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. SPARSEMEM divides the whole memory
92 into chunks of the same size. The chunk is called a "section". The size of
93 a section is architecture dependent. For example, power uses 16MiB, ia64 uses
94 1GiB. The unit of online/offline operation is "one section". (see Section 3.)
95
96 To determine the size of sections, please read this file:
97
98 /sys/devices/system/memory/block_size_bytes
99
100 This file shows the size of sections in byte.
101
102 -----------------------
103 2. Kernel Configuration
104 -----------------------
105 To use memory hotplug feature, kernel must be compiled with following
106 config 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 --------------------------------
127 4 sysfs files for memory hotplug
128 --------------------------------
129 All sections have their device information under /sys/devices/system/memory as
130
131 /sys/devices/system/memory/memoryXXX
132 (XXX is section id.)
133
134 Now, XXX is defined as start_address_of_section / section_size.
135
136 For example, assume 1GiB section size. A device for a memory starting at
137 0x100000000 is /sys/device/system/memory/memory4
138 (0x100000000 / 1Gib = 4)
139 This device covers address range [0x100000000 ... 0x140000000)
140
141 Under 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
160 NOTE:
161 These directories/files appear after physical memory hotplug phase.
162
163 If CONFIG_NUMA is enabled the
164 /sys/devices/system/memory/memoryXXX memory section
165 directories can also be accessed via symbolic links located in
166 the /sys/devices/system/node/node* directories. For example:
167 /sys/devices/system/node/node0/memory9 -> ../../memory/memory9
168
169 --------------------------------
170 4. Physical memory hot-add phase
171 --------------------------------
172
173 4.1 Hardware(Firmware) Support
174 ------------
175 On x86_64/ia64 platform, memory hotplug by ACPI is supported.
176
177 In general, the firmware (ACPI) which supports memory hotplug defines
178 memory class object of _HID "PNP0C80". When a notify is asserted to PNP0C80,
179 Linux's ACPI handler does hot-add memory to the system and calls a hotplug udev
180 script. This will be done automatically.
181
182 But scripts for memory hotplug are not contained in generic udev package(now).
183 You may have to write it by yourself or online/offline memory by hand.
184 Please see "How to online memory", "How to offline memory" in this text.
185
186 If firmware supports NUMA-node hotplug, and defines an object _HID "ACPI0004",
187 "PNP0A05", or "PNP0A06", notification is asserted to it, and ACPI handler
188 calls hotplug code for all of objects which are defined in it.
189 If memory device is found, memory hotplug code will be called.
190
191
192 4.2 Notify memory hot-add event by hand
193 ------------
194 In some environments, especially virtualized environment, firmware will not
195 notify memory hotplug event to the kernel. For such environment, "probe"
196 interface is supported. This interface depends on CONFIG_ARCH_MEMORY_PROBE.
197
198 Now, CONFIG_ARCH_MEMORY_PROBE is supported only by powerpc but it does not
199 contain highly architecture codes. Please add config if you need "probe"
200 interface.
201
202 Probe interface is located at
203 /sys/devices/system/memory/probe
204
205 You 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
209 Then, [start_address_of_new_memory, start_address_of_new_memory + section_size)
210 memory range is hot-added. In this case, hotplug script is not called (in
211 current implementation). You'll have to online memory by yourself.
212 Please see "How to online memory" in this text.
213
214
215
216 ------------------------------
217 5. Logical Memory hot-add phase
218 ------------------------------
219
220 5.1. State of memory
221 ------------
222 To see (online/offline) state of memory section, read 'state' file.
223
224 % cat /sys/device/system/memory/memoryXXX/state
225
226
227 If the memory section is online, you'll read "online".
228 If the memory section is offline, you'll read "offline".
229
230
231 5.2. How to online memory
232 ------------
233 Even if the memory is hot-added, it is not at ready-to-use state.
234 For using newly added memory, you have to "online" the memory section.
235
236 For onlining, you have to write "online" to the section's state file as:
237
238 % echo online > /sys/devices/system/memory/memoryXXX/state
239
240 After this, section memoryXXX's state will be 'online' and the amount of
241 available memory will be increased.
