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
  76avaiable/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 triggred 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. SPARSEMEM divides the whole memory
  92into chunks of the same size. The chunk is called a "section". The size of
  93a section is architecture dependent. For example, power uses 16MiB, ia64 uses
  941GiB. The unit of online/offline operation is "one section". (see Section 3.)
  95
  96To determine the size of sections, please read this file:
  97
  98/sys/devices/system/memory/block_size_bytes
  99
 100This file shows the size of sections in byte.
 101
 102-----------------------
 1032. Kernel Configuration
 104-----------------------
 105To use memory hotplug feature, kernel must be compiled with following
 106config options.
 107
 108- For all memory hotplug
 109    Memory model -> Sparse Memory  (CONFIG_SPARSEMEM)
 110    Allow for memory hot-add       (CONFIG_MEMORY_HOTPLUG)
 111
 112- To enable memory removal, the followings are also necessary
 113    Allow for memory hot remove    (CONFIG_MEMORY_HOTREMOVE)
 114    Page Migration                 (CONFIG_MIGRATION)
 115
 116- For ACPI memory hotplug, the followings are also necessary
 117    Memory hotplug (under ACPI Support menu) (CONFIG_ACPI_HOTPLUG_MEMORY)
 118    This option can be kernel module.
 119
 120- As a related configuration, if your box has a feature of NUMA-node hotplug
 121  via ACPI, then this option is necessary too.
 122    ACPI0004,PNP0A05 and PNP0A06 Container Driver (under ACPI Support menu)
 123    (CONFIG_ACPI_CONTAINER).
 124    This option can be kernel module too.
 125
 126--------------------------------
 1274 sysfs files for memory hotplug
 128--------------------------------
 129All sections have their device information under /sys/devices/system/memory as
 130
 131/sys/devices/system/memory/memoryXXX
 132(XXX is section id.)
 133
 134Now, XXX is defined as start_address_of_section / section_size.
 135
 136For example, assume 1GiB section size. A device for a memory starting at
 1370x100000000 is /sys/device/system/memory/memory4
 138(0x100000000 / 1Gib = 4)
 139This device covers address range [0x100000000 ... 0x140000000)
 140
 141Under each section, you can see 4 files.
 142
 143/sys/devices/system/memory/memoryXXX/phys_index
 144/sys/devices/system/memory/memoryXXX/phys_device
 145/sys/devices/system/memory/memoryXXX/state
 146/sys/devices/system/memory/memoryXXX/removable
 147
 148'phys_index' : read-only and contains section id, same as XXX.
 149'state'      : read-write
 150               at read:  contains online/offline state of memory.
 151               at write: user can specify "online", "offline" command
 152'phys_device': read-only: designed to show the name of physical memory device.
 153               This is not well implemented now.
 154'removable'  : read-only: contains an integer value indicating
 155               whether the memory section is removable or not
 156               removable.  A value of 1 indicates that the memory
 157               section is removable and a value of 0 indicates that
 158               it is not removable.
 159
 160NOTE:
 161  These directories/files appear after physical memory hotplug phase.
 162
 163If CONFIG_NUMA is enabled the
 164/sys/devices/system/memory/memoryXXX memory section
 165directories can also be accessed via symbolic links located in
 166the /sys/devices/system/node/node* directories.  For example:
 167/sys/devices/system/node/node0/memory9 -> ../../memory/memory9
 168
 169--------------------------------
 1704. Physical memory hot-add phase
 171--------------------------------
 172
 1734.1 Hardware(Firmware) Support
 174------------
 175On x86_64/ia64 platform, memory hotplug by ACPI is supported.
 176
 177In general, the firmware (ACPI) which supports memory hotplug defines
 178memory class object of _HID "PNP0C80". When a notify is asserted to PNP0C80,
 179Linux's ACPI handler does hot-add memory to the system and calls a hotplug udev
 180script. This will be done automatically.
 181
 182But scripts for memory hotplug are not contained in generic udev package(now).
