linux/Documentation/memory-hotplug.txt
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   1==============
   2Memory Hotplug
   3==============
   4
   5Created:                                        Jul 28 2007
   6Add description of notifier of memory hotplug   Oct 11 2007
   7
   8This document is about memory hotplug including how-to-use and current status.
   9Because Memory Hotplug is still under development, contents of this text will
  10be changed often.
  11
  121. Introduction
  13  1.1 purpose of memory hotplug
  14  1.2. Phases of memory hotplug
  15  1.3. Unit of Memory online/offline operation
  162. Kernel Configuration
  173. sysfs files for memory hotplug
  184. Physical memory hot-add phase
  19  4.1 Hardware(Firmware) Support
  20  4.2 Notify memory hot-add event by hand
  215. Logical Memory hot-add phase
  22  5.1. State of memory
  23  5.2. How to online memory
  246. Logical memory remove
  25  6.1 Memory offline and ZONE_MOVABLE
  26  6.2. How to offline memory
  277. Physical memory remove
  288. Memory hotplug event notifier
  299. Future Work List
  30
  31Note(1): x86_64's has special implementation for memory hotplug.
  32         This text does not describe it.
  33Note(2): This text assumes that sysfs is mounted at /sys.
  34
  35
  36---------------
  371. Introduction
  38---------------
  39
  401.1 purpose of memory hotplug
  41------------
  42Memory Hotplug allows users to increase/decrease the amount of memory.
  43Generally, there are two purposes.
  44
  45(A) For changing the amount of memory.
  46    This is to allow a feature like capacity on demand.
  47(B) For installing/removing DIMMs or NUMA-nodes physically.
  48    This is to exchange DIMMs/NUMA-nodes, reduce power consumption, etc.
  49
  50(A) is required by highly virtualized environments and (B) is required by
  51hardware which supports memory power management.
  52
  53Linux memory hotplug is designed for both purpose.
  54
  55
  561.2. Phases of memory hotplug
  57---------------
  58There are 2 phases in Memory Hotplug.
  59  1) Physical Memory Hotplug phase
  60  2) Logical Memory Hotplug phase.
  61
  62The First phase is to communicate hardware/firmware and make/erase
  63environment for hotplugged memory. Basically, this phase is necessary
  64for the purpose (B), but this is good phase for communication between
  65highly virtualized environments too.
  66
  67When memory is hotplugged, the kernel recognizes new memory, makes new memory
  68management tables, and makes sysfs files for new memory's operation.
  69
  70If firmware supports notification of connection of new memory to OS,
  71this phase is triggered automatically. ACPI can notify this event. If not,
  72"probe" operation by system administration is used instead.
  73(see Section 4.).
  74
  75Logical Memory Hotplug phase is to change memory state into
  76available/unavailable for users. Amount of memory from user's view is
  77changed by this phase. The kernel makes all memory in it as free pages
  78when a memory range is available.
  79
  80In this document, this phase is described as online/offline.
  81
  82Logical Memory Hotplug phase is triggered by write of sysfs file by system
  83administrator. For the hot-add case, it must be executed after Physical Hotplug
  84phase by hand.
  85(However, if you writes udev's hotplug scripts for memory hotplug, these
  86 phases can be execute in seamless way.)
  87
  88
  891.3. Unit of Memory online/offline operation
  90------------
  91Memory hotplug uses SPARSEMEM memory model which allows memory to be divided
  92into chunks of the same size. These chunks are called "sections". The size of
  93a memory section is architecture dependent. For example, power uses 16MiB, ia64
  94uses 1GiB.
  95
  96Memory sections are combined into chunks referred to as "memory blocks". The
  97size of a memory block is architecture dependent and represents the logical
  98unit upon which memory online/offline operations are to be performed. The
  99default size of a memory block is the same as memory section size unless an
 100architecture specifies otherwise. (see Section 3.)
