1================== 2Memory Hot(Un)Plug 3================== 4 5This document describes generic Linux support for memory hot(un)plug with 6a focus on System RAM, including ZONE_MOVABLE support. 7 8.. contents:: :local: 9 10Introduction 11============ 12 13Memory hot(un)plug allows for increasing and decreasing the size of physical 14memory available to a machine at runtime. In the simplest case, it consists of 15physically plugging or unplugging a DIMM at runtime, coordinated with the 16operating system. 17 18Memory hot(un)plug is used for various purposes: 19 20- The physical memory available to a machine can be adjusted at runtime, up- or 21 downgrading the memory capacity. This dynamic memory resizing, sometimes 22 referred to as "capacity on demand", is frequently used with virtual machines 23 and logical partitions. 24 25- Replacing hardware, such as DIMMs or whole NUMA nodes, without downtime. One 26 example is replacing failing memory modules. 27 28- Reducing energy consumption either by physically unplugging memory modules or 29 by logically unplugging (parts of) memory modules from Linux. 30 31Further, the basic memory hot(un)plug infrastructure in Linux is nowadays also 32used to expose persistent memory, other performance-differentiated memory and 33reserved memory regions as ordinary system RAM to Linux. 34 35Linux only supports memory hot(un)plug on selected 64 bit architectures, such as 36x86_64, arm64, ppc64 and s390x. 37 38Memory Hot(Un)Plug Granularity 39------------------------------ 40 41Memory hot(un)plug in Linux uses the SPARSEMEM memory model, which divides the 42physical memory address space into chunks of the same size: memory sections. The 43size of a memory section is architecture dependent. For example, x86_64 uses 44128 MiB and ppc64 uses 16 MiB. 45 46Memory sections are combined into chunks referred to as "memory blocks". The 47size of a memory block is architecture dependent and corresponds to the smallest 48granularity that can be hot(un)plugged. The default size of a memory block is 49the same as memory section size, unless an architecture specifies otherwise. 50 51All memory blocks have the same size. 52 53Phases of Memory Hotplug 54------------------------ 55 56Memory hotplug consists of two phases: 57 58(1) Adding the memory to Linux 59(2) Onlining memory blocks 60 61In the first phase, metadata, such as the memory map ("memmap") and page tables 62for the direct mapping, is allocated and initialized, and memory blocks are 63created; the latter also creates sysfs files for managing newly created memory 64blocks. 65 66In the second phase, added memory is exposed to the page allocator. After this 67phase, the memory is visible in memory statistics, such as free and total 68memory, of the system. 69 70Phases of Memory Hotunplug 71-------------------------- 72 73Memory hotunplug consists of two phases: 74 75(1) Offlining memory blocks 76(2) Removing the memory from Linux 77 78In the first phase, memory is "hidden" from the page allocator again, for 79example, by migrating busy memory to other memory locations and removing all 80relevant free pages from the page allocator After this phase, the memory is no 81longer visible in memory statistics of the system. 82 83In the second phase, the memory blocks are removed and metadata is freed. 84 85Memory Hotplug Notifications 86============================ 87 88There are various ways how Linux is notified about memory hotplug events such 89that it can start adding hotplugged memory. This description is limited to 90systems that support ACPI; mechanisms specific to other firmware interfaces or 91virtual machines are not described. 92 93ACPI Notifications 94------------------ 95 96Platforms that support ACPI, such as x86_64, can support memory hotplug 97notifications via ACPI. 98 99In general, a firmware supporting memory hotplug defines a memory class object 100HID "PNP0C80". When notified about hotplug of a new memory device, the ACPI 101driver will hotplug the memory to Linux. 102 103If the firmware supports hotplug of NUMA nodes, it defines an object _HID 104"ACPI0004", "PNP0A05", or "PNP0A06". When notified about an hotplug event, all 105assigned memory devices are added to Linux by the ACPI driver. 106 107Similarly, Linux can be notified about requests to hotunplug a memory device or 108a NUMA node via ACPI. The ACPI driver will try offlining all relevant memory 109blocks, and, if successful, hotunplug the memory from Linux. 110 111Manual Probing 112-------------- 113 114On some architectures, the firmware may not be able to notify the operating 115system about a memory hotplug event. Instead, the memory has to be manually 116probed from user space. 117 118The probe interface is located at:: 119 120 /sys/devices/system/memory/probe 121 122Only complete memory blocks can be probed. Individual memory blocks are probed 123by providing the physical start address of the memory block:: 124 125 % echo addr > /sys/devices/system/memory/probe 126 127Which results in a memory block for the range [addr, addr + memory_block_size) 128being created. 