Root/Documentation/memory-hotplug.txt

1==============
2Memory Hotplug
3==============
4
5Created: Jul 28 2007
6Add description of notifier of memory hotplug Oct 11 2007
7
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
288. Memory hotplug event notifier
299. Future Work List
30
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
76available/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 triggered 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--------------------------------
1274 sysfs files for memory hotplug
128--------------------------------
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
141Under 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
160NOTE:
161  These directories/files appear after physical memory hotplug phase.
162
163If CONFIG_NUMA is enabled the memoryXXX/ directories can also be accessed
164via symbolic links located in the /sys/devices/system/node/node* directories.
165
166For example:
167/sys/devices/system/node/node0/memory9 -> ../../memory/memory9
168
169A backlink will also be created:
170/sys/devices/system/memory/memory9/node0 -> ../../node/node0
171
172--------------------------------
1734. Physical memory hot-add phase
174--------------------------------
175
1764.1 Hardware(Firmware) Support
177------------
178On x86_64/ia64 platform, memory hotplug by ACPI is supported.
179
180In general, the firmware (ACPI) which supports memory hotplug defines
181memory class object of _HID "PNP0C80". When a notify is asserted to PNP0C80,
182Linux's ACPI handler does hot-add memory to the system and calls a hotplug udev
183script. This will be done automatically.
184
185But scripts for memory hotplug are not contained in generic udev package(now).
186You may have to write it by yourself or online/offline memory by hand.
187Please see "How to online memory", "How to offline memory" in this text.
188
189If firmware supports NUMA-node hotplug, and defines an object _HID "ACPI0004",
190"PNP0A05", or "PNP0A06", notification is asserted to it, and ACPI handler
191calls hotplug code for all of objects which are defined in it.
192If memory device is found, memory hotplug code will be called.
193
194
1954.2 Notify memory hot-add event by hand
196------------
197In some environments, especially virtualized environment, firmware will not
198notify memory hotplug event to the kernel. For such environment, "probe"
199interface is supported. This interface depends on CONFIG_ARCH_MEMORY_PROBE.
200
201Now, CONFIG_ARCH_MEMORY_PROBE is supported only by powerpc but it does not
202contain highly architecture codes. Please add config if you need "probe"
203interface.
204
205Probe interface is located at
206/sys/devices/system/memory/probe
207
208You can tell the physical address of new memory to the kernel by
209
210% echo start_address_of_new_memory > /sys/devices/system/memory/probe
211
212Then, [start_address_of_new_memory, start_address_of_new_memory + section_size)
213memory range is hot-added. In this case, hotplug script is not called (in
214current implementation). You'll have to online memory by yourself.
215Please see "How to online memory" in this text.
216
217
218
219------------------------------
2205. Logical Memory hot-add phase
221------------------------------
222
2235.1. State of memory
224------------
225To see (online/offline) state of memory section, read 'state' file.
226
227% cat /sys/device/system/memory/memoryXXX/state
228
229
230If the memory section is online, you'll read "online".
231If the memory section is offline, you'll read "offline".
232
233
2345.2. How to online memory
235------------
236Even if the memory is hot-added, it is not at ready-to-use state.
237For using newly added memory, you have to "online" the memory section.
238
239For onlining, you have to write "online" to the section's state file as:
240
241% echo online > /sys/devices/system/memory/memoryXXX/state
242
243After this, section memoryXXX's state will be 'online' and the amount of
244available memory will be increased.
245
246Currently, newly added memory is added as ZONE_NORMAL (for powerpc, ZONE_DMA).
247This may be changed in future.
248
249
250
251------------------------
2526. Logical memory remove
253------------------------
254
2556.1 Memory offline and ZONE_MOVABLE
256------------
257Memory offlining is more complicated than memory online. Because memory offline
258has to make the whole memory section be unused, memory offline can fail if
259the section includes memory which cannot be freed.
260
261In general, memory offline can use 2 techniques.
262
263(1) reclaim and free all memory in the section.
264(2) migrate all pages in the section.
265
266In the current implementation, Linux's memory offline uses method (2), freeing
267all pages in the section by page migration. But not all pages are
268migratable. Under current Linux, migratable pages are anonymous pages and
269page caches. For offlining a section by migration, the kernel has to guarantee
270that the section contains only migratable pages.
271
272Now, a boot option for making a section which consists of migratable pages is
273supported. By specifying "kernelcore=" or "movablecore=" boot option, you can
274create ZONE_MOVABLE...a zone which is just used for movable pages.
