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Based on kernel version 3.16. Page generated on 2014-08-06 21:40 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 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 4 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	
159	'phys_index'      : read-only and contains memory block id, same as XXX.
160	'state'           : read-write
161	                    at read:  contains online/offline state of memory.
162	                    at write: user can specify "online_kernel",
163	                    "online_movable", "online", "offline" command
164	                    which will be performed on all sections in the block.
165	'phys_device'     : read-only: designed to show the name of physical memory
166	                    device.  This is not well implemented now.
167	'removable'       : read-only: contains an integer value indicating
168	                    whether the memory block is removable or not
169	                    removable.  A value of 1 indicates that the memory
170	                    block is removable and a value of 0 indicates that
171	                    it is not removable. A memory block is removable only if
172	                    every section in the block is removable.
173	
174	NOTE:
175	  These directories/files appear after physical memory hotplug phase.
176	
177	If CONFIG_NUMA is enabled the memoryXXX/ directories can also be accessed
178	via symbolic links located in the /sys/devices/system/node/node* directories.
179	
180	For example:
181	/sys/devices/system/node/node0/memory9 -> ../../memory/memory9
182	
183	A backlink will also be created:
184	/sys/devices/system/memory/memory9/node0 -> ../../node/node0
185	
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	On some architectures, the firmware may not notify the kernel of a memory
213	hotplug event.  Therefore, the memory "probe" interface is supported to
214	explicitly notify the kernel.  This interface depends on
215	CONFIG_ARCH_MEMORY_PROBE and can be configured on powerpc, sh, and x86
216	if hotplug is supported, although for x86 this should be handled by ACPI
217	notification.
218	
219	Probe interface is located at
220	/sys/devices/system/memory/probe
221	
222	You can tell the physical address of new memory to the kernel by
223	
224	% echo start_address_of_new_memory > /sys/devices/system/memory/probe
225	
226	Then, [start_address_of_new_memory, start_address_of_new_memory +
227	memory_block_size] memory range is hot-added. In this case, hotplug script is
228	not called (in current implementation). You'll have to online memory by
229	yourself.  Please see "How to online memory" in this text.
230	
231	
232	------------------------------
233	5. Logical Memory hot-add phase
234	------------------------------
235	
236	5.1. State of memory
237	------------
238	To see (online/offline) state of a memory block, read 'state' file.
239	
240	% cat /sys/device/system/memory/memoryXXX/state
241	
242	
243	If the memory block is online, you'll read "online".
244	If the memory block is offline, you'll read "offline".
245	
246	
247	5.2. How to online memory
248	------------
249	Even if the memory is hot-added, it is not at ready-to-use state.
250	For using newly added memory, you have to "online" the memory block.
251	
252	For onlining, you have to write "online" to the memory block's state file as:
253	
254	% echo online > /sys/devices/system/memory/memoryXXX/state
255	
256	This onlining will not change the ZONE type of the target memory block,
257	If the memory block is in ZONE_NORMAL, you can change it to ZONE_MOVABLE:
258	
259	% echo online_movable > /sys/devices/system/memory/memoryXXX/state
260	(NOTE: current limit: this memory block must be adjacent to ZONE_MOVABLE)
261	
262	And if the memory block is in ZONE_MOVABLE, you can change it to ZONE_NORMAL:
263	
264	% echo online_kernel > /sys/devices/system/memory/memoryXXX/state
265	(NOTE: current limit: this memory block must be adjacent to ZONE_NORMAL)
266	
267	After this, memory block XXX's state will be 'online' and the amount of
268	available memory will be increased.
269	
270	Currently, newly added memory is added as ZONE_NORMAL (for powerpc, ZONE_DMA).
271	This may be changed in future.
272	
273	
274	
275	------------------------
276	6. Logical memory remove
277	------------------------
278	
279	6.1 Memory offline and ZONE_MOVABLE
280	------------
281	Memory offlining is more complicated than memory online. Because memory offline
282	has to make the whole memory block be unused, memory offline can fail if
283	the memory block includes memory which cannot be freed.
284	
285	In general, memory offline can use 2 techniques.
286	
287	(1) reclaim and free all memory in the memory block.
288	(2) migrate all pages in the memory block.
289	
290	In the current implementation, Linux's memory offline uses method (2), freeing
291	all  pages in the memory block by page migration. But not all pages are
292	migratable. Under current Linux, migratable pages are anonymous pages and
293	page caches. For offlining a memory block by migration, the kernel has to
294	guarantee that the memory block contains only migratable pages.
295	
296	Now, a boot option for making a memory block which consists of migratable pages
297	is supported. By specifying "kernelcore=" or "movablecore=" boot option, you can
298	create ZONE_MOVABLE...a zone which is just used for movable pages.
299	(See also Documentation/kernel-parameters.txt)
300	
301	Assume the system has "TOTAL" amount of memory at boot time, this boot option
302	creates ZONE_MOVABLE as following.