129 130.. note:: 131 132 Using the probe interface is discouraged as it is easy to crash the kernel, 133 because Linux cannot validate user input; this interface might be removed in 134 the future. 135 136Onlining and Offlining Memory Blocks 137==================================== 138 139After a memory block has been created, Linux has to be instructed to actually 140make use of that memory: the memory block has to be "online". 141 142Before a memory block can be removed, Linux has to stop using any memory part of 143the memory block: the memory block has to be "offlined". 144 145The Linux kernel can be configured to automatically online added memory blocks 146and drivers automatically trigger offlining of memory blocks when trying 147hotunplug of memory. Memory blocks can only be removed once offlining succeeded 148and drivers may trigger offlining of memory blocks when attempting hotunplug of 149memory. 150 151Onlining Memory Blocks Manually 152------------------------------- 153 154If auto-onlining of memory blocks isn't enabled, user-space has to manually 155trigger onlining of memory blocks. Often, udev rules are used to automate this 156task in user space. 157 158Onlining of a memory block can be triggered via:: 159 160 % echo online > /sys/devices/system/memory/memoryXXX/state 161 162Or alternatively:: 163 164 % echo 1 > /sys/devices/system/memory/memoryXXX/online 165 166The kernel will select the target zone automatically, depending on the 167configured ``online_policy``. 168 169One can explicitly request to associate an offline memory block with 170ZONE_MOVABLE by:: 171 172 % echo online_movable > /sys/devices/system/memory/memoryXXX/state 173 174Or one can explicitly request a kernel zone (usually ZONE_NORMAL) by:: 175 176 % echo online_kernel > /sys/devices/system/memory/memoryXXX/state 177 178In any case, if onlining succeeds, the state of the memory block is changed to 179be "online". If it fails, the state of the memory block will remain unchanged 180and the above commands will fail. 181 182Onlining Memory Blocks Automatically 183------------------------------------ 184 185The kernel can be configured to try auto-onlining of newly added memory blocks. 186If this feature is disabled, the memory blocks will stay offline until 187explicitly onlined from user space. 188 189The configured auto-online behavior can be observed via:: 190 191 % cat /sys/devices/system/memory/auto_online_blocks 192 193Auto-onlining can be enabled by writing ``online``, ``online_kernel`` or 194``online_movable`` to that file, like:: 195 196 % echo online > /sys/devices/system/memory/auto_online_blocks 197 198Similarly to manual onlining, with ``online`` the kernel will select the 199target zone automatically, depending on the configured ``online_policy``. 200 201Modifying the auto-online behavior will only affect all subsequently added 202memory blocks only. 203 204.. note:: 205 206 In corner cases, auto-onlining can fail. The kernel won't retry. Note that 207 auto-onlining is not expected to fail in default configurations. 208 209.. note:: 210 211 DLPAR on ppc64 ignores the ``offline`` setting and will still online added 212 memory blocks; if onlining fails, memory blocks are removed again. 213 214Offlining Memory Blocks 215----------------------- 216 217In the current implementation, Linux's memory offlining will try migrating all 218movable pages off the affected memory block. As most kernel allocations, such as 219page tables, are unmovable, page migration can fail and, therefore, inhibit 220memory offlining from succeeding. 221 222Having the memory provided by memory block managed by ZONE_MOVABLE significantly 223increases memory offlining reliability; still, memory offlining can fail in 224some corner cases. 225 226Further, memory offlining might retry for a long time (or even forever), until 227aborted by the user. 228 229Offlining of a memory block can be triggered via:: 230 231 % echo offline > /sys/devices/system/memory/memoryXXX/state 232 233Or alternatively:: 234 235 % echo 0 > /sys/devices/system/memory/memoryXXX/online 236 237If offlining succeeds, the state of the memory block is changed to be "offline". 