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Linux/Documentation/memory-hotplug.txt

  1 ==============
  2 Memory Hotplug
  3 ==============
  4 
  5 Created:                                        Jul 28 2007
  6 Add description of notifier of memory hotplug   Oct 11 2007
  7 
  8 This document is about memory hotplug including how-to-use and current status.
  9 Because Memory Hotplug is still under development, contents of this text will
 10 be changed often.
 11 
 12 1. Introduction
 13   1.1 purpose of memory hotplug
 14   1.2. Phases of memory hotplug
 15   1.3. Unit of Memory online/offline operation
 16 2. Kernel Configuration
 17 3. sysfs files for memory hotplug
 18 4. Physical memory hot-add phase
 19   4.1 Hardware(Firmware) Support
 20   4.2 Notify memory hot-add event by hand
 21 5. Logical Memory hot-add phase
 22   5.1. State of memory
 23   5.2. How to online memory
 24 6. Logical memory remove
 25   6.1 Memory offline and ZONE_MOVABLE
 26   6.2. How to offline memory
 27 7. Physical memory remove
 28 8. Memory hotplug event notifier
 29 9. Future Work List
 30 
 31 Note(1): x86_64's has special implementation for memory hotplug.
 32          This text does not describe it.
 33 Note(2): This text assumes that sysfs is mounted at /sys.
 34 
 35 
 36 ---------------
 37 1. Introduction
 38 ---------------
 39 
 40 1.1 purpose of memory hotplug
 41 ------------
 42 Memory Hotplug allows users to increase/decrease the amount of memory.
 43 Generally, 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
 51 hardware which supports memory power management.
 52 
 53 Linux memory hotplug is designed for both purpose.
 54 
 55 
 56 1.2. Phases of memory hotplug
 57 ---------------
 58 There are 2 phases in Memory Hotplug.
 59   1) Physical Memory Hotplug phase
 60   2) Logical Memory Hotplug phase.
 61 
 62 The First phase is to communicate hardware/firmware and make/erase
 63 environment for hotplugged memory. Basically, this phase is necessary
 64 for the purpose (B), but this is good phase for communication between
 65 highly virtualized environments too.
 66 
 67 When memory is hotplugged, the kernel recognizes new memory, makes new memory
 68 management tables, and makes sysfs files for new memory's operation.
 69 
 70 If firmware supports notification of connection of new memory to OS,
 71 this 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 
 75 Logical Memory Hotplug phase is to change memory state into
 76 available/unavailable for users. Amount of memory from user's view is
 77 changed by this phase. The kernel makes all memory in it as free pages
 78 when a memory range is available.
 79 
 80 In this document, this phase is described as online/offline.
 81 
 82 Logical Memory Hotplug phase is triggered by write of sysfs file by system
 83 administrator. For the hot-add case, it must be executed after Physical Hotplug
 84 phase 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 
 89 1.3. Unit of Memory online/offline operation
 90 ------------
 91 Memory hotplug uses SPARSEMEM memory model which allows memory to be divided
 92 into chunks of the same size. These chunks are called "sections". The size of
 93 a memory section is architecture dependent. For example, power uses 16MiB, ia64
 94 uses 1GiB.
 95 
 96 Memory sections are combined into chunks referred to as "memory blocks". The
 97 size of a memory block is architecture dependent and represents the logical
 98 unit upon which memory online/offline operations are to be performed. The
 99 default size of a memory block is the same as memory section size unless an
100 architecture specifies otherwise. (see Section 3.)
101 
102 To 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 -----------------------
108 2. Kernel Configuration
109 -----------------------
110 To use memory hotplug feature, kernel must be compiled with following
111 config 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 --------------------------------
133 3 sysfs files for memory hotplug
134 --------------------------------
135 All memory blocks have their device information in sysfs.  Each memory block
136 is described under /sys/devices/system/memory as
137 
138 /sys/devices/system/memory/memoryXXX
139 (XXX is the memory block id.)
140 
141 For the memory block covered by the sysfs directory.  It is expected that all
142 memory sections in this range are present and no memory holes exist in the
143 range. Currently there is no way to determine if there is a memory hole, but
144 the existence of one should not affect the hotplug capabilities of the memory
145 block.
146 
147 For example, assume 1GiB memory block size. A device for a memory starting at
148 0x100000000 is /sys/device/system/memory/memory4
149 (0x100000000 / 1Gib = 4)
150 This device covers address range [0x100000000 ... 0x140000000)
151 
152 Under 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 
184 NOTE:
185   These directories/files appear after physical memory hotplug phase.
186 
187 If CONFIG_NUMA is enabled the memoryXXX/ directories can also be accessed
188 via symbolic links located in the /sys/devices/system/node/node* directories.
