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Based on kernel version 3.4. Page generated on 2012-05-21 22:07 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", "offline" command
165	                    which will be performed on al 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	
175	NOTE:
176	  These directories/files appear after physical memory hotplug phase.
177	
178	If CONFIG_NUMA is enabled the memoryXXX/ directories can also be accessed
179	via symbolic links located in the /sys/devices/system/node/node* directories.
180	
181	For example:
182	/sys/devices/system/node/node0/memory9 -> ../../memory/memory9
183	
184	A backlink will also be created:
185	/sys/devices/system/memory/memory9/node0 -> ../../node/node0
186	
187	--------------------------------
188	4. Physical memory hot-add phase
189	--------------------------------
190	
191	4.1 Hardware(Firmware) Support
192	------------
193	On x86_64/ia64 platform, memory hotplug by ACPI is supported.
194	
195	In general, the firmware (ACPI) which supports memory hotplug defines
196	memory class object of _HID "PNP0C80". When a notify is asserted to PNP0C80,
197	Linux's ACPI handler does hot-add memory to the system and calls a hotplug udev
198	script. This will be done automatically.
199	
200	But scripts for memory hotplug are not contained in generic udev package(now).
201	You may have to write it by yourself or online/offline memory by hand.
202	Please see "How to online memory", "How to offline memory" in this text.
203	
204	If firmware supports NUMA-node hotplug, and defines an object _HID "ACPI0004",
205	"PNP0A05", or "PNP0A06", notification is asserted to it, and ACPI handler
206	calls hotplug code for all of objects which are defined in it.
207	If memory device is found, memory hotplug code will be called.
208	
209	
210	4.2 Notify memory hot-add event by hand
211	------------
212	In some environments, especially virtualized environment, firmware will not
213	notify memory hotplug event to the kernel. For such environment, "probe"
214	interface is supported. This interface depends on CONFIG_ARCH_MEMORY_PROBE.
215	
216	Now, CONFIG_ARCH_MEMORY_PROBE is supported only by powerpc but it does not
217	contain highly architecture codes. Please add config if you need "probe"
218	interface.
219	
220	Probe interface is located at
221	/sys/devices/system/memory/probe
222	
223	You can tell the physical address of new memory to the kernel by
224	
225	% echo start_address_of_new_memory > /sys/devices/system/memory/probe
226	
227	Then, [start_address_of_new_memory, start_address_of_new_memory + section_size)
228	memory range is hot-added. In this case, hotplug script is not called (in
229	current implementation). You'll have to online memory by yourself.
230	Please see "How to online memory" in this text.
231	
232	
233	
234	------------------------------
235	5. Logical Memory hot-add phase
236	------------------------------
237	
238	5.1. State of memory
239	------------
240	To see (online/offline) state of memory section, read 'state' file.
241	
242	% cat /sys/device/system/memory/memoryXXX/state
243	
244	
245	If the memory section is online, you'll read "online".
246	If the memory section is offline, you'll read "offline".
247	
248	
249	5.2. How to online memory
250	------------
251	Even if the memory is hot-added, it is not at ready-to-use state.
252	For using newly added memory, you have to "online" the memory section.
253	
254	For onlining, you have to write "online" to the section's state file as:
255	
256	% echo online > /sys/devices/system/memory/memoryXXX/state
257	
258	After this, section memoryXXX's state will be 'online' and the amount of
259	available memory will be increased.
260	
261	Currently, newly added memory is added as ZONE_NORMAL (for powerpc, ZONE_DMA).
262	This may be changed in future.
263	
264	
265	
266	------------------------
267	6. Logical memory remove
268	------------------------
269	
270	6.1 Memory offline and ZONE_MOVABLE
271	------------
272	Memory offlining is more complicated than memory online. Because memory offline
273	has to make the whole memory section be unused, memory offline can fail if
274	the section includes memory which cannot be freed.
275	
276	In general, memory offline can use 2 techniques.
277	
278	(1) reclaim and free all memory in the section.
279	(2) migrate all pages in the section.
280	
281	In the current implementation, Linux's memory offline uses method (2), freeing
282	all  pages in the section by page migration. But not all pages are
283	migratable. Under current Linux, migratable pages are anonymous pages and
284	page caches. For offlining a section by migration, the kernel has to guarantee
285	that the section contains only migratable pages.
286	
287	Now, a boot option for making a section which consists of migratable pages is
288	supported. By specifying "kernelcore=" or "movablecore=" boot option, you can
289	create ZONE_MOVABLE...a zone which is just used for movable pages.
