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Yasunori Goto6867c932007-08-10 13:00:59 -07001==============
2Memory Hotplug
3==============
4
Yasunori Goto10020ca2007-10-21 16:41:36 -07005Created: Jul 28 2007
6Add description of notifier of memory hotplug Oct 11 2007
Yasunori Goto6867c932007-08-10 13:00:59 -07007
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
Yasunori Goto10020ca2007-10-21 16:41:36 -0700288. Memory hotplug event notifier
299. Future Work List
Yasunori Goto6867c932007-08-10 13:00:59 -070030
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
Matt LaPlante19f59462009-04-27 15:06:31 +020076available/unavailable for users. Amount of memory from user's view is
Yasunori Goto6867c932007-08-10 13:00:59 -070077changed 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
Matt LaPlante19f59462009-04-27 15:06:31 +020082Logical Memory Hotplug phase is triggered by write of sysfs file by system
Yasunori Goto6867c932007-08-10 13:00:59 -070083administrator. 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--------------------------------
Gary Hadec04fc582009-01-06 14:39:14 -08001274 sysfs files for memory hotplug
Yasunori Goto6867c932007-08-10 13:00:59 -0700128--------------------------------
Nathan Fontenot0c2c99b2011-01-20 10:43:34 -0600129All sections have their device information in sysfs. Each section is part of
130a memory block under /sys/devices/system/memory as
Yasunori Goto6867c932007-08-10 13:00:59 -0700131
132/sys/devices/system/memory/memoryXXX
Nathan Fontenot0c2c99b2011-01-20 10:43:34 -0600133(XXX is the section id.)
Yasunori Goto6867c932007-08-10 13:00:59 -0700134
Nathan Fontenot0c2c99b2011-01-20 10:43:34 -0600135Now, XXX is defined as (start_address_of_section / section_size) of the first
136section contained in the memory block. The files 'phys_index' and
137'end_phys_index' under each directory report the beginning and end section id's
138for the memory block covered by the sysfs directory. It is expected that all
139memory sections in this range are present and no memory holes exist in the
140range. Currently there is no way to determine if there is a memory hole, but
141the existence of one should not affect the hotplug capabilities of the memory
142block.
Yasunori Goto6867c932007-08-10 13:00:59 -0700143
144For example, assume 1GiB section size. A device for a memory starting at
1450x100000000 is /sys/device/system/memory/memory4
146(0x100000000 / 1Gib = 4)
147This device covers address range [0x100000000 ... 0x140000000)
148
Nathan Fontenot0c2c99b2011-01-20 10:43:34 -0600149Under each section, you can see 4 or 5 files, the end_phys_index file being
150a recent addition and not present on older kernels.
Yasunori Goto6867c932007-08-10 13:00:59 -0700151
Nathan Fontenot0c2c99b2011-01-20 10:43:34 -0600152/sys/devices/system/memory/memoryXXX/start_phys_index
153/sys/devices/system/memory/memoryXXX/end_phys_index
Yasunori Goto6867c932007-08-10 13:00:59 -0700154/sys/devices/system/memory/memoryXXX/phys_device
155/sys/devices/system/memory/memoryXXX/state
Gary Hadec04fc582009-01-06 14:39:14 -0800156/sys/devices/system/memory/memoryXXX/removable
Yasunori Goto6867c932007-08-10 13:00:59 -0700157
Nathan Fontenot0c2c99b2011-01-20 10:43:34 -0600158'phys_index' : read-only and contains section id of the first section
159 in the memory block, same as XXX.
160'end_phys_index' : read-only and contains section id of the last section
161 in the memory block.
162'state' : read-write
163 at read: contains online/offline state of memory.
Lai Jiangshan511c2ab2012-12-11 16:03:16 -0800164 at write: user can specify "online_kernel",
165 "online_movable", "online", "offline" command
Nathan Fontenot0c2c99b2011-01-20 10:43:34 -0600166 which will be performed on al sections in the block.
