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memory-hotplug.rst: complete admin-guide overhaul 

The memory hot(un)plug documentation is outdated and incomplete.  Most of
the content dates back to 2007, so it's time for a major overhaul.

Let's rewrite, reorganize and update most parts of the documentation.  In
addition to memory hot(un)plug, also add some details regarding
ZONE_MOVABLE, with memory hotunplug being one of its main consumers.

Drop the file history, that information can more reliably be had from the
git log.

The style of the document is also properly fixed that e.g., "restview"
renders it cleanly now.

In the future, we might add some more details about virt users like
virtio-mem, the XEN balloon, the Hyper-V balloon and ppc64 dlpar.

Link: https://lkml.kernel.org/r/20210707073205.3835-3-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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David Hildenbrand 2021-09-07 19:54:49 -07:00 committed by Linus Torvalds
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.. _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::
(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 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 Hotplug allows users to increase/decrease the amount of memory.
Generally, there are two purposes.
Memory hot(un)plug is used for various 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.
- 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.
(A) is required by highly virtualized environments and (B) is required by
hardware which supports memory power management.
- Replacing hardware, such as DIMMs or whole NUMA nodes, without downtime. One
example is replacing failing memory modules.
Linux memory hotplug is designed for both purpose.
- Reducing energy consumption either by physically unplugging memory modules or
by logically unplugging (parts of) memory modules from Linux.
Phases of memory hotplug
------------------------
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.
There are 2 phases in Memory Hotplug:
Linux only supports memory hot(un)plug on selected 64 bit architectures, such as
x86_64, arm64, ppc64, s390x and ia64.
1) Physical Memory Hotplug phase
2) Logical Memory Hotplug phase.
Memory Hot(Un)Plug Granularity
------------------------------
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 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.
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`.)
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.
To determine the size (in bytes) of a memory block please read this file::
All memory blocks have the same size.
/sys/devices/system/memory/block_size_bytes
Phases of Memory Hotplug
------------------------
Kernel Configuration
====================
Memory hotplug consists of two phases:
To use memory hotplug feature, kernel must be compiled with following
config options.
(1) Adding the memory to Linux
(2) Onlining memory blocks
- For all memory hotplug:
- Memory model -> Sparse Memory (``CONFIG_SPARSEMEM``)
- Allow for memory hot-add (``CONFIG_MEMORY_HOTPLUG``)
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.
- To enable memory removal, the following are also necessary:
- Allow for memory hot remove (``CONFIG_MEMORY_HOTREMOVE``)
- Page Migration (``CONFIG_MIGRATION``)
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.
- 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.
Phases of Memory Hotunplug
--------------------------
- As a related configuration, if your box has a feature of NUMA-node hotplug
via ACPI, then this option is necessary too.
Memory hotunplug consists of two phases:
- ACPI0004,PNP0A05 and PNP0A06 Container Driver (under ACPI Support menu)
(``CONFIG_ACPI_CONTAINER``).
(1) Offlining memory blocks
(2) Removing the memory from Linux
This option can be kernel module too.
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.
In the second phase, the memory blocks are removed and metadata is freed.
.. _memory_hotplug_sysfs_files:
Memory Hotplug Notifications
============================
sysfs files for memory hotplug
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.
ACPI Notifications
------------------
Platforms that support ACPI, such as x86_64, can support memory hotplug
notifications via ACPI.
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.
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.
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.
Manual Probing
--------------
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.
The probe interface is located at::
/sys/devices/system/memory/probe
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
Which results in a memory block for the range [addr, addr + memory_block_size)
being created.
.. note::
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.
Onlining and Offlining Memory Blocks
====================================
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".
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".
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.
Onlining Memory Blocks Manually
-------------------------------
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.
Onlining of a memory block can be triggered via::
% echo online > /sys/devices/system/memory/memoryXXX/state
Or alternatively::
% echo 1 > /sys/devices/system/memory/memoryXXX/online
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.
One can explicitly request to associate an offline memory block with
ZONE_MOVABLE by::
% echo online_movable > /sys/devices/system/memory/memoryXXX/state
Or one can explicitly request a kernel zone (usually ZONE_NORMAL) by::
% echo online_kernel > /sys/devices/system/memory/memoryXXX/state
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.
Onlining Memory Blocks Automatically
------------------------------------
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.
The configured auto-online behavior can be observed via::
% cat /sys/devices/system/memory/auto_online_blocks
Auto-onlining can be enabled by writing ``online``, ``online_kernel`` or
``online_movable`` to that file, like::
% echo online > /sys/devices/system/memory/auto_online_blocks
Modifying the auto-online behavior will only affect all subsequently added
memory blocks only.
.. note::
In corner cases, auto-onlining can fail. The kernel won't retry. Note that
auto-onlining is not expected to fail in default configurations.
.. note::
DLPAR on ppc64 ignores the ``offline`` setting and will still online added
memory blocks; if onlining fails, memory blocks are removed again.
Offlining Memory Blocks
-----------------------
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.
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.
Further, memory offlining might retry for a long time (or even forever), until
aborted by the user.
Offlining of a memory block can be triggered via::
% echo offline > /sys/devices/system/memory/memoryXXX/state
Or alternatively::
% echo 0 > /sys/devices/system/memory/memoryXXX/online
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
or via::
bash: echo: write error: Invalid argument
Observing the State of Memory Blocks
------------------------------------
The state (online/offline/going-offline) of a memory block can be observed
either via::
% cat /sys/device/system/memory/memoryXXX/state
Or alternatively (1/0) via::
% cat /sys/device/system/memory/memoryXXX/online
For an online memory block, the managing zone can be observed via::
% cat /sys/device/system/memory/memoryXXX/valid_zones
Configuring Memory Hot(Un)Plug
==============================