189 
190 For example:
191 /sys/devices/system/node/node0/memory9 -> ../../memory/memory9
192 
193 A backlink will also be created:
194 /sys/devices/system/memory/memory9/node0 -> ../../node/node0
195 
196 
197 --------------------------------
198 4. Physical memory hot-add phase
199 --------------------------------
200 
201 4.1 Hardware(Firmware) Support
202 ------------
203 On x86_64/ia64 platform, memory hotplug by ACPI is supported.
204 
205 In general, the firmware (ACPI) which supports memory hotplug defines
206 memory class object of _HID "PNP0C80". When a notify is asserted to PNP0C80,
207 Linux's ACPI handler does hot-add memory to the system and calls a hotplug udev
208 script. This will be done automatically.
209 
210 But scripts for memory hotplug are not contained in generic udev package(now).
211 You may have to write it by yourself or online/offline memory by hand.
212 Please see "How to online memory", "How to offline memory" in this text.
213 
214 If firmware supports NUMA-node hotplug, and defines an object _HID "ACPI0004",
215 "PNP0A05", or "PNP0A06", notification is asserted to it, and ACPI handler
216 calls hotplug code for all of objects which are defined in it.
217 If memory device is found, memory hotplug code will be called.
218 
219 
220 4.2 Notify memory hot-add event by hand
221 ------------
222 On some architectures, the firmware may not notify the kernel of a memory
223 hotplug event.  Therefore, the memory "probe" interface is supported to
224 explicitly notify the kernel.  This interface depends on
225 CONFIG_ARCH_MEMORY_PROBE and can be configured on powerpc, sh, and x86
226 if hotplug is supported, although for x86 this should be handled by ACPI
227 notification.
228 
229 Probe interface is located at
230 /sys/devices/system/memory/probe
231 
232 You 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 
236 Then, [start_address_of_new_memory, start_address_of_new_memory +
237 memory_block_size] memory range is hot-added. In this case, hotplug script is
238 not called (in current implementation). You'll have to online memory by
239 yourself.  Please see "How to online memory" in this text.
240 
241 
242 ------------------------------
243 5. Logical Memory hot-add phase
244 ------------------------------
245 
246 5.1. State of memory
247 ------------
248 To see (online/offline) state of a memory block, read 'state' file.
249 
250 % cat /sys/device/system/memory/memoryXXX/state
251 
252 
253 If the memory block is online, you'll read "online".
254 If the memory block is offline, you'll read "offline".
255 
256 
257 5.2. How to online memory
258 ------------
259 When the memory is hot-added, the kernel decides whether or not to "online"
260 it according to the policy which can be read from "auto_online_blocks" file:
261 
262 % cat /sys/devices/system/memory/auto_online_blocks
263 
264 The default depends on the CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE kernel config
265 option. If it is disabled the default is "offline" which means the newly added
266 memory is not in a ready-to-use state and you have to "online" the newly added
267 memory blocks manually. Automatic onlining can be requested by writing "online"
268 to "auto_online_blocks" file:
269 
270 % echo online > /sys/devices/system/memory/auto_online_blocks
271 
272 This sets a global policy and impacts all memory blocks that will subsequently
273 be hotplugged. Currently offline blocks keep their state. It is possible, under
274 certain circumstances, that some memory blocks will be added but will fail to
275 online. User space tools can check their "state" files
276 (/sys/devices/system/memory/memoryXXX/state) and try to online them manually.
277 
278 If the automatic onlining wasn't requested, failed, or some memory block was
279 offlined 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 
284 This onlining will not change the ZONE type of the target memory block,
285 If 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 
290 And 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 
295 After this, memory block XXX's state will be 'online' and the amount of
296 available memory will be increased.
297 
298 Currently, newly added memory is added as ZONE_NORMAL (for powerpc, ZONE_DMA).
299 This may be changed in future.
300 
301 
302 
303 ------------------------
304 6. Logical memory remove
305 ------------------------
306 
307 6.1 Memory offline and ZONE_MOVABLE
308 ------------
309 Memory offlining is more complicated than memory online. Because memory offline
310 has to make the whole memory block be unused, memory offline can fail if
311 the memory block includes memory which cannot be freed.
312 
313 In 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 
318 In the current implementation, Linux's memory offline uses method (2), freeing
319 all  pages in the memory block by page migration. But not all pages are
320 migratable. Under current Linux, migratable pages are anonymous pages and
321 page caches. For offlining a memory block by migration, the kernel has to
322 guarantee that the memory block contains only migratable pages.