 101
 102To determine the size (in bytes) of a memory block please read this file:
 103
 104/sys/devices/system/memory/block_size_bytes
 105
 106
 107-----------------------
 1082. Kernel Configuration
 109-----------------------
 110To use memory hotplug feature, kernel must be compiled with following
 111config options.
 112
 113- For all memory hotplug
 114    Memory model -> Sparse Memory  (CONFIG_SPARSEMEM)
 115    Allow for memory hot-add       (CONFIG_MEMORY_HOTPLUG)
 116
 117- To enable memory removal, the followings are also necessary
 118    Allow for memory hot remove    (CONFIG_MEMORY_HOTREMOVE)
 119    Page Migration                 (CONFIG_MIGRATION)
 120
 121- For ACPI memory hotplug, the followings are also necessary
 122    Memory hotplug (under ACPI Support menu) (CONFIG_ACPI_HOTPLUG_MEMORY)
 123    This option can be kernel module.
 124
 125- As a related configuration, if your box has a feature of NUMA-node hotplug
 126  via ACPI, then this option is necessary too.
 127    ACPI0004,PNP0A05 and PNP0A06 Container Driver (under ACPI Support menu)
 128    (CONFIG_ACPI_CONTAINER).
 129    This option can be kernel module too.
 130
 131
 132--------------------------------
 1333 sysfs files for memory hotplug
 134--------------------------------
 135All memory blocks have their device information in sysfs.  Each memory block
 136is described under /sys/devices/system/memory as
 137
 138/sys/devices/system/memory/memoryXXX
 139(XXX is the memory block id.)
 140
 141For the memory block covered by the sysfs directory.  It is expected that all
 142memory sections in this range are present and no memory holes exist in the
 143range. Currently there is no way to determine if there is a memory hole, but
 144the existence of one should not affect the hotplug capabilities of the memory
 145block.
 146
 147For example, assume 1GiB memory block size. A device for a memory starting at
 1480x100000000 is /sys/device/system/memory/memory4
 149(0x100000000 / 1Gib = 4)
 150This device covers address range [0x100000000 ... 0x140000000)
 151
 152Under each memory block, you can see 5 files:
 153
 154/sys/devices/system/memory/memoryXXX/phys_index
 155/sys/devices/system/memory/memoryXXX/phys_device
 156/sys/devices/system/memory/memoryXXX/state
 157/sys/devices/system/memory/memoryXXX/removable
 158/sys/devices/system/memory/memoryXXX/valid_zones
 159
 160'phys_index'      : read-only and contains memory block id, same as XXX.
 161'state'           : read-write
 162                    at read:  contains online/offline state of memory.
 163                    at write: user can specify "online_kernel",
 164                    "online_movable", "online", "offline" command
 165                    which will be performed on all sections in the block.
 166'phys_device'     : read-only: designed to show the name of physical memory
 167                    device.  This is not well implemented now.
 168'removable'       : read-only: contains an integer value indicating
 169                    whether the memory block is removable or not
 170                    removable.  A value of 1 indicates that the memory
 171                    block is removable and a value of 0 indicates that
 172                    it is not removable. A memory block is removable only if
 173                    every section in the block is removable.
 174'valid_zones'     : read-only: designed to show which zones this memory block
 175                    can be onlined to.
 176                    The first column shows it's default zone.
 177                    "memory6/valid_zones: Normal Movable" shows this memoryblock
 178                    can be onlined to ZONE_NORMAL by default and to ZONE_MOVABLE
 179                    by online_movable.
 180                    "memory7/valid_zones: Movable Normal" shows this memoryblock
 181                    can be onlined to ZONE_MOVABLE by default and to ZONE_NORMAL
 182                    by online_kernel.
 183
 184NOTE:
 185  These directories/files appear after physical memory hotplug phase.
 186
 187If CONFIG_NUMA is enabled the memoryXXX/ directories can also be accessed
 188via symbolic links located in the /sys/devices/system/node/node* directories.