275(See also Documentation/kernel-parameters.txt)
276
277Assume the system has "TOTAL" amount of memory at boot time, this boot option
278creates ZONE_MOVABLE as following.
279
2801) When kernelcore=YYYY boot option is used,
281  Size of memory not for movable pages (not for offline) is YYYY.
282  Size of memory for movable pages (for offline) is TOTAL-YYYY.
283
2842) When movablecore=ZZZZ boot option is used,
285  Size of memory not for movable pages (not for offline) is TOTAL - ZZZZ.
286  Size of memory for movable pages (for offline) is ZZZZ.
287
288
289Note) Unfortunately, there is no information to show which section belongs
290to ZONE_MOVABLE. This is TBD.
291
292
2936.2. How to offline memory
294------------
295You can offline a section by using the same sysfs interface that was used in
296memory onlining.
297
298% echo offline > /sys/devices/system/memory/memoryXXX/state
299
300If offline succeeds, the state of the memory section is changed to be "offline".
301If it fails, some error core (like -EBUSY) will be returned by the kernel.
302Even if a section does not belong to ZONE_MOVABLE, you can try to offline it.
303If it doesn't contain 'unmovable' memory, you'll get success.
304
305A section under ZONE_MOVABLE is considered to be able to be offlined easily.
306But under some busy state, it may return -EBUSY. Even if a memory section
307cannot be offlined due to -EBUSY, you can retry offlining it and may be able to
308offline it (or not).
309(For example, a page is referred to by some kernel internal call and released
310 soon.)
311
312Consideration:
313Memory hotplug's design direction is to make the possibility of memory offlining
314higher and to guarantee unplugging memory under any situation. But it needs
315more work. Returning -EBUSY under some situation may be good because the user
316can decide to retry more or not by himself. Currently, memory offlining code
317does some amount of retry with 120 seconds timeout.
318
319-------------------------
3207. Physical memory remove
321-------------------------
322Need more implementation yet....
323 - Notification completion of remove works by OS to firmware.
324 - Guard from remove if not yet.
325
326--------------------------------
3278. Memory hotplug event notifier
328--------------------------------
329Memory hotplug has event notifer. There are 6 types of notification.
330
331MEMORY_GOING_ONLINE
332  Generated before new memory becomes available in order to be able to
333  prepare subsystems to handle memory. The page allocator is still unable
334  to allocate from the new memory.
335
336MEMORY_CANCEL_ONLINE
337  Generated if MEMORY_GOING_ONLINE fails.
338
339MEMORY_ONLINE
340  Generated when memory has successfully brought online. The callback may
341  allocate pages from the new memory.
342
343MEMORY_GOING_OFFLINE
344  Generated to begin the process of offlining memory. Allocations are no
345  longer possible from the memory but some of the memory to be offlined
346  is still in use. The callback can be used to free memory known to a
347  subsystem from the indicated memory section.
348
349MEMORY_CANCEL_OFFLINE
350  Generated if MEMORY_GOING_OFFLINE fails. Memory is available again from
351  the section that we attempted to offline.
352
353MEMORY_OFFLINE
354  Generated after offlining memory is complete.
355
356A callback routine can be registered by
357  hotplug_memory_notifier(callback_func, priority)
358
359The second argument of callback function (action) is event types of above.
360The third argument is passed by pointer of struct memory_notify.
361
362struct memory_notify {
363       unsigned long start_pfn;
364       unsigned long nr_pages;
365       int status_change_nid;
366}
367
368start_pfn is start_pfn of online/offline memory.
369nr_pages is # of pages of online/offline memory.
370status_change_nid is set node id when N_HIGH_MEMORY of nodemask is (will be)
371set/clear. It means a new(memoryless) node gets new memory by online and a
372node loses all memory. If this is -1, then nodemask status is not changed.
373If status_changed_nid >= 0, callback should create/discard structures for the
374node if necessary.
375
376--------------
3779. Future Work
378--------------
379  - allowing memory hot-add to ZONE_MOVABLE. maybe we need some switch like
380    sysctl or new control file.
381  - showing memory section and physical device relationship.
382  - showing memory section is under ZONE_MOVABLE or not
383  - test and make it better memory offlining.
384  - support HugeTLB page migration and offlining.
385  - memmap removing at memory offline.
386  - physical remove memory.
387
388

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