303	
304	1) When kernelcore=YYYY boot option is used,
305	  Size of memory not for movable pages (not for offline) is YYYY.
306	  Size of memory for movable pages (for offline) is TOTAL-YYYY.
307	
308	2) When movablecore=ZZZZ boot option is used,
309	  Size of memory not for movable pages (not for offline) is TOTAL - ZZZZ.
310	  Size of memory for movable pages (for offline) is ZZZZ.
311	
312	
313	Note: Unfortunately, there is no information to show which memory block belongs
314	to ZONE_MOVABLE. This is TBD.
315	
316	
317	6.2. How to offline memory
318	------------
319	You can offline a memory block by using the same sysfs interface that was used
320	in memory onlining.
321	
322	% echo offline > /sys/devices/system/memory/memoryXXX/state
323	
324	If offline succeeds, the state of the memory block is changed to be "offline".
325	If it fails, some error core (like -EBUSY) will be returned by the kernel.
326	Even if a memory block does not belong to ZONE_MOVABLE, you can try to offline
327	it.  If it doesn't contain 'unmovable' memory, you'll get success.
328	
329	A memory block under ZONE_MOVABLE is considered to be able to be offlined
330	easily.  But under some busy state, it may return -EBUSY. Even if a memory
331	block cannot be offlined due to -EBUSY, you can retry offlining it and may be
332	able to offline it (or not). (For example, a page is referred to by some kernel
333	internal call and released soon.)
334	
335	Consideration:
336	Memory hotplug's design direction is to make the possibility of memory offlining
337	higher and to guarantee unplugging memory under any situation. But it needs
338	more work. Returning -EBUSY under some situation may be good because the user
339	can decide to retry more or not by himself. Currently, memory offlining code
340	does some amount of retry with 120 seconds timeout.
341	
342	-------------------------
343	7. Physical memory remove
344	-------------------------
345	Need more implementation yet....
346	 - Notification completion of remove works by OS to firmware.
347	 - Guard from remove if not yet.
348	
349	--------------------------------
350	8. Memory hotplug event notifier
351	--------------------------------
352	Memory hotplug has event notifier. There are 6 types of notification.
353	
354	MEMORY_GOING_ONLINE
355	  Generated before new memory becomes available in order to be able to
356	  prepare subsystems to handle memory. The page allocator is still unable
357	  to allocate from the new memory.
358	
359	MEMORY_CANCEL_ONLINE
360	  Generated if MEMORY_GOING_ONLINE fails.
361	
362	MEMORY_ONLINE
363	  Generated when memory has successfully brought online. The callback may
364	  allocate pages from the new memory.
365	
366	MEMORY_GOING_OFFLINE
367	  Generated to begin the process of offlining memory. Allocations are no
368	  longer possible from the memory but some of the memory to be offlined
369	  is still in use. The callback can be used to free memory known to a
370	  subsystem from the indicated memory block.
371	
372	MEMORY_CANCEL_OFFLINE
373	  Generated if MEMORY_GOING_OFFLINE fails. Memory is available again from
374	  the memory block that we attempted to offline.
375	
376	MEMORY_OFFLINE
377	  Generated after offlining memory is complete.
378	
379	A callback routine can be registered by
380	  hotplug_memory_notifier(callback_func, priority)
381	
382	The second argument of callback function (action) is event types of above.
383	The third argument is passed by pointer of struct memory_notify.
384	
385	struct memory_notify {
386	       unsigned long start_pfn;
387	       unsigned long nr_pages;
388	       int status_change_nid_normal;
389	       int status_change_nid_high;
390	       int status_change_nid;
391	}
392	
393	start_pfn is start_pfn of online/offline memory.
394	nr_pages is # of pages of online/offline memory.
395	status_change_nid_normal is set node id when N_NORMAL_MEMORY of nodemask
396	is (will be) set/clear, if this is -1, then nodemask status is not changed.
397	status_change_nid_high is set node id when N_HIGH_MEMORY of nodemask
398	is (will be) set/clear, if this is -1, then nodemask status is not changed.
399	status_change_nid is set node id when N_MEMORY of nodemask is (will be)
400	set/clear. It means a new(memoryless) node gets new memory by online and a
401	node loses all memory. If this is -1, then nodemask status is not changed.
402	If status_changed_nid* >= 0, callback should create/discard structures for the
403	node if necessary.
404	
405	--------------
406	9. Future Work
407	--------------
408	  - allowing memory hot-add to ZONE_MOVABLE. maybe we need some switch like
409	    sysctl or new control file.
410	  - showing memory block and physical device relationship.
411	  - showing memory block is under ZONE_MOVABLE or not
412	  - test and make it better memory offlining.
413	  - support HugeTLB page migration and offlining.
414	  - memmap removing at memory offline.
415	  - physical remove memory.
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