242
243 Currently, newly added memory is added as ZONE_NORMAL (for powerpc, ZONE_DMA).
244 This may be changed in future.
245
246
247
248 ------------------------
249 6. Logical memory remove
250 ------------------------
251
252 6.1 Memory offline and ZONE_MOVABLE
253 ------------
254 Memory offlining is more complicated than memory online. Because memory offline
255 has to make the whole memory section be unused, memory offline can fail if
256 the section includes memory which cannot be freed.
257
258 In 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
263 In the current implementation, Linux's memory offline uses method (2), freeing
264 all pages in the section by page migration. But not all pages are
265 migratable. Under current Linux, migratable pages are anonymous pages and
266 page caches. For offlining a section by migration, the kernel has to guarantee
267 that the section contains only migratable pages.
268
269 Now, a boot option for making a section which consists of migratable pages is
270 supported. By specifying "kernelcore=" or "movablecore=" boot option, you can
271 create ZONE_MOVABLE...a zone which is just used for movable pages.
272 (See also Documentation/kernel-parameters.txt)
273
274 Assume the system has "TOTAL" amount of memory at boot time, this boot option
275 creates ZONE_MOVABLE as following.
276
277 1) 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
281 2) 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
286 Note) Unfortunately, there is no information to show which section belongs
287 to ZONE_MOVABLE. This is TBD.
288
289
290 6.2. How to offline memory
291 ------------
292 You can offline a section by using the same sysfs interface that was used in
293 memory onlining.
294
295 % echo offline > /sys/devices/system/memory/memoryXXX/state
296
297 If offline succeeds, the state of the memory section is changed to be "offline".
298 If it fails, some error core (like -EBUSY) will be returned by the kernel.
299 Even if a section does not belong to ZONE_MOVABLE, you can try to offline it.
300 If it doesn't contain 'unmovable' memory, you'll get success.
301
302 A section under ZONE_MOVABLE is considered to be able to be offlined easily.
303 But under some busy state, it may return -EBUSY. Even if a memory section
304 cannot be offlined due to -EBUSY, you can retry offlining it and may be able to
305 offline it (or not).
306 (For example, a page is referred to by some kernel internal call and released
307 soon.)
308
309 Consideration:
310 Memory hotplug's design direction is to make the possibility of memory offlining
311 higher and to guarantee unplugging memory under any situation. But it needs
312 more work. Returning -EBUSY under some situation may be good because the user
313 can decide to retry more or not by himself. Currently, memory offlining code
314 does some amount of retry with 120 seconds timeout.
315
316 -------------------------
317 7. Physical memory remove
318 -------------------------
319 Need more implementation yet....
320 - Notification completion of remove works by OS to firmware.
321 - Guard from remove if not yet.
322
323 --------------------------------
324 8. Memory hotplug event notifier
325 --------------------------------
326 Memory hotplug has event notifer. There are 6 types of notification.
327
328 MEMORY_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
333 MEMORY_CANCEL_ONLINE
334 Generated if MEMORY_GOING_ONLINE fails.
335
336 MEMORY_ONLINE
337 Generated when memory has successfully brought online. The callback may
338 allocate pages from the new memory.
339
340 MEMORY_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
346 MEMORY_CANCEL_OFFLINE
347 Generated if MEMORY_GOING_OFFLINE fails. Memory is available again from
348 the section that we attempted to offline.
349
350 MEMORY_OFFLINE
351 Generated after offlining memory is complete.
352
353 A callback routine can be registered by
354 hotplug_memory_notifier(callback_func, priority)
355
356 The second argument of callback function (action) is event types of above.
357 The third argument is passed by pointer of struct memory_notify.
358
359 struct memory_notify {
360 unsigned long start_pfn;
361 unsigned long nr_pages;
362 int status_change_nid;
363 }
364
365 start_pfn is start_pfn of online/offline memory.
366 nr_pages is # of pages of online/offline memory.
367 status_change_nid is set node id when N_HIGH_MEMORY of nodemask is (will be)
368 set/clear. It means a new(memoryless) node gets new memory by online and a
369 node loses all memory. If this is -1, then nodemask status is not changed.
370 If status_changed_nid >= 0, callback should create/discard structures for the
371 node if necessary.
372
373 --------------
374 9. 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
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