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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 | Even if the memory is hot-added, it is not at ready-to-use state. | |
260 | For using newly added memory, you have to "online" the memory block. | |
261 | ||
262 | For onlining, you have to write "online" to the memory block's state file as: | |
263 | ||
264 | % echo online > /sys/devices/system/memory/memoryXXX/state | |
265 | ||
266 | This onlining will not change the ZONE type of the target memory block, | |
267 | If the memory block is in ZONE_NORMAL, you can change it to ZONE_MOVABLE: | |
268 | ||
269 | % echo online_movable > /sys/devices/system/memory/memoryXXX/state | |
270 | (NOTE: current limit: this memory block must be adjacent to ZONE_MOVABLE) | |
271 | ||
272 | And if the memory block is in ZONE_MOVABLE, you can change it to ZONE_NORMAL: | |
273 | ||
274 | % echo online_kernel > /sys/devices/system/memory/memoryXXX/state | |
275 | (NOTE: current limit: this memory block must be adjacent to ZONE_NORMAL) | |
276 | ||
277 | After this, memory block XXX's state will be 'online' and the amount of | |
278 | available memory will be increased. | |
279 | ||
280 | Currently, newly added memory is added as ZONE_NORMAL (for powerpc, ZONE_DMA). | |
281 | This may be changed in future. | |
282 | ||
283 | ||
284 | ||
285 | ------------------------ | |
286 | 6. Logical memory remove | |
287 | ------------------------ | |
288 | ||
289 | 6.1 Memory offline and ZONE_MOVABLE | |
290 | ------------ | |
291 | Memory offlining is more complicated than memory online. Because memory offline | |
292 | has to make the whole memory block be unused, memory offline can fail if | |
293 | the memory block includes memory which cannot be freed. | |
294 | ||
295 | In general, memory offline can use 2 techniques. | |
296 | ||
297 | (1) reclaim and free all memory in the memory block. | |
298 | (2) migrate all pages in the memory block. | |
299 | ||
300 | In the current implementation, Linux's memory offline uses method (2), freeing | |
301 | all pages in the memory block by page migration. But not all pages are | |
302 | migratable. Under current Linux, migratable pages are anonymous pages and | |
303 | page caches. For offlining a memory block by migration, the kernel has to | |
304 | guarantee that the memory block contains only migratable pages. | |
305 | ||
306 | Now, a boot option for making a memory block which consists of migratable pages | |
307 | is supported. By specifying "kernelcore=" or "movablecore=" boot option, you can | |
308 | create ZONE_MOVABLE...a zone which is just used for movable pages. | |
309 | (See also Documentation/kernel-parameters.txt) | |
310 | ||
311 | Assume the system has "TOTAL" amount of memory at boot time, this boot option | |
312 | creates ZONE_MOVABLE as following. | |
313 | ||
314 | 1) When kernelcore=YYYY boot option is used, | |
315 | Size of memory not for movable pages (not for offline) is YYYY. | |
316 | Size of memory for movable pages (for offline) is TOTAL-YYYY. | |
317 | ||
318 | 2) When movablecore=ZZZZ boot option is used, | |
319 | Size of memory not for movable pages (not for offline) is TOTAL - ZZZZ. | |
320 | Size of memory for movable pages (for offline) is ZZZZ. | |
321 | ||
322 | ||
323 | Note: Unfortunately, there is no information to show which memory block belongs | |
324 | to ZONE_MOVABLE. This is TBD. | |
325 | ||
326 | ||
327 | 6.2. How to offline memory | |
328 | ------------ | |
329 | You can offline a memory block by using the same sysfs interface that was used | |
330 | in memory onlining. | |
331 | ||
332 | % echo offline > /sys/devices/system/memory/memoryXXX/state | |
333 | ||
334 | If offline succeeds, the state of the memory block is changed to be "offline". | |
335 | If it fails, some error core (like -EBUSY) will be returned by the kernel. | |
336 | Even if a memory block does not belong to ZONE_MOVABLE, you can try to offline | |
337 | it. If it doesn't contain 'unmovable' memory, you'll get success. | |
338 | ||
339 | A memory block under ZONE_MOVABLE is considered to be able to be offlined | |
340 | easily. But under some busy state, it may return -EBUSY. Even if a memory | |
341 | block cannot be offlined due to -EBUSY, you can retry offlining it and may be | |
342 | able to offline it (or not). (For example, a page is referred to by some kernel | |
343 | internal call and released soon.) | |
344 | ||
345 | Consideration: | |
346 | Memory hotplug's design direction is to make the possibility of memory offlining | |
347 | higher and to guarantee unplugging memory under any situation. But it needs | |
348 | more work. Returning -EBUSY under some situation may be good because the user | |