238If it fails, the state of the memory block will remain unchanged and the above 239commands will fail, for example, via:: 240 241 bash: echo: write error: Device or resource busy 242 243or via:: 244 245 bash: echo: write error: Invalid argument 246 247Observing the State of Memory Blocks 248------------------------------------ 249 250The state (online/offline/going-offline) of a memory block can be observed 251either via:: 252 253 % cat /sys/devices/system/memory/memoryXXX/state 254 255Or alternatively (1/0) via:: 256 257 % cat /sys/devices/system/memory/memoryXXX/online 258 259For an online memory block, the managing zone can be observed via:: 260 261 % cat /sys/devices/system/memory/memoryXXX/valid_zones 262 263Configuring Memory Hot(Un)Plug 264============================== 265 266There are various ways how system administrators can configure memory 267hot(un)plug and interact with memory blocks, especially, to online them. 268 269Memory Hot(Un)Plug Configuration via Sysfs 270------------------------------------------ 271 272Some memory hot(un)plug properties can be configured or inspected via sysfs in:: 273 274 /sys/devices/system/memory/ 275 276The following files are currently defined: 277 278====================== ========================================================= 279``auto_online_blocks`` read-write: set or get the default state of new memory 280 blocks; configure auto-onlining. 281 282 The default value depends on the 283 CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE kernel configuration 284 option. 285 286 See the ``state`` property of memory blocks for details. 287``block_size_bytes`` read-only: the size in bytes of a memory block. 288``probe`` write-only: add (probe) selected memory blocks manually 289 from user space by supplying the physical start address. 290 291 Availability depends on the CONFIG_ARCH_MEMORY_PROBE 292 kernel configuration option. 293``uevent`` read-write: generic udev file for device subsystems. 294``crash_hotplug`` read-only: when changes to the system memory map 295 occur due to hot un/plug of memory, this file contains 296 '1' if the kernel updates the kdump capture kernel memory 297 map itself (via elfcorehdr and other relevant kexec 298 segments), or '0' if userspace must update the kdump 299 capture kernel memory map. 300 301 Availability depends on the CONFIG_MEMORY_HOTPLUG kernel 302 configuration option. 303====================== ========================================================= 304 305.. note:: 306 307 When the CONFIG_MEMORY_FAILURE kernel configuration option is enabled, two 308 additional files ``hard_offline_page`` and ``soft_offline_page`` are available 309 to trigger hwpoisoning of pages, for example, for testing purposes. Note that 310 this functionality is not really related to memory hot(un)plug or actual 311 offlining of memory blocks. 312 313Memory Block Configuration via Sysfs 314------------------------------------ 315 316Each memory block is represented as a memory block device that can be 317onlined or offlined. All memory blocks have their device information located in 318sysfs. Each present memory block is listed under 319``/sys/devices/system/memory`` as:: 320 321 /sys/devices/system/memory/memoryXXX 322 323where XXX is the memory block id; the number of digits is variable. 324 325A present memory block indicates that some memory in the range is present; 326however, a memory block might span memory holes. A memory block spanning memory 327holes cannot be offlined. 328 329For example, assume 1 GiB memory block size. A device for a memory starting at 3300x100000000 is ``/sys/devices/system/memory/memory4``:: 331 332 (0x100000000 / 1Gib = 4) 333 334This device covers address range [0x100000000 ... 0x140000000) 335 336The following files are currently defined: 337 338=================== ============================================================ 339``online`` read-write: simplified interface to trigger onlining / 340 offlining and to observe the state of a memory block. 341 When onlining, the zone is selected automatically. 342``phys_device`` read-only: legacy interface only ever used on s390x to 343 expose the covered storage increment. 344``phys_index`` read-only: the memory block id (XXX). 345``removable`` read-only: legacy interface that indicated whether a memory 346 block was likely to be offlineable or not. Nowadays, the 347 kernel return ``1`` if and only if it supports memory 348 offlining. 349``state`` read-write: advanced interface to trigger onlining / 350 offlining and to observe the state of a memory block. 351 352 When writing, ``online``, ``offline``, ``online_kernel`` and 353 ``online_movable`` are supported. 354 355 ``online_movable`` specifies onlining to ZONE_MOVABLE. 356 ``online_kernel`` specifies onlining to the default kernel 357 zone for the memory block, such as ZONE_NORMAL. 358 ``online`` let's the kernel select the zone automatically. 359 360 When reading, ``online``, ``offline`` and ``going-offline`` 361 may be returned. 362``uevent`` read-write: generic uevent file for devices. 363``valid_zones`` read-only: when a block is online, shows the zone it 364 belongs to; when a block is offline, shows what zone will 365 manage it when the block will be onlined. 366 367 For online memory blocks, ``DMA``, ``DMA32``, ``Normal``, 368 ``Movable`` and ``none`` may be returned. ``none`` indicates 369 that memory provided by a memory block is managed by 370 multiple zones or spans multiple nodes; such memory blocks 371 cannot be offlined. ``Movable`` indicates ZONE_MOVABLE. 