 189
 190For example:
 191/sys/devices/system/node/node0/memory9 -> ../../memory/memory9
 192
 193A backlink will also be created:
 194/sys/devices/system/memory/memory9/node0 -> ../../node/node0
 195
 196
 197--------------------------------
 1984. Physical memory hot-add phase
 199--------------------------------
 200
 2014.1 Hardware(Firmware) Support
 202------------
 203On x86_64/ia64 platform, memory hotplug by ACPI is supported.
 204
 205In general, the firmware (ACPI) which supports memory hotplug defines
 206memory class object of _HID "PNP0C80". When a notify is asserted to PNP0C80,
 207Linux's ACPI handler does hot-add memory to the system and calls a hotplug udev
 208script. This will be done automatically.
 209
 210But scripts for memory hotplug are not contained in generic udev package(now).
 211You may have to write it by yourself or online/offline memory by hand.
 212Please see "How to online memory", "How to offline memory" in this text.
 213
 214If firmware supports NUMA-node hotplug, and defines an object _HID "ACPI0004",
 215"PNP0A05", or "PNP0A06", notification is asserted to it, and ACPI handler
 216calls hotplug code for all of objects which are defined in it.
 217If memory device is found, memory hotplug code will be called.
 218
 219
 2204.2 Notify memory hot-add event by hand
 221------------
 222On some architectures, the firmware may not notify the kernel of a memory
 223hotplug event.  Therefore, the memory "probe" interface is supported to
 224explicitly notify the kernel.  This interface depends on
 225CONFIG_ARCH_MEMORY_PROBE and can be configured on powerpc, sh, and x86
 226if hotplug is supported, although for x86 this should be handled by ACPI
 227notification.
 228
 229Probe interface is located at
 230/sys/devices/system/memory/probe
 231
 232You can tell the physical address of new memory to the kernel by
 233
 234% echo start_address_of_new_memory > /sys/devices/system/memory/probe
 235
 236Then, [start_address_of_new_memory, start_address_of_new_memory +
 237memory_block_size] memory range is hot-added. In this case, hotplug script is
 238not called (in current implementation). You'll have to online memory by
 239yourself.  Please see "How to online memory" in this text.
 240
 241
 242------------------------------
 2435. Logical Memory hot-add phase
 244------------------------------
 245
 2465.1. State of memory
 247------------
 248To see (online/offline) state of a memory block, read 'state' file.
 249
 250% cat /sys/device/system/memory/memoryXXX/state
 251
 252
 253If the memory block is online, you'll read "online".
 254If the memory block is offline, you'll read "offline".
 255
 256
 2575.2. How to online memory
 258------------
 259When the memory is hot-added, the kernel decides whether or not to "online"
 260it according to the policy which can be read from "auto_online_blocks" file:
 261
 262% cat /sys/devices/system/memory/auto_online_blocks
 263
 264The default depends on the CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE kernel config
 265option. If it is disabled the default is "offline" which means the newly added
 266memory is not in a ready-to-use state and you have to "online" the newly added
 267memory blocks manually. Automatic onlining can be requested by writing "online"
 268to "auto_online_blocks" file:
 269
 270% echo online > /sys/devices/system/memory/auto_online_blocks
 271
 272This sets a global policy and impacts all memory blocks that will subsequently
 273be hotplugged. Currently offline blocks keep their state. It is possible, under
 274certain circumstances, that some memory blocks will be added but will fail to
 275online. User space tools can check their "state" files
 276(/sys/devices/system/memory/memoryXXX/state) and try to online them manually.