349 | can decide to retry more or not by himself. Currently, memory offlining code | |
350 | does some amount of retry with 120 seconds timeout. | |
351 | ||
352 | ------------------------- | |
353 | 7. Physical memory remove | |
354 | ------------------------- | |
355 | Need more implementation yet.... | |
356 | - Notification completion of remove works by OS to firmware. | |
357 | - Guard from remove if not yet. | |
358 | ||
359 | -------------------------------- | |
360 | 8. Memory hotplug event notifier | |
361 | -------------------------------- | |
362 | Hotplugging events are sent to a notification queue. | |
363 | ||
364 | There are six types of notification defined in include/linux/memory.h: | |
365 | ||
366 | MEM_GOING_ONLINE | |
367 | Generated before new memory becomes available in order to be able to | |
368 | prepare subsystems to handle memory. The page allocator is still unable | |
369 | to allocate from the new memory. | |
370 | ||
371 | MEM_CANCEL_ONLINE | |
372 | Generated if MEMORY_GOING_ONLINE fails. | |
373 | ||
374 | MEM_ONLINE | |
375 | Generated when memory has successfully brought online. The callback may | |
376 | allocate pages from the new memory. | |
377 | ||
378 | MEM_GOING_OFFLINE | |
379 | Generated to begin the process of offlining memory. Allocations are no | |
380 | longer possible from the memory but some of the memory to be offlined | |
381 | is still in use. The callback can be used to free memory known to a | |
382 | subsystem from the indicated memory block. | |
383 | ||
384 | MEM_CANCEL_OFFLINE | |
385 | Generated if MEMORY_GOING_OFFLINE fails. Memory is available again from | |
386 | the memory block that we attempted to offline. | |
387 | ||
388 | MEM_OFFLINE | |
389 | Generated after offlining memory is complete. | |
390 | ||
391 | A callback routine can be registered by calling | |
392 | ||
393 | hotplug_memory_notifier(callback_func, priority) | |
394 | ||
395 | Callback functions with higher values of priority are called before callback | |
396 | functions with lower values. | |
397 | ||
398 | A callback function must have the following prototype: | |
399 | ||
400 | int callback_func( | |
401 | struct notifier_block *self, unsigned long action, void *arg); | |
402 | ||
403 | The first argument of the callback function (self) is a pointer to the block | |
404 | of the notifier chain that points to the callback function itself. | |
405 | The second argument (action) is one of the event types described above. | |
406 | The third argument (arg) passes a pointer of struct memory_notify. | |
407 | ||
408 | struct memory_notify { | |
409 | unsigned long start_pfn; | |
410 | unsigned long nr_pages; | |
411 | int status_change_nid_normal; | |
412 | int status_change_nid_high; | |
413 | int status_change_nid; | |
414 | } | |
415 | ||
416 | start_pfn is start_pfn of online/offline memory. | |
417 | nr_pages is # of pages of online/offline memory. | |
418 | status_change_nid_normal is set node id when N_NORMAL_MEMORY of nodemask | |
419 | is (will be) set/clear, if this is -1, then nodemask status is not changed. | |
420 | status_change_nid_high is set node id when N_HIGH_MEMORY of nodemask | |
421 | is (will be) set/clear, if this is -1, then nodemask status is not changed. | |
422 | status_change_nid is set node id when N_MEMORY of nodemask is (will be) | |
423 | set/clear. It means a new(memoryless) node gets new memory by online and a | |
424 | node loses all memory. If this is -1, then nodemask status is not changed. | |
425 | If status_changed_nid* >= 0, callback should create/discard structures for the | |
426 | node if necessary. | |
427 | ||
428 | The callback routine shall return one of the values | |
429 | NOTIFY_DONE, NOTIFY_OK, NOTIFY_BAD, NOTIFY_STOP | |
430 | defined in include/linux/notifier.h | |
431 | ||
432 | NOTIFY_DONE and NOTIFY_OK have no effect on the further processing. | |
433 | ||
434 | NOTIFY_BAD is used as response to the MEM_GOING_ONLINE, MEM_GOING_OFFLINE, | |
435 | MEM_ONLINE, or MEM_OFFLINE action to cancel hotplugging. It stops | |
436 | further processing of the notification queue. | |
437 | ||
438 | NOTIFY_STOP stops further processing of the notification queue. | |
439 | ||
440 | -------------- | |
441 | 9. Future Work | |
442 | -------------- | |
443 | - allowing memory hot-add to ZONE_MOVABLE. maybe we need some switch like | |
444 | sysctl or new control file. | |
445 | - showing memory block and physical device relationship. | |
446 | - test and make it better memory offlining. | |
447 | - support HugeTLB page migration and offlining. | |
448 | - memmap removing at memory offline. | |
449 | - physical remove memory. | |
450 |