323 
324 Now, a boot option for making a memory block which consists of migratable pages
325 is supported. By specifying "kernelcore=" or "movablecore=" boot option, you can
326 create ZONE_MOVABLE...a zone which is just used for movable pages.
327 (See also Documentation/admin-guide/kernel-parameters.rst)
328 
329 Assume the system has "TOTAL" amount of memory at boot time, this boot option
330 creates ZONE_MOVABLE as following.
331 
332 1) 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 
336 2) 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 
341 Note: Unfortunately, there is no information to show which memory block belongs
342 to ZONE_MOVABLE. This is TBD.
343 
344 
345 6.2. How to offline memory
346 ------------
347 You can offline a memory block by using the same sysfs interface that was used
348 in memory onlining.
349 
350 % echo offline > /sys/devices/system/memory/memoryXXX/state
351 
352 If offline succeeds, the state of the memory block is changed to be "offline".
353 If it fails, some error core (like -EBUSY) will be returned by the kernel.
354 Even if a memory block does not belong to ZONE_MOVABLE, you can try to offline
355 it.  If it doesn't contain 'unmovable' memory, you'll get success.
356 
357 A memory block under ZONE_MOVABLE is considered to be able to be offlined
358 easily.  But under some busy state, it may return -EBUSY. Even if a memory
359 block cannot be offlined due to -EBUSY, you can retry offlining it and may be
360 able to offline it (or not). (For example, a page is referred to by some kernel
361 internal call and released soon.)
362 
363 Consideration:
364 Memory hotplug's design direction is to make the possibility of memory offlining
365 higher and to guarantee unplugging memory under any situation. But it needs
366 more work. Returning -EBUSY under some situation may be good because the user
367 can decide to retry more or not by himself. Currently, memory offlining code
368 does some amount of retry with 120 seconds timeout.
369 
370 -------------------------
371 7. Physical memory remove
372 -------------------------
373 Need more implementation yet....
374  - Notification completion of remove works by OS to firmware.
375  - Guard from remove if not yet.
376 
377 --------------------------------
378 8. Memory hotplug event notifier
379 --------------------------------
380 Hotplugging events are sent to a notification queue.
381 
382 There are six types of notification defined in include/linux/memory.h:
383 
384 MEM_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 
389 MEM_CANCEL_ONLINE
390   Generated if MEMORY_GOING_ONLINE fails.
391 
392 MEM_ONLINE
393   Generated when memory has successfully brought online. The callback may
394   allocate pages from the new memory.
395 
396 MEM_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 
402 MEM_CANCEL_OFFLINE
403   Generated if MEMORY_GOING_OFFLINE fails. Memory is available again from
404   the memory block that we attempted to offline.
405 
406 MEM_OFFLINE
407   Generated after offlining memory is complete.
408 
409 A callback routine can be registered by calling
410 
411   hotplug_memory_notifier(callback_func, priority)
412 
413 Callback functions with higher values of priority are called before callback
414 functions with lower values.
415 
416 A callback function must have the following prototype:
417 
418   int callback_func(
419     struct notifier_block *self, unsigned long action, void *arg);
420 
421 The first argument of the callback function (self) is a pointer to the block
422 of the notifier chain that points to the callback function itself.
423 The second argument (action) is one of the event types described above.
424 The third argument (arg) passes a pointer of struct memory_notify.
425 
426 struct 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 
434 start_pfn is start_pfn of online/offline memory.
435 nr_pages is # of pages of online/offline memory.
436 status_change_nid_normal is set node id when N_NORMAL_MEMORY of nodemask
437 is (will be) set/clear, if this is -1, then nodemask status is not changed.
438 status_change_nid_high is set node id when N_HIGH_MEMORY of nodemask
439 is (will be) set/clear, if this is -1, then nodemask status is not changed.
440 status_change_nid is set node id when N_MEMORY of nodemask is (will be)
441 set/clear. It means a new(memoryless) node gets new memory by online and a
442 node loses all memory. If this is -1, then nodemask status is not changed.
443 If status_changed_nid* >= 0, callback should create/discard structures for the
444 node if necessary.
445 
446 The callback routine shall return one of the values
447 NOTIFY_DONE, NOTIFY_OK, NOTIFY_BAD, NOTIFY_STOP
448 defined in include/linux/notifier.h
449 
450 NOTIFY_DONE and NOTIFY_OK have no effect on the further processing.
451 
452 NOTIFY_BAD is used as response to the MEM_GOING_ONLINE, MEM_GOING_OFFLINE,
453 MEM_ONLINE, or MEM_OFFLINE action to cancel hotplugging. It stops
454 further processing of the notification queue.
455 
456 NOTIFY_STOP stops further processing of the notification queue.
457 
458 --------------
459 9. 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 

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