On Tue, May 25, 2021 at 12:26:04PM +0200, David Hildenbrand wrote:
> The memory hot(un)plug documentation is outdated and incomplete.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.
>
> 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.
>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Oscar Salvador <osalvador@xxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> Cc: Mike Rapoport <rppt@xxxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> Cc: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> Cc: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
> Cc: Jonathan Corbet <corbet@xxxxxxx>
> Cc: Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx>
> Cc: linux-doc@xxxxxxxxxxxxxxx
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
>
> Based on linux-next, which includes hugetlb vmemmap changes to the doc
> that are not upstream yet.
>
> ---
> .../admin-guide/mm/memory-hotplug.rst | 738 +++++++++++-------
> 1 file changed, 440 insertions(+), 298 deletions(-)
>
> diff --git a/Documentation/admin-guide/mm/memory-hotplug.rst b/Documentation/admin-guide/mm/memory-hotplug.rst
> index c6bae2d77160..c95f5c2b30dd 100644
> --- a/Documentation/admin-guide/mm/memory-hotplug.rst
> +++ b/Documentation/admin-guide/mm/memory-hotplug.rst
> @@ -1,419 +1,573 @@
> .. _admin_guide_memory_hotplug:
>
> -==============
> -Memory Hotplug
> -==============
> +==================
> +Memory Hot(Un)Plug
> +==================
>
> :Created: Jul 28 2007
> :Updated: Add some details about locking internals: Aug 20 2018
> +:Updated: Complete overhaul: May 18 2021
I'd drop all three, we have git log...
>
> -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 physical memory
Maybe: 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.
> +(A) 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.
I like more the bulleted lists, so you just put * or - in the beginning of
the line and then you don't need to know neither the alphabet nor how to
count :)
More seriously, if the letters (or numbers) have no particular meaning it's
easier to maintain a list with neutral bullets.
> -(A) is required by highly virtualized environments and (B) is required by
> -hardware which supports memory power management.
> +(B) 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.
> +(C) Reducing memory consumption either by physically unplugging
> + memory modules or by logically unplugging (parts of) memory modules
> + from Linux.
It feels like some part of explanation is missing. My understanding of the
above paragraph is "we remove a DIMM and thus the memory consumption
drops".
My guess is that you refer here to VM environments, in this case some more
details would help.
> -Phases of memory hotplug
> +Further, the basic memory hot(un)plug infrastructure in Linux is nowadays
> +also used to expose PMEM, other performance-differentiated
^ persistent memory (PMEM)
> +memory and reserved memory regions as ordinary system RAM to Linux.
> +
> +Phases of Memory Hotplug
> ------------------------
>
> -There are 2 phases in Memory Hotplug:
> +Memory hotplug consists of two phases:
> +
> +(1) Adding the memory to Linux
> +(2) Onlining memory blocks
>
> - 1) Physical Memory Hotplug phase
> - 2) Logical Memory Hotplug phase.
> +In the first phase, metadata (such as the memmap) is allocated, page tables
> +for the direct mapping are allocated and initialized, and memory blocks
User/administrator should not care about memmap or direct map and these
details are better suited for Documentation/vm but since we don't have it
how about:
... metadata, such as the memory map and page tables for the direct map,
are allocated and initialized, ...
> +are created; the latter also creates sysfs files for managing
The reader doesn't know what are memory blocks in this context yet. I'd
suggest to move "Unit of Memory Hot(Un)Plug" before the phases.
> +newly created 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 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.
>
> -When memory is hotplugged, the kernel recognizes new memory, makes new memory
> -management tables, and makes sysfs files for new memory's operation.
> +Phases of Memory Hotunplug
> +--------------------------
>
> -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`).
> +Memory hotunplug consists of two phases:
>
> -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.
> +(1) Offlining memory blocks
> +(2) Removing the memory from Linux
>
> -In this document, this phase is described as online/offline.
> +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.
>
> -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 second phase, the memory blocks are removed, the direct map is torn
> +down, and metadata is freed.
>
> -Unit of Memory online/offline operation
> ----------------------------------------
> +Unit of Memory Hot(Un)Plug
Units?
> +--------------------------
>
> -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 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
> +Memory Hotplug Notifications
> +============================
>
> -Kernel Configuration
> -====================
> +There are various ways how Linux is notified about memory hotplug events
> +such that it can start adding hotplugged memory. This description is
> +mostly limited to mechanisms present on physical machines; mechanisms specific
> +to virtual machines or logical partitions are not described.