167'phys_device' : read-only: designed to show the name of physical memory
168 device. This is not well implemented now.
169'removable' : read-only: contains an integer value indicating
170 whether the memory block is removable or not
171 removable. A value of 1 indicates that the memory
172 block is removable and a value of 0 indicates that
173 it is not removable. A memory block is removable only if
174 every section in the block is removable.
Yasunori Goto6867c932007-08-10 13:00:59 -0700175
176NOTE:
177 These directories/files appear after physical memory hotplug phase.
178
Alex Chiangdee5d0d2009-12-14 17:59:05 -0800179If CONFIG_NUMA is enabled the memoryXXX/ directories can also be accessed
180via symbolic links located in the /sys/devices/system/node/node* directories.
181
182For example:
Gary Hadec04fc582009-01-06 14:39:14 -0800183/sys/devices/system/node/node0/memory9 -> ../../memory/memory9
Yasunori Goto6867c932007-08-10 13:00:59 -0700184
Alex Chiangdee5d0d2009-12-14 17:59:05 -0800185A backlink will also be created:
186/sys/devices/system/memory/memory9/node0 -> ../../node/node0
187
Yasunori Goto6867c932007-08-10 13:00:59 -0700188--------------------------------
1894. Physical memory hot-add phase
190--------------------------------
191
1924.1 Hardware(Firmware) Support
193------------
194On x86_64/ia64 platform, memory hotplug by ACPI is supported.
195
196In general, the firmware (ACPI) which supports memory hotplug defines
197memory class object of _HID "PNP0C80". When a notify is asserted to PNP0C80,
198Linux's ACPI handler does hot-add memory to the system and calls a hotplug udev
199script. This will be done automatically.
200
201But scripts for memory hotplug are not contained in generic udev package(now).
202You may have to write it by yourself or online/offline memory by hand.
203Please see "How to online memory", "How to offline memory" in this text.
204
205If firmware supports NUMA-node hotplug, and defines an object _HID "ACPI0004",
206"PNP0A05", or "PNP0A06", notification is asserted to it, and ACPI handler
207calls hotplug code for all of objects which are defined in it.
208If memory device is found, memory hotplug code will be called.
209
210
2114.2 Notify memory hot-add event by hand
212------------
213In some environments, especially virtualized environment, firmware will not
214notify memory hotplug event to the kernel. For such environment, "probe"
215interface is supported. This interface depends on CONFIG_ARCH_MEMORY_PROBE.
216
217Now, CONFIG_ARCH_MEMORY_PROBE is supported only by powerpc but it does not
218contain highly architecture codes. Please add config if you need "probe"
219interface.
220
221Probe interface is located at
222/sys/devices/system/memory/probe
223
224You can tell the physical address of new memory to the kernel by
225
226% echo start_address_of_new_memory > /sys/devices/system/memory/probe
227
228Then, [start_address_of_new_memory, start_address_of_new_memory + section_size)
229memory range is hot-added. In this case, hotplug script is not called (in
230current implementation). You'll have to online memory by yourself.
231Please see "How to online memory" in this text.
232
233
234
235------------------------------
2365. Logical Memory hot-add phase
237------------------------------
238
2395.1. State of memory
240------------
241To see (online/offline) state of memory section, read 'state' file.
242
243% cat /sys/device/system/memory/memoryXXX/state
244
245
246If the memory section is online, you'll read "online".
247If the memory section is offline, you'll read "offline".
248
249
2505.2. How to online memory
251------------
252Even if the memory is hot-added, it is not at ready-to-use state.
253For using newly added memory, you have to "online" the memory section.
