Adding upstream version 6.1.76.upstream/6.1.76

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+.. _admin_guide_memory_hotplug:
+
+==================
+Memory Hot(Un)Plug
+==================
+
+This document describes generic Linux support for memory hot(un)plug with
+a focus on System RAM, including ZONE_MOVABLE support.
+
+.. contents:: :local:
+
+Introduction
+============
+
+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:
+
+- 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.
+
+- Reducing energy consumption either by physically unplugging memory modules or
+  by logically unplugging (parts of) memory modules from Linux.
+
+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.
+
+Linux only supports memory hot(un)plug on selected 64 bit architectures, such as
+x86_64, arm64, ppc64, s390x and ia64.
+
+Memory Hot(Un)Plug Granularity
+------------------------------
+
+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 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
+------------------------
+
+Memory hotplug consists of two phases:
+
+(1) Adding the memory to Linux
+(2) Onlining memory blocks
+
+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.
+
+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.
+
+Phases of Memory Hotunplug
+--------------------------
+
+Memory hotunplug consists of two phases:
+
+(1) Offlining memory blocks
+(2) Removing the memory from Linux
+
+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 Notifications
+============================
+
+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, depending on the
+configured ``online_policy``.
+
+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
+
+Similarly to manual onlining, with ``online`` the kernel will select the
+target zone automatically, depending on the configured ``online_policy``.
+
+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
+==============================
+
+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; the number of digits is variable.
+
+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 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)
+
+The following files are currently defined:
+
+=================== ============================================================
+``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::
+
+  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``.
+``movable_node``	 configure automatic zone selection in the kernel when
+			 using the ``contig-zones`` online policy. When
+			 set, the kernel will default to ZONE_MOVABLE when
+			 onlining a memory block, unless other zones can be kept
+			 contiguous.
+======================== =======================================================
+
+See Documentation/admin-guide/kernel-parameters.txt for a more generic
+description of these command line parameters.
+
+Module Parameters
+------------------
+
+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/module/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.
+``online_policy``		 read-write: Set the basic policy used for
+				 automatic zone selection when onlining memory
+				 blocks without specifying a target zone.
+				 ``contig-zones`` has been the kernel default
+				 before this parameter was added. After an
+				 online policy was configured and memory was
+				 online, the policy should not be changed
+				 anymore.
+
+				 When set to ``contig-zones``, the kernel will
+				 try keeping zones contiguous. If a memory block
+				 intersects multiple zones or no zone, the
+				 behavior depends on the ``movable_node`` kernel
+				 command line parameter: default to ZONE_MOVABLE
+				 if set, default to the applicable kernel zone
+				 (usually ZONE_NORMAL) if not set.
+
+				 When set to ``auto-movable``, the kernel will
+				 try onlining memory blocks to ZONE_MOVABLE if
+				 possible according to the configuration and
+				 memory device details. With this policy, one
+				 can avoid zone imbalances when eventually
+				 hotplugging a lot of memory later and still
+				 wanting to be able to hotunplug as much as
+				 possible reliably, very desirable in
+				 virtualized environments. This policy ignores
+				 the ``movable_node`` kernel command line
+				 parameter and isn't really applicable in
+				 environments that require it (e.g., bare metal
+				 with hotunpluggable nodes) where hotplugged
+				 memory might be exposed via the
+				 firmware-provided memory map early during boot
+				 to the system instead of getting detected,
+				 added and onlined  later during boot (such as
+				 done by virtio-mem or by some hypervisors
+				 implementing emulated DIMMs). As one example, a
+				 hotplugged DIMM will be onlined either
+				 completely to ZONE_MOVABLE or completely to
+				 ZONE_NORMAL, not a mixture.
+				 As another example, as many memory blocks
+				 belonging to a virtio-mem device will be
+				 onlined to ZONE_MOVABLE as possible,
+				 special-casing units of memory blocks that can
+				 only get hotunplugged together. *This policy
+				 does not protect from setups that are
+				 problematic with ZONE_MOVABLE and does not
+				 change the zone of memory blocks dynamically
+				 after they were onlined.*
+``auto_movable_ratio``		 read-write: Set the maximum MOVABLE:KERNEL
+				 memory ratio in % for the ``auto-movable``
+				 online policy. Whether the ratio applies only
+				 for the system across all NUMA nodes or also
+				 per NUMA nodes depends on the
+				 ``auto_movable_numa_aware`` configuration.
+
+				 All accounting is based on present memory pages
+				 in the zones combined with accounting per
+				 memory device. Memory dedicated to the CMA
+				 allocator is accounted as MOVABLE, although
+				 residing on one of the kernel zones. The
+				 possible ratio depends on the actual workload.
+				 The kernel default is "301" %, for example,
+				 allowing for hotplugging 24 GiB to a 8 GiB VM
+				 and automatically onlining all hotplugged
+				 memory to ZONE_MOVABLE in many setups. The
+				 additional 1% deals with some pages being not
+				 present, for example, because of some firmware
+				 allocations.
+
+				 Note that ZONE_NORMAL memory provided by one
+				 memory device does not allow for more
+				 ZONE_MOVABLE memory for a different memory
+				 device. As one example, onlining memory of a
+				 hotplugged DIMM to ZONE_NORMAL will not allow
+				 for another hotplugged DIMM to get onlined to
+				 ZONE_MOVABLE automatically. In contrast, memory
+				 hotplugged by a virtio-mem device that got
+				 onlined to ZONE_NORMAL will allow for more
+				 ZONE_MOVABLE memory within *the same*
+				 virtio-mem device.
+``auto_movable_numa_aware``	 read-write: Configure whether the
+				 ``auto_movable_ratio`` in the ``auto-movable``
+				 online policy also applies per NUMA
+				 node in addition to the whole system across all
+				 NUMA nodes. The kernel default is "Y".
+
+				 Disabling NUMA awareness can be helpful when
+				 dealing with NUMA nodes that should be
+				 completely hotunpluggable, onlining the memory
+				 completely to ZONE_MOVABLE automatically if
+				 possible.
+
+				 Parameter availability depends on CONFIG_NUMA.
+================================ ===============================================
+
+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
+---------------
+
+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::
+
+  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.
+
+ZONE_MOVABLE Sizing Considerations
+----------------------------------
+
+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.
+
+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 HugeTLB Vmemmap
+  Optimization (HVO) 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