... This description is limited to systems that support ACPI; mechanisms
specific to other firmware interfaces or virtual machines are not
described.
> -To use memory hotplug feature, kernel must be compiled with following
> -config options.
> +ACPI Notifications
> +------------------
>
> -- For all memory hotplug:
> - - Memory model -> Sparse Memory (``CONFIG_SPARSEMEM``)
> - - Allow for memory hot-add (``CONFIG_MEMORY_HOTPLUG``)
> +Platforms that support ACPI, such as x86_64, can support memory hotplug
> +notifications via ACPI.
>
> -- To enable memory removal, the following are also necessary:
> - - Allow for memory hot remove (``CONFIG_MEMORY_HOTREMOVE``)
> - - Page Migration (``CONFIG_MIGRATION``)
> +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.
>
> -- 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.
> +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.
>
> -- As a related configuration, if your box has a feature of NUMA-node hotplug
> - via ACPI, then this option is necessary too.
> +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.
>
> - - ACPI0004,PNP0A05 and PNP0A06 Container Driver (under ACPI Support menu)
> - (``CONFIG_ACPI_CONTAINER``).
> +Manual Probing
> +--------------
>
> - This option can be kernel module too.
> +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::
>
> -.. _memory_hotplug_sysfs_files:
> + /sys/devices/system/memory/probe
>
> -sysfs files for memory hotplug
> -==============================
> +Only complete memory blocks can be probed. Individual memory blocks are
> +probed by providing the physical start address of the memory block::
>
> -All memory blocks have their device information in sysfs. Each memory block
> -is described under ``/sys/devices/system/memory`` as::
> + % echo addr > /sys/devices/system/memory/probe
>
> - /sys/devices/system/memory/memoryXXX
> +Which results in a memory block for the range [addr, addr + memory_block_size)
> +being created.
>
> -where XXX is the memory block id.
> +.. note::
>
> -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.
> + 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.
>
> -For example, assume 1GiB memory block size. A device for a memory starting at
> -0x100000000 is ``/sys/device/system/memory/memory4``::
> +Onlining and Offlining Memory Blocks
> +====================================
>
> - (0x100000000 / 1Gib = 4)
> +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".
>
> -This device covers address range [0x100000000 ... 0x140000000)
> +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".
>
> -Under each memory block, you can see 5 files:
> +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
... and drivers may trigger offlining of memory blocks when they attempt of
hotunplug the memory.
> +succeeded.
>
> -- ``/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``
> +Onlining Memory Blocks Manually
> +-------------------------------
>
> -=================== ============================================================
> -``phys_index`` read-only and contains memory block id, same as XXX.
> -``state`` read-write
> +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.
>
> - - at read: contains online/offline state of memory.
> - - at write: user can specify "online_kernel",
> +Onlining of a memory block can be triggered via::
>
> - "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.
> + % echo online > /sys/devices/system/memory/memoryXXX/state
>
> - The first column shows it`s default zone.
> +Or alternatively::
>
> - "memory6/valid_zones: Normal Movable" shows this memoryblock
> - can be onlined to ZONE_NORMAL by default and to ZONE_MOVABLE
> - by online_movable.
> + % echo 1 > /sys/devices/system/memory/memoryXXX/online
>
> - "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
> +cmdline or if the memory block would intersect the ZONE_MOVABLE already.
>
> -.. note::
> +One can explicitly request to associate it with ZONE_MOVABLE by::
s/it/added memory block/
>
> - 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 offline succeeds, the state of the memory block is changed to
> +be "online". If it fails, an error will be returned by the kernel.
>
> -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 disabled, the memory blocks will stay offline until explicitly onlined
^ If this feature is disabled
> +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 severely
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 offline succeeds, the state of the memory block is changed to be "offline".
> +If it fails, an error will be returned by the kernel.
I think elaborating here how the error is returned would be nice.
> +Observing the State of Memory Blocks
> +------------------------------------
>
> -- If the memory block is online, you'll read "online".
> -- If the memory block is offline, you'll read "offline".
> +The state (online/offline/going-offline) of a memory block can be observed
> +either via::
>
> + % cat /sys/device/system/memory/memoryXXX/state
>
> -.. _memory_hotplug_how_to_online_memory:
> +Or alternatively (1/0) via::
>
> -How to online memory
> ---------------------
> + % cat /sys/device/system/memory/memoryXXX/online
>
> -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::
> +For an online memory block, the managing zone van be observed via::
typo: ^ can
>
> - % cat /sys/devices/system/memory/auto_online_blocks
> + % cat /sys/device/system/memory/memoryXXX/valid_zones
>
> -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::
> +Configuring Memory Hot(Un)Plug
> +==============================
>
> - % echo online > /sys/devices/system/memory/auto_online_blocks
> +There are various ways how admins can configure memory hot(un)plug and interact
^ system administrators
> +with memory blocks, especially, to online them.