 277
 278If the automatic onlining wasn't requested, failed, or some memory block was
 279offlined it is possible to change the individual block's state by writing to the
 280"state" file:
 281
 282% echo online > /sys/devices/system/memory/memoryXXX/state
 283
 284This onlining will not change the ZONE type of the target memory block,
 285If the memory block is in ZONE_NORMAL, you can change it to ZONE_MOVABLE:
 286
 287% echo online_movable > /sys/devices/system/memory/memoryXXX/state
 288(NOTE: current limit: this memory block must be adjacent to ZONE_MOVABLE)
 289
 290And if the memory block is in ZONE_MOVABLE, you can change it to ZONE_NORMAL:
 291
 292% echo online_kernel > /sys/devices/system/memory/memoryXXX/state
 293(NOTE: current limit: this memory block must be adjacent to ZONE_NORMAL)
 294
 295After this, memory block XXX's state will be 'online' and the amount of
 296available memory will be increased.
 297
 298Currently, newly added memory is added as ZONE_NORMAL (for powerpc, ZONE_DMA).
 299This may be changed in future.
 300
 301
 302
 303------------------------
 3046. Logical memory remove
 305------------------------
 306
 3076.1 Memory offline and ZONE_MOVABLE
 308------------
 309Memory offlining is more complicated than memory online. Because memory offline
 310has to make the whole memory block be unused, memory offline can fail if
 311the memory block includes memory which cannot be freed.
 312
 313In general, memory offline can use 2 techniques.
 314
 315(1) reclaim and free all memory in the memory block.
 316(2) migrate all pages in the memory block.
 317
 318In the current implementation, Linux's memory offline uses method (2), freeing
 319all  pages in the memory block by page migration. But not all pages are
 320migratable. Under current Linux, migratable pages are anonymous pages and
 321page caches. For offlining a memory block by migration, the kernel has to
 322guarantee that the memory block contains only migratable pages.
 323
 324Now, a boot option for making a memory block which consists of migratable pages
 325is supported. By specifying "kernelcore=" or "movablecore=" boot option, you can
 326create ZONE_MOVABLE...a zone which is just used for movable pages.
 327(See also Documentation/kernel-parameters.txt)
 328
 329Assume the system has "TOTAL" amount of memory at boot time, this boot option
 330creates ZONE_MOVABLE as following.
 331
 3321) When kernelcore=YYYY boot option is used,
 333  Size of memory not for movable pages (not for offline) is YYYY.
 334  Size of memory for movable pages (for offline) is TOTAL-YYYY.
 335
 3362) When movablecore=ZZZZ boot option is used,
 337  Size of memory not for movable pages (not for offline) is TOTAL - ZZZZ.
 338  Size of memory for movable pages (for offline) is ZZZZ.
 339
 340
 341Note: Unfortunately, there is no information to show which memory block belongs
 342to ZONE_MOVABLE. This is TBD.
 343
 344
 3456.2. How to offline memory
 346------------
 347You can offline a memory block by using the same sysfs interface that was used
 348in memory onlining.
 349
 350% echo offline > /sys/devices/system/memory/memoryXXX/state
 351
 352If offline succeeds, the state of the memory block is changed to be "offline".
 353If it fails, some error core (like -EBUSY) will be returned by the kernel.
 354Even if a memory block does not belong to ZONE_MOVABLE, you can try to offline
 355it.  If it doesn't contain 'unmovable' memory, you'll get success.
 356
 357A memory block under ZONE_MOVABLE is considered to be able to be offlined
 358easily.  But under some busy state, it may return -EBUSY. Even if a memory
 359block cannot be offlined due to -EBUSY, you can retry offlining it and may be
 360able to offline it (or not). (For example, a page is referred to by some kernel
 361internal call and released soon.)
 362
 363Consideration:
 364Memory hotplug's design direction is to make the possibility of memory offlining
 365higher and to guarantee unplugging memory under any situation. But it needs
 366more work. Returning -EBUSY under some situation may be good because the user
 367can decide to retry more or not by himself. Currently, memory offlining code
 368does some amount of retry with 120 seconds timeout.
 369
 370-------------------------
 3717. Physical memory remove
 372-------------------------
 373Need more implementation yet....