372 Other values indicate a kernel zone. 373 374 For offline memory blocks, the first column shows the 375 zone the kernel would select when onlining the memory block 376 right now without further specifying a zone. 377 378 Availability depends on the CONFIG_MEMORY_HOTREMOVE 379 kernel configuration option. 380=================== ============================================================ 381 382.. note:: 383 384 If the CONFIG_NUMA kernel configuration option is enabled, the memoryXXX/ 385 directories can also be accessed via symbolic links located in the 386 ``/sys/devices/system/node/node*`` directories. 387 388 For example:: 389 390 /sys/devices/system/node/node0/memory9 -> ../../memory/memory9 391 392 A backlink will also be created:: 393 394 /sys/devices/system/memory/memory9/node0 -> ../../node/node0 395 396Command Line Parameters 397----------------------- 398 399Some command line parameters affect memory hot(un)plug handling. The following 400command line parameters are relevant: 401 402======================== ======================================================= 403``memhp_default_state`` configure auto-onlining by essentially setting 404 ``/sys/devices/system/memory/auto_online_blocks``. 405``movable_node`` configure automatic zone selection in the kernel when 406 using the ``contig-zones`` online policy. When 407 set, the kernel will default to ZONE_MOVABLE when 408 onlining a memory block, unless other zones can be kept 409 contiguous. 410======================== ======================================================= 411 412See Documentation/admin-guide/kernel-parameters.txt for a more generic 413description of these command line parameters. 414 415Module Parameters 416------------------ 417 418Instead of additional command line parameters or sysfs files, the 419``memory_hotplug`` subsystem now provides a dedicated namespace for module 420parameters. Module parameters can be set via the command line by predicating 421them with ``memory_hotplug.`` such as:: 422 423 memory_hotplug.memmap_on_memory=1 424 425and they can be observed (and some even modified at runtime) via:: 426 427 /sys/module/memory_hotplug/parameters/ 428 429The following module parameters are currently defined: 430 431================================ =============================================== 432``memmap_on_memory`` read-write: Allocate memory for the memmap from 433 the added memory block itself. Even if enabled, 434 actual support depends on various other system 435 properties and should only be regarded as a 436 hint whether the behavior would be desired. 437 438 While allocating the memmap from the memory 439 block itself makes memory hotplug less likely 440 to fail and keeps the memmap on the same NUMA 441 node in any case, it can fragment physical 442 memory in a way that huge pages in bigger 443 granularity cannot be formed on hotplugged 444 memory. 445 446 With value "force" it could result in memory 447 wastage due to memmap size limitations. For 448 example, if the memmap for a memory block 449 requires 1 MiB, but the pageblock size is 2 450 MiB, 1 MiB of hotplugged memory will be wasted. 451 Note that there are still cases where the 452 feature cannot be enforced: for example, if the 453 memmap is smaller than a single page, or if the 454 architecture does not support the forced mode 455 in all configurations. 456 457``online_policy`` read-write: Set the basic policy used for 458 automatic zone selection when onlining memory 459 blocks without specifying a target zone. 460 ``contig-zones`` has been the kernel default 461 before this parameter was added. After an 462 online policy was configured and memory was 463 online, the policy should not be changed 464 anymore. 465 466 When set to ``contig-zones``, the kernel will 467 try keeping zones contiguous. If a memory block 468 intersects multiple zones or no zone, the 469 behavior depends on the ``movable_node`` kernel 470 command line parameter: default to ZONE_MOVABLE 471 if set, default to the applicable kernel zone 472 (usually ZONE_NORMAL) if not set. 473 474 When set to ``auto-movable``, the kernel will 475 try onlining memory blocks to ZONE_MOVABLE if 476 possible according to the configuration and 477 memory device details. With this policy, one 478 can avoid zone imbalances when eventually 479 hotplugging a lot of memory later and still 480 wanting to be able to hotunplug as much as 481 possible reliably, very desirable in 482 virtualized environments. This policy ignores 483 the ``movable_node`` kernel command line 484 parameter and isn't really applicable in 485 environments that require it (e.g., bare metal 486 with hotunpluggable nodes) where hotplugged 487 memory might be exposed via the 488 firmware-provided memory map early during boot 489 to the system instead of getting detected, 490 added and onlined later during boot (such as 491 done by virtio-mem or by some hypervisors 492 implementing emulated DIMMs). As one example, a 493 hotplugged DIMM will be onlined either 494 completely to ZONE_MOVABLE or completely to 495 ZONE_NORMAL, not a mixture. 