 183You may have to write it by yourself or online/offline memory by hand.
 184Please see "How to online memory", "How to offline memory" in this text.
 185
 186If firmware supports NUMA-node hotplug, and defines an object _HID "ACPI0004",
 187"PNP0A05", or "PNP0A06", notification is asserted to it, and ACPI handler
 188calls hotplug code for all of objects which are defined in it.
 189If memory device is found, memory hotplug code will be called.
 190
 191
 1924.2 Notify memory hot-add event by hand
 193------------
 194In some environments, especially virtualized environment, firmware will not
 195notify memory hotplug event to the kernel. For such environment, "probe"
 196interface is supported. This interface depends on CONFIG_ARCH_MEMORY_PROBE.
 197
 198Now, CONFIG_ARCH_MEMORY_PROBE is supported only by powerpc but it does not
 199contain highly architecture codes. Please add config if you need "probe"
 200interface.
 201
 202Probe interface is located at
 203/sys/devices/system/memory/probe
 204
 205You can tell the physical address of new memory to the kernel by
 206
 207% echo start_address_of_new_memory > /sys/devices/system/memory/probe
 208
 209Then, [start_address_of_new_memory, start_address_of_new_memory + section_size)
 210memory range is hot-added. In this case, hotplug script is not called (in
 211current implementation). You'll have to online memory by yourself.
 212Please see "How to online memory" in this text.
 213
 214
 215
 216------------------------------
 2175. Logical Memory hot-add phase
 218------------------------------
 219
 2205.1. State of memory
 221------------
 222To see (online/offline) state of memory section, read 'state' file.
 223
 224% cat /sys/device/system/memory/memoryXXX/state
 225
 226
 227If the memory section is online, you'll read "online".
 228If the memory section is offline, you'll read "offline".
 229
 230
 2315.2. How to online memory
 232------------
 233Even if the memory is hot-added, it is not at ready-to-use state.
 234For using newly added memory, you have to "online" the memory section.
 235
 236For onlining, you have to write "online" to the section's state file as:
 237
 238% echo online > /sys/devices/system/memory/memoryXXX/state
 239
 240After this, section memoryXXX's state will be 'online' and the amount of
 241available memory will be increased.
 242
 243Currently, newly added memory is added as ZONE_NORMAL (for powerpc, ZONE_DMA).
 244This may be changed in future.
 245
 246
 247
 248------------------------
 2496. Logical memory remove
 250------------------------
 251
 2526.1 Memory offline and ZONE_MOVABLE
 253------------
 254Memory offlining is more complicated than memory online. Because memory offline
 255has to make the whole memory section be unused, memory offline can fail if
 256the section includes memory which cannot be freed.
 257
 258In general, memory offline can use 2 techniques.
 259
 260(1) reclaim and free all memory in the section.
 261(2) migrate all pages in the section.
 262
 263In the current implementation, Linux's memory offline uses method (2), freeing
 264all  pages in the section by page migration. But not all pages are
 265migratable. Under current Linux, migratable pages are anonymous pages and
 266page caches. For offlining a section by migration, the kernel has to guarantee
 267that the section contains only migratable pages.
 268
 269Now, a boot option for making a section which consists of migratable pages is
 270supported. By specifying "kernelcore=" or "movablecore=" boot option, you can
 271create ZONE_MOVABLE...a zone which is just used for movable pages.
 272(See also Documentation/kernel-parameters.txt)
 273
 274Assume the system has "TOTAL" amount of memory at boot time, this boot option
 275creates ZONE_MOVABLE as following.
 276
 2771) When kernelcore=YYYY boot option is used,
 278  Size of memory not for movable pages (not for offline) is YYYY.
 279  Size of memory for movable pages (for offline) is TOTAL-YYYY.
 280
 2812) When movablecore=ZZZZ boot option is used,
 282  Size of memory not for movable pages (not for offline) is TOTAL - ZZZZ.
 283  Size of memory for movable pages (for offline) is ZZZZ.
 284
 285
 286Note) Unfortunately, there is no information to show which section belongs
 287to ZONE_MOVABLE. This is TBD.