290	(See also Documentation/kernel-parameters.txt)
291	
292	Assume the system has "TOTAL" amount of memory at boot time, this boot option
293	creates ZONE_MOVABLE as following.
294	
295	1) When kernelcore=YYYY boot option is used,
296	  Size of memory not for movable pages (not for offline) is YYYY.
297	  Size of memory for movable pages (for offline) is TOTAL-YYYY.
298	
299	2) When movablecore=ZZZZ boot option is used,
300	  Size of memory not for movable pages (not for offline) is TOTAL - ZZZZ.
301	  Size of memory for movable pages (for offline) is ZZZZ.
302	
303	
304	Note) Unfortunately, there is no information to show which section belongs
305	to ZONE_MOVABLE. This is TBD.
306	
307	
308	6.2. How to offline memory
309	------------
310	You can offline a section by using the same sysfs interface that was used in
311	memory onlining.
312	
313	% echo offline > /sys/devices/system/memory/memoryXXX/state
314	
315	If offline succeeds, the state of the memory section is changed to be "offline".
316	If it fails, some error core (like -EBUSY) will be returned by the kernel.
317	Even if a section does not belong to ZONE_MOVABLE, you can try to offline it.
318	If it doesn't contain 'unmovable' memory, you'll get success.
319	
320	A section under ZONE_MOVABLE is considered to be able to be offlined easily.
321	But under some busy state, it may return -EBUSY. Even if a memory section
322	cannot be offlined due to -EBUSY, you can retry offlining it and may be able to
323	offline it (or not).
324	(For example, a page is referred to by some kernel internal call and released
325	 soon.)
326	
327	Consideration:
328	Memory hotplug's design direction is to make the possibility of memory offlining
329	higher and to guarantee unplugging memory under any situation. But it needs
330	more work. Returning -EBUSY under some situation may be good because the user
331	can decide to retry more or not by himself. Currently, memory offlining code
332	does some amount of retry with 120 seconds timeout.
333	
334	-------------------------
335	7. Physical memory remove
336	-------------------------
337	Need more implementation yet....
338	 - Notification completion of remove works by OS to firmware.
339	 - Guard from remove if not yet.
340	
341	--------------------------------
342	8. Memory hotplug event notifier
343	--------------------------------
344	Memory hotplug has event notifer. There are 6 types of notification.
345	
346	MEMORY_GOING_ONLINE
347	  Generated before new memory becomes available in order to be able to
348	  prepare subsystems to handle memory. The page allocator is still unable
349	  to allocate from the new memory.
350	
351	MEMORY_CANCEL_ONLINE
352	  Generated if MEMORY_GOING_ONLINE fails.
353	
354	MEMORY_ONLINE
355	  Generated when memory has successfully brought online. The callback may
356	  allocate pages from the new memory.
357	
358	MEMORY_GOING_OFFLINE
359	  Generated to begin the process of offlining memory. Allocations are no
360	  longer possible from the memory but some of the memory to be offlined
361	  is still in use. The callback can be used to free memory known to a
362	  subsystem from the indicated memory section.
363	
364	MEMORY_CANCEL_OFFLINE
365	  Generated if MEMORY_GOING_OFFLINE fails. Memory is available again from
366	  the section that we attempted to offline.
367	
368	MEMORY_OFFLINE
369	  Generated after offlining memory is complete.
370	
371	A callback routine can be registered by
372	  hotplug_memory_notifier(callback_func, priority)
373	
374	The second argument of callback function (action) is event types of above.
375	The third argument is passed by pointer of struct memory_notify.
376	
377	struct memory_notify {
378	       unsigned long start_pfn;
379	       unsigned long nr_pages;
380	       int status_change_nid;
381	}
382	
383	start_pfn is start_pfn of online/offline memory.
384	nr_pages is # of pages of online/offline memory.
385	status_change_nid is set node id when N_HIGH_MEMORY of nodemask is (will be)
386	set/clear. It means a new(memoryless) node gets new memory by online and a
387	node loses all memory. If this is -1, then nodemask status is not changed.
388	If status_changed_nid >= 0, callback should create/discard structures for the
389	node if necessary.
390	
391	--------------
392	9. Future Work
393	--------------
394	  - allowing memory hot-add to ZONE_MOVABLE. maybe we need some switch like
395	    sysctl or new control file.
396	  - showing memory section and physical device relationship.
397	  - showing memory section is under ZONE_MOVABLE or not
398	  - test and make it better memory offlining.
399	  - support HugeTLB page migration and offlining.
400	  - memmap removing at memory offline.
401	  - physical remove memory.
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