254
255For onlining, you have to write "online" to the section's state file as:
256
257% echo online > /sys/devices/system/memory/memoryXXX/state
258
Lai Jiangshan511c2ab2012-12-11 16:03:16 -0800259This onlining will not change the ZONE type of the target memory section,
260If the memory section is in ZONE_NORMAL, you can change it to ZONE_MOVABLE:
261
262% echo online_movable > /sys/devices/system/memory/memoryXXX/state
263(NOTE: current limit: this memory section must be adjacent to ZONE_MOVABLE)
264
265And if the memory section is in ZONE_MOVABLE, you can change it to ZONE_NORMAL:
266
267% echo online_kernel > /sys/devices/system/memory/memoryXXX/state
268(NOTE: current limit: this memory section must be adjacent to ZONE_NORMAL)
269
Yasunori Goto6867c932007-08-10 13:00:59 -0700270After this, section memoryXXX's state will be 'online' and the amount of
271available memory will be increased.
272
273Currently, newly added memory is added as ZONE_NORMAL (for powerpc, ZONE_DMA).
274This may be changed in future.
275
276
277
278------------------------
2796. Logical memory remove
280------------------------
281
2826.1 Memory offline and ZONE_MOVABLE
283------------
284Memory offlining is more complicated than memory online. Because memory offline
285has to make the whole memory section be unused, memory offline can fail if
286the section includes memory which cannot be freed.
287
288In general, memory offline can use 2 techniques.
289
290(1) reclaim and free all memory in the section.
291(2) migrate all pages in the section.
292
293In the current implementation, Linux's memory offline uses method (2), freeing
294all pages in the section by page migration. But not all pages are
295migratable. Under current Linux, migratable pages are anonymous pages and
296page caches. For offlining a section by migration, the kernel has to guarantee
297that the section contains only migratable pages.
298
299Now, a boot option for making a section which consists of migratable pages is
300supported. By specifying "kernelcore=" or "movablecore=" boot option, you can
301create ZONE_MOVABLE...a zone which is just used for movable pages.
302(See also Documentation/kernel-parameters.txt)
303
304Assume the system has "TOTAL" amount of memory at boot time, this boot option
305creates ZONE_MOVABLE as following.
306
3071) When kernelcore=YYYY boot option is used,
308 Size of memory not for movable pages (not for offline) is YYYY.
309 Size of memory for movable pages (for offline) is TOTAL-YYYY.
310
3112) When movablecore=ZZZZ boot option is used,
312 Size of memory not for movable pages (not for offline) is TOTAL - ZZZZ.
313 Size of memory for movable pages (for offline) is ZZZZ.
314
315
316Note) Unfortunately, there is no information to show which section belongs
317to ZONE_MOVABLE. This is TBD.
318
319
3206.2. How to offline memory
321------------
322You can offline a section by using the same sysfs interface that was used in
323memory onlining.
324
325% echo offline > /sys/devices/system/memory/memoryXXX/state
326
327If offline succeeds, the state of the memory section is changed to be "offline".
328If it fails, some error core (like -EBUSY) will be returned by the kernel.
329Even if a section does not belong to ZONE_MOVABLE, you can try to offline it.
330If it doesn't contain 'unmovable' memory, you'll get success.
331
332A section under ZONE_MOVABLE is considered to be able to be offlined easily.
333But under some busy state, it may return -EBUSY. Even if a memory section
334cannot be offlined due to -EBUSY, you can retry offlining it and may be able to
335offline it (or not).
336(For example, a page is referred to by some kernel internal call and released
337 soon.)
338
339Consideration:
340Memory hotplug's design direction is to make the possibility of memory offlining
341higher and to guarantee unplugging memory under any situation. But it needs
342more work. Returning -EBUSY under some situation may be good because the user
343can decide to retry more or not by himself. Currently, memory offlining code
344does some amount of retry with 120 seconds timeout.
345
346-------------------------
3477. Physical memory remove
348-------------------------
349Need more implementation yet....
350 - Notification completion of remove works by OS to firmware.
351 - Guard from remove if not yet.
