnings
+ (especially, RDMA and vfio/mdev) are fundamentally problematic with
+ ZONE_MOVABLE, and therefore, memory offlining. Pinned pages cannot reside
+ on ZONE_MOVABLE as that would turn these pages unmovable. Therefore, they
+ have to be migrated off that zone while pinning. Pinning a page can fail
+ even if there is plenty of free memory in ZONE_MOVABLE.
+
+ In addition, using ZONE_MOVABLE might make page pinning more expensive,
+ because of the page migration overhead.
+
+By default, all the memory configured at boot time is managed by the kernel
+zones and ZONE_MOVABLE is not used.
+
+To enable ZONE_MOVABLE to include the memory present at boot and to control the
+ratio between movable and kernel zones there are two command line options:
+``kernelcore=`` and ``movablecore=``. See
+Documentation/admin-guide/kernel-parameters.rst for their description.
+
+Memory Offlining and ZONE_MOVABLE
+---------------------------------
+
+Even with ZONE_MOVABLE, there are some corner cases where offlining a memory
+block might fail:
+
+- Memory blocks with memory holes; this applies to memory blocks present during
+ boot and can apply to memory blocks hotplugged via the XEN balloon and the
+ Hyper-V balloon.
+
+- Mixed NUMA nodes and mixed zones within a single memory block prevent memory
+ offlining; this applies to memory blocks present during boot only.
+
+- Special memory blocks prevented by the system from getting offlined. Examples
+ include any memory available during boot on arm64 or memory blocks spanning
+ the crashkernel area on s390x; this usually applies to memory blocks present
+ during boot only.
+
+- Memory blocks overlapping with CMA areas cannot be offlined, this applies to
+ memory blocks present during boot only.
+
+- Concurrent activity that operates on the same physical memory area, such as
+ allocating gigantic pages, can result in temporary offlining failures.
+
+- Out of memory when dissolving huge pages, especially when freeing unused
+ vmemmap pages associated with each hugetlb page is enabled.
+
+ Offlining code may be able to migrate huge page contents, but may not be able
+ to dissolve the source huge page because it fails allocating (unmovable) pages
+ for the vmemmap, because the system might not have free memory in the kernel
+ zones left.
+
+ Users that depend on memory offlining to succeed for movable zones should
+ carefully consider whether the memory savings gained from this feature are
+ worth the risk of possibly not being able to offline memory in certain
+ situations.
+
+Further, when running into out of memory situations while migrating pages, or
+when still encountering permanently unmovable pages within ZONE_MOVABLE
+(-> BUG), memory offlining will keep retrying until it eventually succeeds.
+
+When offlining is triggered from user space, the offlining context can be
+terminated by sending a fatal signal. A timeout based offlining can easily be
+implemented via::
-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.
+ % timeout $TIMEOUT offline_block | failure_handling
projects / linux-2.6-microblaze.git / blobdiff