>
> -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.
> +Memory Hot(Un)Plug Configuration via Sysfs
> +------------------------------------------
>
> -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::
> +Some memory hot(un)plug properties can be configured or inspected via sysfs in::
>
> - % echo online > /sys/devices/system/memory/memoryXXX/state
> + /sys/devices/system/memory/
>
> -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.
> +The following files are currently defined:
>
> -You can explicitly request to associate it with ZONE_MOVABLE by::
> +====================== =========================================================
> +``auto_online_blocks`` read-write: set or get the default state of new memory
> + blocks; configure auto-onlining.
>
> - % echo online_movable > /sys/devices/system/memory/memoryXXX/state
> + 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.
> +``uevent`` read-write: generic udev file for device subsystems.
> +====================== =========================================================
>
> -.. note:: current limit: this memory block must be adjacent to ZONE_MOVABLE
> +.. note::
> + The default of ``auto_online_blocks`` depends on
> + CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE.
... depends on the CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE kernel
configuration option.
> -Or you can explicitly request a kernel zone (usually ZONE_NORMAL) by::
> +.. note::
>
> - % echo online_kernel > /sys/devices/system/memory/memoryXXX/state
> + With CONFIG_MEMORY_FAILURE, two additional files ``hard_offline_page`` and
When the kernel is built with CONFIG_MEMORY_FAILURE option enabled
Maybe add a subsection about the configuration options that define sysfs
behaviour and group all the notes there as simple paragraphs?
> + ``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.
>
> -.. note:: current limit: this memory block must be adjacent to ZONE_NORMAL
> +.. note::
> +
> + ``probe`` is only available with CONFIG_ARCH_MEMORY_PROBE.
>
> -An explicit zone onlining can fail (e.g. when the range is already within
> -and existing and incompatible zone already).
> +Memory Block Configuration via Sysfs
> +------------------------------------
>
> -After this, memory block XXX's state will be 'online' and the amount of
> -available memory will be increased.
> +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::
>
> -This may be changed in future.
> + /sys/devices/system/memory/memoryXXX
>
> -Logical memory remove
> -=====================
> +where XXX is the memory block id; the number of digits is variable.
>
> -Memory offline and ZONE_MOVABLE
> --------------------------------
> +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.
>
> -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.
> +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)
>
> -In general, memory offline can use 2 techniques.
> +This device covers address range [0x100000000 ... 0x140000000)
>
> -(1) reclaim and free all memory in the memory block.
> -(2) migrate all pages in the memory block.
> +The following files are currently defined:
>
> -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.
> +=================== ============================================================
> +``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: shows by which zone memory provided by an
> + online memory block is managed, and by which zone memory
> + provided by an offline memory block could be managed when
> + onlining.
Sounds a bit awkward to me. Maybe
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.
>
> -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)
> + For online memory blocks, ``DMA``, ``DMA32``, ``Normal``,
> + ``Movable`` and ``none`` may be returned. ``none`` indicates
Highmem? Or we don't support hotplug on 32 bits?
> + 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.
>
> -Assume the system has "TOTAL" amount of memory at boot time, this boot option
> -creates ZONE_MOVABLE as following.
> + 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.
> +=================== ============================================================
>
> -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.
> +.. note::
>
> -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.
> + ``valid_zones`` is only available with CONFIG_MEMORY_HOTREMOVE.
>
> .. note::
>
> - Unfortunately, there is no information to show which memory block belongs
> - to ZONE_MOVABLE. This is TBD.
> + 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 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.
> +Cmdline Parameters
Command line
> +------------------
>
> - 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.
> +Some cmdline parameters affect memory hot(un)plug handling. The following
^ command line
> +cmdline parameters are relevant:
and here ^
> +
> +======================== =======================================================
> +``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
> +------------------
> +
> +Instead of additional cmdline parameters or sysfs files, the ``memory_hotplug``
> +subsystem now provides a dedicated namespace for module parameters. Module
> +parameters can be set via the cmdline by predicating them with
^ command line
> +``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
> +============
This will take some more time.
--
Sincerely yours,
Mike.