All memory blocks have their device information in sysfs. Each memory block
is described under ``/sys/devices/system/memory`` as::
There are various ways how system administrators can configure memory
hot(un)plug and interact with memory blocks, especially, to online them.
Memory Hot(Un)Plug Configuration via Sysfs
------------------------------------------
Some memory hot(un)plug properties can be configured or inspected via sysfs in::
/sys/devices/system/memory/
The following files are currently defined:
====================== =========================================================
``auto_online_blocks`` read-write: set or get the default state of new memory
blocks; configure auto-onlining.
The default value depends on the
CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE kernel configuration
option.
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.
Availability depends on the CONFIG_ARCH_MEMORY_PROBE
kernel configuration option.
``uevent`` read-write: generic udev file for device subsystems.
====================== =========================================================
.. note::
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.
Memory Block Configuration via Sysfs
------------------------------------
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::
/sys/devices/system/memory/memoryXXX
where XXX is the memory block id.
where XXX is the memory block id; the number of digits is variable.
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.
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.
For example, assume 1GiB memory block size. A device for a memory starting at
For example, assume 1 GiB 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``
The following files are currently defined:
=================== ============================================================
``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.
``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. 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.
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.
The first column shows it`s default zone.
When writing, ``online``, ``offline``, ``online_kernel`` and
``online_movable`` are supported.
"memory6/valid_zones: Normal Movable" shows this memoryblock
can be onlined to ZONE_NORMAL by default and to ZONE_MOVABLE
by online_movable.
``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.
"memory7/valid_zones: Movable Normal" shows this memoryblock
can be onlined to ZONE_MOVABLE by default and to ZONE_NORMAL
by online_kernel.
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::
These directories/files appear after physical memory hotplug phase.
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.
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::
For example::
/sys/devices/system/node/node0/memory9 -> ../../memory/memory9
A backlink will also be created::
A backlink will also be created::
/sys/devices/system/memory/memory9/node0 -> ../../node/node0
.. _memory_hotplug_physical_mem:
Command Line Parameters
-----------------------
Physical memory hot-add phase
=============================
Some command line parameters affect memory hot(un)plug handling. The following
command line parameters are relevant:
Hardware(Firmware) Support
--------------------------
======================== =======================================================
``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.
======================== =======================================================
On x86_64/ia64 platform, memory hotplug by ACPI is supported.
Module Parameters
------------------
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.
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::
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`.
memory_hotplug.memmap_on_memory=1
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.
and they can be observed (and some even modified at runtime) via::
Notify memory hot-add event by hand
-----------------------------------
/sys/modules/memory_hotplug/parameters/
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.
The following module parameters are currently defined:
Probe interface is located at::
======================== =======================================================
``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.
/sys/devices/system/memory/probe
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.
======================== =======================================================
You can tell the physical address of new memory to the kernel by::
ZONE_MOVABLE
============
% echo start_address_of_new_memory > /sys/devices/system/memory/probe
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.
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`.
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.
Logical Memory hot-add phase
============================
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.
State of memory
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
---------------
To see (online/offline) state of a memory block, read 'state' file::
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.
% cat /sys/device/system/memory/memoryXXX/state
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.
- 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.
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::
Unfortunately, there is no information to show which memory block belongs
to ZONE_MOVABLE. This is TBD.
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.
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.
ZONE_MOVABLE Sizing Considerations
----------------------------------
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.
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.
.. 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.
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:
.. _memory_hotplug_how_to_offline_memory:
- 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.
How to offline memory
---------------------
- Memory ballooning without balloon compaction is incompatible with
ZONE_MOVABLE. Only some implementations, such as virtio-balloon and
pseries CMM, fully support balloon compaction.
You can offline a memory block by using the same sysfs interface that was used
in memory onlining::
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.
% echo offline > /sys/devices/system/memory/memoryXXX/state
- Gigantic pages are unmovable, resulting in user space consuming a
lot of unmovable memory.
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.
- Huge pages are unmovable when an architectures does not support huge
page migration, resulting in a similar issue as with gigantic pages.
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.)
- 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.
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.
- In certain DAX configurations the memory map for the device memory will be
allocated from the kernel zones.
Physical memory remove
======================
- KASAN can have a significant memory overhead, for example, consuming 1/8th of
the total system memory size as (unmovable) tracking metadata.
Need more implementation yet....
- Notification completion of remove works by OS to firmware.
- Guard from remove if not yet.
- 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.
Future Work
===========
By default, all the memory configured at boot time is managed by the kernel
zones and ZONE_MOVABLE is not used.
- 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.
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::
% timeout $TIMEOUT offline_block | failure_handling