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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:03 EST.

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 in sysfs.  Each section is part of
130	a memory block under /sys/devices/system/memory as
131	
132	/sys/devices/system/memory/memoryXXX
133	(XXX is the section id.)
134	
135	Now, XXX is defined as (start_address_of_section / section_size) of the first
136	section 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
138	for the memory block covered by the sysfs directory.  It is expected that all
139	memory sections in this range are present and no memory holes exist in the
140	range. Currently there is no way to determine if there is a memory hole, but
141	the existence of one should not affect the hotplug capabilities of the memory
142	block.
143	
144	For example, assume 1GiB section size. A device for a memory starting at
145	0x100000000 is /sys/device/system/memory/memory4
146	(0x100000000 / 1Gib = 4)
147	This device covers address range [0x100000000 ... 0x140000000)
148	
149	Under each section, you can see 4 or 5 files, the end_phys_index file being
150	a recent addition and not present on older kernels.
151	
152	/sys/devices/system/memory/memoryXXX/start_phys_index
153	/sys/devices/system/memory/memoryXXX/end_phys_index
154	/sys/devices/system/memory/memoryXXX/phys_device
155	/sys/devices/system/memory/memoryXXX/state
156	/sys/devices/system/memory/memoryXXX/removable
157	
158	'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.
164	                    at write: user can specify "online_kernel",
165	                    "online_movable", "online", "offline" command
166	                    which will be performed on all 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.
175	
176	NOTE:
177	  These directories/files appear after physical memory hotplug phase.
178	
179	If CONFIG_NUMA is enabled the memoryXXX/ directories can also be accessed
180	via symbolic links located in the /sys/devices/system/node/node* directories.
181	
182	For example:
183	/sys/devices/system/node/node0/memory9 -> ../../memory/memory9
184	
185	A backlink will also be created:
186	/sys/devices/system/memory/memory9/node0 -> ../../node/node0
187	
188	--------------------------------
189	4. Physical memory hot-add phase
190	--------------------------------
191	
192	4.1 Hardware(Firmware) Support
193	------------
194	On x86_64/ia64 platform, memory hotplug by ACPI is supported.
195	
196	In general, the firmware (ACPI) which supports memory hotplug defines
197	memory class object of _HID "PNP0C80". When a notify is asserted to PNP0C80,
198	Linux's ACPI handler does hot-add memory to the system and calls a hotplug udev
199	script. This will be done automatically.
200	
201	But scripts for memory hotplug are not contained in generic udev package(now).
202	You may have to write it by yourself or online/offline memory by hand.
203	Please see "How to online memory", "How to offline memory" in this text.
204	
205	If firmware supports NUMA-node hotplug, and defines an object _HID "ACPI0004",
206	"PNP0A05", or "PNP0A06", notification is asserted to it, and ACPI handler
207	calls hotplug code for all of objects which are defined in it.
208	If memory device is found, memory hotplug code will be called.
209	
210	
211	4.2 Notify memory hot-add event by hand
212	------------
213	On powerpc, the firmware does not notify a memory hotplug event to the kernel.
214	Therefore, "probe" interface is supported to notify the event to the kernel.
215	This interface depends on CONFIG_ARCH_MEMORY_PROBE.
216	
217	CONFIG_ARCH_MEMORY_PROBE is supported on powerpc only. On x86, this config
218	option is disabled by default since ACPI notifies a memory hotplug event to
219	the kernel, which performs its hotplug operation as the result. Please
220	enable this option if you need the "probe" interface for testing purposes
221	on x86.
222	
223	Probe interface is located at
224	/sys/devices/system/memory/probe
225	
226	You can tell the physical address of new memory to the kernel by
227	
228	% echo start_address_of_new_memory > /sys/devices/system/memory/probe
229	
230	Then, [start_address_of_new_memory, start_address_of_new_memory + section_size)
231	memory range is hot-added. In this case, hotplug script is not called (in
232	current implementation). You'll have to online memory by yourself.
233	Please see "How to online memory" in this text.
234	
235	
236	
237	------------------------------
238	5. Logical Memory hot-add phase
239	------------------------------
240	
241	5.1. State of memory
242	------------
243	To see (online/offline) state of memory section, read 'state' file.
244	
245	% cat /sys/device/system/memory/memoryXXX/state
246	
247	
248	If the memory section is online, you'll read "online".
249	If the memory section is offline, you'll read "offline".
250	
251	
252	5.2. How to online memory
253	------------
254	Even if the memory is hot-added, it is not at ready-to-use state.
255	For using newly added memory, you have to "online" the memory section.
256	
257	For onlining, you have to write "online" to the section's state file as:
258	
259	% echo online > /sys/devices/system/memory/memoryXXX/state
260	
261	This onlining will not change the ZONE type of the target memory section,
262	If the memory section is in ZONE_NORMAL, you can change it to ZONE_MOVABLE:
263	
264	% echo online_movable > /sys/devices/system/memory/memoryXXX/state
265	(NOTE: current limit: this memory section must be adjacent to ZONE_MOVABLE)
266	
267	And if the memory section is in ZONE_MOVABLE, you can change it to ZONE_NORMAL:
268	
269	% echo online_kernel > /sys/devices/system/memory/memoryXXX/state
270	(NOTE: current limit: this memory section must be adjacent to ZONE_NORMAL)
271	
272	After this, section memoryXXX's state will be 'online' and the amount of
273	available memory will be increased.
274	
275	Currently, newly added memory is added as ZONE_NORMAL (for powerpc, ZONE_DMA).
276	This may be changed in future.
277	
278	
279	
280	------------------------
281	6. Logical memory remove
282	------------------------
283	
284	6.1 Memory offline and ZONE_MOVABLE
285	------------
286	Memory offlining is more complicated than memory online. Because memory offline
287	has to make the whole memory section be unused, memory offline can fail if
288	the section includes memory which cannot be freed.