496 As another example, as many memory blocks 497 belonging to a virtio-mem device will be 498 onlined to ZONE_MOVABLE as possible, 499 special-casing units of memory blocks that can 500 only get hotunplugged together. *This policy 501 does not protect from setups that are 502 problematic with ZONE_MOVABLE and does not 503 change the zone of memory blocks dynamically 504 after they were onlined.* 505``auto_movable_ratio`` read-write: Set the maximum MOVABLE:KERNEL 506 memory ratio in % for the ``auto-movable`` 507 online policy. Whether the ratio applies only 508 for the system across all NUMA nodes or also 509 per NUMA nodes depends on the 510 ``auto_movable_numa_aware`` configuration. 511 512 All accounting is based on present memory pages 513 in the zones combined with accounting per 514 memory device. Memory dedicated to the CMA 515 allocator is accounted as MOVABLE, although 516 residing on one of the kernel zones. The 517 possible ratio depends on the actual workload. 518 The kernel default is "301" %, for example, 519 allowing for hotplugging 24 GiB to a 8 GiB VM 520 and automatically onlining all hotplugged 521 memory to ZONE_MOVABLE in many setups. The 522 additional 1% deals with some pages being not 523 present, for example, because of some firmware 524 allocations. 525 526 Note that ZONE_NORMAL memory provided by one 527 memory device does not allow for more 528 ZONE_MOVABLE memory for a different memory 529 device. As one example, onlining memory of a 530 hotplugged DIMM to ZONE_NORMAL will not allow 531 for another hotplugged DIMM to get onlined to 532 ZONE_MOVABLE automatically. In contrast, memory 533 hotplugged by a virtio-mem device that got 534 onlined to ZONE_NORMAL will allow for more 535 ZONE_MOVABLE memory within *the same* 536 virtio-mem device. 537``auto_movable_numa_aware`` read-write: Configure whether the 538 ``auto_movable_ratio`` in the ``auto-movable`` 539 online policy also applies per NUMA 540 node in addition to the whole system across all 541 NUMA nodes. The kernel default is "Y". 542 543 Disabling NUMA awareness can be helpful when 544 dealing with NUMA nodes that should be 545 completely hotunpluggable, onlining the memory 546 completely to ZONE_MOVABLE automatically if 547 possible. 548 549 Parameter availability depends on CONFIG_NUMA. 550================================ =============================================== 551 552ZONE_MOVABLE 553============ 554 555ZONE_MOVABLE is an important mechanism for more reliable memory offlining. 556Further, having system RAM managed by ZONE_MOVABLE instead of one of the 557kernel zones can increase the number of possible transparent huge pages and 558dynamically allocated huge pages. 559 560Most kernel allocations are unmovable. Important examples include the memory 561map (usually 1/64ths of memory), page tables, and kmalloc(). Such allocations 562can only be served from the kernel zones. 563 564Most user space pages, such as anonymous memory, and page cache pages are 565movable. Such allocations can be served from ZONE_MOVABLE and the kernel zones. 566 567Only movable allocations are served from ZONE_MOVABLE, resulting in unmovable 568allocations being limited to the kernel zones. Without ZONE_MOVABLE, there is 569absolutely no guarantee whether a memory block can be offlined successfully. 570 571Zone Imbalances 572--------------- 573 574Having too much system RAM managed by ZONE_MOVABLE is called a zone imbalance, 575which can harm the system or degrade performance. As one example, the kernel 576might crash because it runs out of free memory for unmovable allocations, 577although there is still plenty of free memory left in ZONE_MOVABLE. 578 579Usually, MOVABLE:KERNEL ratios of up to 3:1 or even 4:1 are fine. Ratios of 63:1 580are definitely impossible due to the overhead for the memory map. 581 582Actual safe zone ratios depend on the workload. Extreme cases, like excessive 583long-term pinning of pages, might not be able to deal with ZONE_MOVABLE at all. 584 585.. note:: 586 587 CMA memory part of a kernel zone essentially behaves like memory in 588 ZONE_MOVABLE and similar considerations apply, especially when combining 589 CMA with ZONE_MOVABLE. 590 591ZONE_MOVABLE Sizing Considerations 592---------------------------------- 593 594We usually expect that a large portion of available system RAM will actually 595be consumed by user space, either directly or indirectly via the page cache. In 596the normal case, ZONE_MOVABLE can be used when allocating such pages just fine. 597 598With that in mind, it makes sense that we can have a big portion of system RAM 599managed by ZONE_MOVABLE. However, there are some things to consider when using 600ZONE_MOVABLE, especially when fine-tuning zone ratios: 601 602- Having a lot of offline memory blocks. Even offline memory blocks consume 603 memory for metadata and page tables in the direct map; having a lot of offline 604 memory blocks is not a typical case, though. 