 374 - Notification completion of remove works by OS to firmware.
 375 - Guard from remove if not yet.
 376
 377--------------------------------
 3788. Memory hotplug event notifier
 379--------------------------------
 380Hotplugging events are sent to a notification queue.
 381
 382There are six types of notification defined in include/linux/memory.h:
 383
 384MEM_GOING_ONLINE
 385  Generated before new memory becomes available in order to be able to
 386  prepare subsystems to handle memory. The page allocator is still unable
 387  to allocate from the new memory.
 388
 389MEM_CANCEL_ONLINE
 390  Generated if MEMORY_GOING_ONLINE fails.
 391
 392MEM_ONLINE
 393  Generated when memory has successfully brought online. The callback may
 394  allocate pages from the new memory.
 395
 396MEM_GOING_OFFLINE
 397  Generated to begin the process of offlining memory. Allocations are no
 398  longer possible from the memory but some of the memory to be offlined
 399  is still in use. The callback can be used to free memory known to a
 400  subsystem from the indicated memory block.
 401
 402MEM_CANCEL_OFFLINE
 403  Generated if MEMORY_GOING_OFFLINE fails. Memory is available again from
 404  the memory block that we attempted to offline.
 405
 406MEM_OFFLINE
 407  Generated after offlining memory is complete.
 408
 409A callback routine can be registered by calling
 410
 411  hotplug_memory_notifier(callback_func, priority)
 412
 413Callback functions with higher values of priority are called before callback
 414functions with lower values.
 415
 416A callback function must have the following prototype:
 417
 418  int callback_func(
 419    struct notifier_block *self, unsigned long action, void *arg);
 420
 421The first argument of the callback function (self) is a pointer to the block
 422of the notifier chain that points to the callback function itself.
 423The second argument (action) is one of the event types described above.
 424The third argument (arg) passes a pointer of struct memory_notify.
 425
 426struct memory_notify {
 427       unsigned long start_pfn;
 428       unsigned long nr_pages;
 429       int status_change_nid_normal;
 430       int status_change_nid_high;
 431       int status_change_nid;
 432}
 433
 434start_pfn is start_pfn of online/offline memory.
 435nr_pages is # of pages of online/offline memory.
 436status_change_nid_normal is set node id when N_NORMAL_MEMORY of nodemask
 437is (will be) set/clear, if this is -1, then nodemask status is not changed.
 438status_change_nid_high is set node id when N_HIGH_MEMORY of nodemask
 439is (will be) set/clear, if this is -1, then nodemask status is not changed.
 440status_change_nid is set node id when N_MEMORY of nodemask is (will be)
 441set/clear. It means a new(memoryless) node gets new memory by online and a
 442node loses all memory. If this is -1, then nodemask status is not changed.
 443If status_changed_nid* >= 0, callback should create/discard structures for the
 444node if necessary.
 445
 446The callback routine shall return one of the values
 447NOTIFY_DONE, NOTIFY_OK, NOTIFY_BAD, NOTIFY_STOP
 448defined in include/linux/notifier.h
 449
 450NOTIFY_DONE and NOTIFY_OK have no effect on the further processing.
 451
 452NOTIFY_BAD is used as response to the MEM_GOING_ONLINE, MEM_GOING_OFFLINE,
 453MEM_ONLINE, or MEM_OFFLINE action to cancel hotplugging. It stops
 454further processing of the notification queue.
 455
 456NOTIFY_STOP stops further processing of the notification queue.
 457
 458--------------
 4599. Future Work
 460--------------
 461  - allowing memory hot-add to ZONE_MOVABLE. maybe we need some switch like
 462    sysctl or new control file.
 463  - showing memory block and physical device relationship.
 464  - test and make it better memory offlining.
 465  - support HugeTLB page migration and offlining.
 466  - memmap removing at memory offline.
 467  - physical remove memory.
 468
 469