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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
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--------------------------------
Gary Hadec04fc582009-01-06 14:39:14 -08001274 sysfs files for memory hotplug
Yasunori Goto6867c932007-08-10 13:00:59 -0700128--------------------------------
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
Gary Hadec04fc582009-01-06 14:39:14 -0800141Under each section, you can see 4 files.
Yasunori Goto6867c932007-08-10 13:00:59 -0700142
143/sys/devices/system/memory/memoryXXX/phys_index
144/sys/devices/system/memory/memoryXXX/phys_device
145/sys/devices/system/memory/memoryXXX/state
Gary Hadec04fc582009-01-06 14:39:14 -0800146/sys/devices/system/memory/memoryXXX/removable
Yasunori Goto6867c932007-08-10 13:00:59 -0700147
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.
Gary Hadec04fc582009-01-06 14:39:14 -0800154'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.
Yasunori Goto6867c932007-08-10 13:00:59 -0700159
160NOTE:
161 These directories/files appear after physical memory hotplug phase.
162
Gary Hadec04fc582009-01-06 14:39:14 -0800163If 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
Yasunori Goto6867c932007-08-10 13:00:59 -0700168
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
Yasunori Goto10020ca2007-10-21 16:41:36 -0700323--------------------------------
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
Yasunori Goto6867c932007-08-10 13:00:59 -0700373--------------
Yasunori Goto10020ca2007-10-21 16:41:36 -07003749. Future Work
Yasunori Goto6867c932007-08-10 13:00:59 -0700375--------------
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.
Yasunori Goto6867c932007-08-10 13:00:59 -0700379 - 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