605 606- Memory ballooning without balloon compaction is incompatible with 607 ZONE_MOVABLE. Only some implementations, such as virtio-balloon and 608 pseries CMM, fully support balloon compaction. 609 610 Further, the CONFIG_BALLOON_COMPACTION kernel configuration option might be 611 disabled. In that case, balloon inflation will only perform unmovable 612 allocations and silently create a zone imbalance, usually triggered by 613 inflation requests from the hypervisor. 614 615- Gigantic pages are unmovable, resulting in user space consuming a 616 lot of unmovable memory. 617 618- Huge pages are unmovable when an architectures does not support huge 619 page migration, resulting in a similar issue as with gigantic pages. 620 621- Page tables are unmovable. Excessive swapping, mapping extremely large 622 files or ZONE_DEVICE memory can be problematic, although only really relevant 623 in corner cases. When we manage a lot of user space memory that has been 624 swapped out or is served from a file/persistent memory/... we still need a lot 625 of page tables to manage that memory once user space accessed that memory. 626 627- In certain DAX configurations the memory map for the device memory will be 628 allocated from the kernel zones. 629 630- KASAN can have a significant memory overhead, for example, consuming 1/8th of 631 the total system memory size as (unmovable) tracking metadata. 632 633- Long-term pinning of pages. Techniques that rely on long-term pinnings 634 (especially, RDMA and vfio/mdev) are fundamentally problematic with 635 ZONE_MOVABLE, and therefore, memory offlining. Pinned pages cannot reside 636 on ZONE_MOVABLE as that would turn these pages unmovable. Therefore, they 637 have to be migrated off that zone while pinning. Pinning a page can fail 638 even if there is plenty of free memory in ZONE_MOVABLE. 639 640 In addition, using ZONE_MOVABLE might make page pinning more expensive, 641 because of the page migration overhead. 642 643By default, all the memory configured at boot time is managed by the kernel 644zones and ZONE_MOVABLE is not used. 645 646To enable ZONE_MOVABLE to include the memory present at boot and to control the 647ratio between movable and kernel zones there are two command line options: 648``kernelcore=`` and ``movablecore=``. See 649Documentation/admin-guide/kernel-parameters.rst for their description. 650 651Memory Offlining and ZONE_MOVABLE 652--------------------------------- 653 654Even with ZONE_MOVABLE, there are some corner cases where offlining a memory 655block might fail: 656 657- Memory blocks with memory holes; this applies to memory blocks present during 658 boot and can apply to memory blocks hotplugged via the XEN balloon and the 659 Hyper-V balloon. 660 661- Mixed NUMA nodes and mixed zones within a single memory block prevent memory 662 offlining; this applies to memory blocks present during boot only. 663 664- Special memory blocks prevented by the system from getting offlined. Examples 665 include any memory available during boot on arm64 or memory blocks spanning 666 the crashkernel area on s390x; this usually applies to memory blocks present 667 during boot only. 668 669- Memory blocks overlapping with CMA areas cannot be offlined, this applies to 670 memory blocks present during boot only. 671 672- Concurrent activity that operates on the same physical memory area, such as 673 allocating gigantic pages, can result in temporary offlining failures. 674 675- Out of memory when dissolving huge pages, especially when HugeTLB Vmemmap 676 Optimization (HVO) is enabled. 677 678 Offlining code may be able to migrate huge page contents, but may not be able 679 to dissolve the source huge page because it fails allocating (unmovable) pages 680 for the vmemmap, because the system might not have free memory in the kernel 681 zones left. 682 683 Users that depend on memory offlining to succeed for movable zones should 684 carefully consider whether the memory savings gained from this feature are 685 worth the risk of possibly not being able to offline memory in certain 686 situations. 687 688Further, when running into out of memory situations while migrating pages, or 689when still encountering permanently unmovable pages within ZONE_MOVABLE 690(-> BUG), memory offlining will keep retrying until it eventually succeeds. 691 692When offlining is triggered from user space, the offlining context can be 693terminated by sending a signal. A timeout based offlining can easily be 694implemented via:: 695 696 % timeout $TIMEOUT offline_block | failure_handling 697