 288
 289
 2906.2. How to offline memory
 291------------
 292You can offline a section by using the same sysfs interface that was used in
 293memory onlining.
 294
 295% echo offline > /sys/devices/system/memory/memoryXXX/state
 296
 297If offline succeeds, the state of the memory section is changed to be "offline".
 298If it fails, some error core (like -EBUSY) will be returned by the kernel.
 299Even if a section does not belong to ZONE_MOVABLE, you can try to offline it.
 300If it doesn't contain 'unmovable' memory, you'll get success.
 301
 302A section under ZONE_MOVABLE is considered to be able to be offlined easily.
 303But under some busy state, it may return -EBUSY. Even if a memory section
 304cannot be offlined due to -EBUSY, you can retry offlining it and may be able to
 305offline it (or not).
 306(For example, a page is referred to by some kernel internal call and released
 307 soon.)
 308
 309Consideration:
 310Memory hotplug's design direction is to make the possibility of memory offlining
 311higher and to guarantee unplugging memory under any situation. But it needs
 312more work. Returning -EBUSY under some situation may be good because the user
 313can decide to retry more or not by himself. Currently, memory offlining code
 314does some amount of retry with 120 seconds timeout.
 315
 316-------------------------
 3177. Physical memory remove
 318-------------------------
 319Need more implementation yet....
 320 - Notification completion of remove works by OS to firmware.
 321 - Guard from remove if not yet.
 322
 323--------------------------------
 3248. Memory hotplug event notifier
 325--------------------------------
 326Memory hotplug has event notifer. There are 6 types of notification.
 327
 328MEMORY_GOING_ONLINE
 329  Generated before new memory becomes available in order to be able to
 330  prepare subsystems to handle memory. The page allocator is still unable
 331  to allocate from the new memory.
 332
 333MEMORY_CANCEL_ONLINE
 334  Generated if MEMORY_GOING_ONLINE fails.
 335
 336MEMORY_ONLINE
 337  Generated when memory has succesfully brought online. The callback may
 338  allocate pages from the new memory.
 339
 340MEMORY_GOING_OFFLINE
 341  Generated to begin the process of offlining memory. Allocations are no
 342  longer possible from the memory but some of the memory to be offlined
 343  is still in use. The callback can be used to free memory known to a
 344  subsystem from the indicated memory section.
 345
 346MEMORY_CANCEL_OFFLINE
 347  Generated if MEMORY_GOING_OFFLINE fails. Memory is available again from
 348  the section that we attempted to offline.
 349
 350MEMORY_OFFLINE
 351  Generated after offlining memory is complete.
 352
 353A callback routine can be registered by
 354  hotplug_memory_notifier(callback_func, priority)
 355
 356The second argument of callback function (action) is event types of above.
 357The third argument is passed by pointer of struct memory_notify.
 358
 359struct memory_notify {
 360       unsigned long start_pfn;
 361       unsigned long nr_pages;
 362       int status_cahnge_nid;
 363}
 364
 365start_pfn is start_pfn of online/offline memory.
 366nr_pages is # of pages of online/offline memory.
 367status_change_nid is set node id when N_HIGH_MEMORY of nodemask is (will be)
 368set/clear. It means a new(memoryless) node gets new memory by online and a
 369node loses all memory. If this is -1, then nodemask status is not changed.
 370If status_changed_nid >= 0, callback should create/discard structures for the
 371node if necessary.
 372
 373--------------
 3749. Future Work
 375--------------
 376  - allowing memory hot-add to ZONE_MOVABLE. maybe we need some switch like
 377    sysctl or new control file.
 378  - showing memory section and physical device relationship.
 379  - showing memory section is under ZONE_MOVABLE or not
 380  - test and make it better memory offlining.
 381  - support HugeTLB page migration and offlining.
 382  - memmap removing at memory offline.
 383  - physical remove memory.
 384
 385
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