289	
290	In general, memory offline can use 2 techniques.
291	
292	(1) reclaim and free all memory in the section.
293	(2) migrate all pages in the section.
294	
295	In the current implementation, Linux's memory offline uses method (2), freeing
296	all  pages in the section by page migration. But not all pages are
297	migratable. Under current Linux, migratable pages are anonymous pages and
298	page caches. For offlining a section by migration, the kernel has to guarantee
299	that the section contains only migratable pages.
300	
301	Now, a boot option for making a section which consists of migratable pages is
302	supported. By specifying "kernelcore=" or "movablecore=" boot option, you can
303	create ZONE_MOVABLE...a zone which is just used for movable pages.
304	(See also Documentation/kernel-parameters.txt)
305	
306	Assume the system has "TOTAL" amount of memory at boot time, this boot option
307	creates ZONE_MOVABLE as following.
308	
309	1) When kernelcore=YYYY boot option is used,
310	  Size of memory not for movable pages (not for offline) is YYYY.
311	  Size of memory for movable pages (for offline) is TOTAL-YYYY.
312	
313	2) When movablecore=ZZZZ boot option is used,
314	  Size of memory not for movable pages (not for offline) is TOTAL - ZZZZ.
315	  Size of memory for movable pages (for offline) is ZZZZ.
316	
317	
318	Note) Unfortunately, there is no information to show which section belongs
319	to ZONE_MOVABLE. This is TBD.
320	
321	
322	6.2. How to offline memory
323	------------
324	You can offline a section by using the same sysfs interface that was used in
325	memory onlining.
326	
327	% echo offline > /sys/devices/system/memory/memoryXXX/state
328	
329	If offline succeeds, the state of the memory section is changed to be "offline".
330	If it fails, some error core (like -EBUSY) will be returned by the kernel.
331	Even if a section does not belong to ZONE_MOVABLE, you can try to offline it.
332	If it doesn't contain 'unmovable' memory, you'll get success.
333	
334	A section under ZONE_MOVABLE is considered to be able to be offlined easily.
335	But under some busy state, it may return -EBUSY. Even if a memory section
336	cannot be offlined due to -EBUSY, you can retry offlining it and may be able to
337	offline it (or not).
338	(For example, a page is referred to by some kernel internal call and released
339	 soon.)
340	
341	Consideration:
342	Memory hotplug's design direction is to make the possibility of memory offlining
343	higher and to guarantee unplugging memory under any situation. But it needs
344	more work. Returning -EBUSY under some situation may be good because the user
345	can decide to retry more or not by himself. Currently, memory offlining code
346	does some amount of retry with 120 seconds timeout.
347	
348	-------------------------
349	7. Physical memory remove
350	-------------------------
351	Need more implementation yet....
352	 - Notification completion of remove works by OS to firmware.
353	 - Guard from remove if not yet.
354	
355	--------------------------------
356	8. Memory hotplug event notifier
357	--------------------------------
358	Memory hotplug has event notifier. There are 6 types of notification.
359	
360	MEMORY_GOING_ONLINE
361	  Generated before new memory becomes available in order to be able to
362	  prepare subsystems to handle memory. The page allocator is still unable
363	  to allocate from the new memory.
364	
365	MEMORY_CANCEL_ONLINE
366	  Generated if MEMORY_GOING_ONLINE fails.
367	
368	MEMORY_ONLINE
369	  Generated when memory has successfully brought online. The callback may
370	  allocate pages from the new memory.
371	
372	MEMORY_GOING_OFFLINE
373	  Generated to begin the process of offlining memory. Allocations are no
374	  longer possible from the memory but some of the memory to be offlined
375	  is still in use. The callback can be used to free memory known to a
376	  subsystem from the indicated memory section.
377	
378	MEMORY_CANCEL_OFFLINE
379	  Generated if MEMORY_GOING_OFFLINE fails. Memory is available again from
380	  the section that we attempted to offline.
381	
382	MEMORY_OFFLINE
383	  Generated after offlining memory is complete.
384	
385	A callback routine can be registered by
386	  hotplug_memory_notifier(callback_func, priority)
387	
388	The second argument of callback function (action) is event types of above.
389	The third argument is passed by pointer of struct memory_notify.
390	
391	struct memory_notify {
392	       unsigned long start_pfn;
393	       unsigned long nr_pages;
394	       int status_change_nid_normal;
395	       int status_change_nid_high;
396	       int status_change_nid;
397	}
398	
399	start_pfn is start_pfn of online/offline memory.
400	nr_pages is # of pages of online/offline memory.
401	status_change_nid_normal is set node id when N_NORMAL_MEMORY of nodemask
402	is (will be) set/clear, if this is -1, then nodemask status is not changed.
403	status_change_nid_high is set node id when N_HIGH_MEMORY of nodemask
404	is (will be) set/clear, if this is -1, then nodemask status is not changed.
405	status_change_nid is set node id when N_MEMORY of nodemask is (will be)
406	set/clear. It means a new(memoryless) node gets new memory by online and a
407	node loses all memory. If this is -1, then nodemask status is not changed.
408	If status_changed_nid* >= 0, callback should create/discard structures for the
409	node if necessary.
410	
411	--------------
412	9. Future Work
413	--------------
414	  - allowing memory hot-add to ZONE_MOVABLE. maybe we need some switch like
415	    sysctl or new control file.
416	  - showing memory section and physical device relationship.
417	  - showing memory section is under ZONE_MOVABLE or not
418	  - test and make it better memory offlining.
419	  - support HugeTLB page migration and offlining.
420	  - memmap removing at memory offline.
421	  - physical remove memory.
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