352
Yasunori Goto10020ca2007-10-21 16:41:36 -0700353--------------------------------
3548. Memory hotplug event notifier
355--------------------------------
Masanari Iidac94bed8e2012-04-10 00:22:13 +0900356Memory hotplug has event notifier. There are 6 types of notification.
Yasunori Goto10020ca2007-10-21 16:41:36 -0700357
358MEMORY_GOING_ONLINE
359 Generated before new memory becomes available in order to be able to
360 prepare subsystems to handle memory. The page allocator is still unable
361 to allocate from the new memory.
362
363MEMORY_CANCEL_ONLINE
364 Generated if MEMORY_GOING_ONLINE fails.
365
366MEMORY_ONLINE
Matt LaPlante19f59462009-04-27 15:06:31 +0200367 Generated when memory has successfully brought online. The callback may
Yasunori Goto10020ca2007-10-21 16:41:36 -0700368 allocate pages from the new memory.
369
370MEMORY_GOING_OFFLINE
371 Generated to begin the process of offlining memory. Allocations are no
372 longer possible from the memory but some of the memory to be offlined
373 is still in use. The callback can be used to free memory known to a
374 subsystem from the indicated memory section.
375
376MEMORY_CANCEL_OFFLINE
377 Generated if MEMORY_GOING_OFFLINE fails. Memory is available again from
378 the section that we attempted to offline.
379
380MEMORY_OFFLINE
381 Generated after offlining memory is complete.
382
383A callback routine can be registered by
384 hotplug_memory_notifier(callback_func, priority)
385
386The second argument of callback function (action) is event types of above.
387The third argument is passed by pointer of struct memory_notify.
388
389struct memory_notify {
390 unsigned long start_pfn;
391 unsigned long nr_pages;
Lai Jiangshand9713672012-12-11 16:01:03 -0800392 int status_change_nid_normal;
Lai Jiangshan6715ddf2012-12-12 13:51:49 -0800393 int status_change_nid_high;
Matt LaPlante19f59462009-04-27 15:06:31 +0200394 int status_change_nid;
Yasunori Goto10020ca2007-10-21 16:41:36 -0700395}
396
397start_pfn is start_pfn of online/offline memory.
398nr_pages is # of pages of online/offline memory.
Lai Jiangshand9713672012-12-11 16:01:03 -0800399status_change_nid_normal is set node id when N_NORMAL_MEMORY of nodemask
400is (will be) set/clear, if this is -1, then nodemask status is not changed.
Lai Jiangshan6715ddf2012-12-12 13:51:49 -0800401status_change_nid_high is set node id when N_HIGH_MEMORY of nodemask
402is (will be) set/clear, if this is -1, then nodemask status is not changed.
403status_change_nid is set node id when N_MEMORY of nodemask is (will be)
Yasunori Goto10020ca2007-10-21 16:41:36 -0700404set/clear. It means a new(memoryless) node gets new memory by online and a
405node loses all memory. If this is -1, then nodemask status is not changed.
Lai Jiangshand9713672012-12-11 16:01:03 -0800406If status_changed_nid* >= 0, callback should create/discard structures for the
Yasunori Goto10020ca2007-10-21 16:41:36 -0700407node if necessary.
408
Yasunori Goto6867c932007-08-10 13:00:59 -0700409--------------
Yasunori Goto10020ca2007-10-21 16:41:36 -07004109. Future Work
Yasunori Goto6867c932007-08-10 13:00:59 -0700411--------------
412 - allowing memory hot-add to ZONE_MOVABLE. maybe we need some switch like
413 sysctl or new control file.
414 - showing memory section and physical device relationship.
Yasunori Goto6867c932007-08-10 13:00:59 -0700415 - showing memory section is under ZONE_MOVABLE or not
416 - test and make it better memory offlining.
417 - support HugeTLB page migration and offlining.
418 - memmap removing at memory offline.
419 - physical remove memory.
420