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Based on kernel version 4.7.2. Page generated on 2016-08-22 22:46 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 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	When the memory is hot-added, the kernel decides whether or not to "online"
260	it according to the policy which can be read from "auto_online_blocks" file:
261	
262	% cat /sys/devices/system/memory/auto_online_blocks
263	
264	The default depends on the CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE kernel config
265	option. If it is disabled the default is "offline" which means the newly added
266	memory is not in a ready-to-use state and you have to "online" the newly added
267	memory blocks manually. Automatic onlining can be requested by writing "online"
268	to "auto_online_blocks" file:
269	
270	% echo online > /sys/devices/system/memory/auto_online_blocks
271	
272	This sets a global policy and impacts all memory blocks that will subsequently
273	be hotplugged. Currently offline blocks keep their state. It is possible, under
274	certain circumstances, that some memory blocks will be added but will fail to
275	online. User space tools can check their "state" files
276	(/sys/devices/system/memory/memoryXXX/state) and try to online them manually.
277	
278	If the automatic onlining wasn't requested, failed, or some memory block was
279	offlined it is possible to change the individual block's state by writing to the
280	"state" file:
281	
282	% echo online > /sys/devices/system/memory/memoryXXX/state
283	
284	This onlining will not change the ZONE type of the target memory block,
285	If the memory block is in ZONE_NORMAL, you can change it to ZONE_MOVABLE:
286	
287	% echo online_movable > /sys/devices/system/memory/memoryXXX/state
288	(NOTE: current limit: this memory block must be adjacent to ZONE_MOVABLE)
289	
290	And if the memory block is in ZONE_MOVABLE, you can change it to ZONE_NORMAL:
291	
292	% echo online_kernel > /sys/devices/system/memory/memoryXXX/state
293	(NOTE: current limit: this memory block must be adjacent to ZONE_NORMAL)
294	
295	After this, memory block XXX's state will be 'online' and the amount of
296	available memory will be increased.
297	
298	Currently, newly added memory is added as ZONE_NORMAL (for powerpc, ZONE_DMA).
299	This may be changed in future.
300	
301	
302	
303	------------------------
304	6. Logical memory remove
305	------------------------
306	
307	6.1 Memory offline and ZONE_MOVABLE
308	------------
309	Memory offlining is more complicated than memory online. Because memory offline
310	has to make the whole memory block be unused, memory offline can fail if
311	the memory block includes memory which cannot be freed.
312	
313	In general, memory offline can use 2 techniques.
314	
315	(1) reclaim and free all memory in the memory block.
316	(2) migrate all pages in the memory block.
317	
318	In the current implementation, Linux's memory offline uses method (2), freeing
319	all  pages in the memory block by page migration. But not all pages are
320	migratable. Under current Linux, migratable pages are anonymous pages and
321	page caches. For offlining a memory block by migration, the kernel has to
322	guarantee that the memory block contains only migratable pages.
323	
324	Now, a boot option for making a memory block which consists of migratable pages
325	is supported. By specifying "kernelcore=" or "movablecore=" boot option, you can
326	create ZONE_MOVABLE...a zone which is just used for movable pages.
327	(See also Documentation/kernel-parameters.txt)
328	
329	Assume the system has "TOTAL" amount of memory at boot time, this boot option
330	creates ZONE_MOVABLE as following.
331	
332	1) When kernelcore=YYYY boot option is used,
333	  Size of memory not for movable pages (not for offline) is YYYY.
334	  Size of memory for movable pages (for offline) is TOTAL-YYYY.
335	
336	2) When movablecore=ZZZZ boot option is used,
337	  Size of memory not for movable pages (not for offline) is TOTAL - ZZZZ.
338	  Size of memory for movable pages (for offline) is ZZZZ.
339	
340	
341	Note: Unfortunately, there is no information to show which memory block belongs
342	to ZONE_MOVABLE. This is TBD.
343	
344	
345	6.2. How to offline memory
346	------------
347	You can offline a memory block by using the same sysfs interface that was used
348	in memory onlining.
349	
350	% echo offline > /sys/devices/system/memory/memoryXXX/state
351	
352	If offline succeeds, the state of the memory block is changed to be "offline".
353	If it fails, some error core (like -EBUSY) will be returned by the kernel.
354	Even if a memory block does not belong to ZONE_MOVABLE, you can try to offline
355	it.  If it doesn't contain 'unmovable' memory, you'll get success.
356	
357	A memory block under ZONE_MOVABLE is considered to be able to be offlined
358	easily.  But under some busy state, it may return -EBUSY. Even if a memory
359	block cannot be offlined due to -EBUSY, you can retry offlining it and may be
360	able to offline it (or not). (For example, a page is referred to by some kernel
361	internal call and released soon.)
362	
363	Consideration:
364	Memory hotplug's design direction is to make the possibility of memory offlining
365	higher and to guarantee unplugging memory under any situation. But it needs
366	more work. Returning -EBUSY under some situation may be good because the user
367	can decide to retry more or not by himself. Currently, memory offlining code
368	does some amount of retry with 120 seconds timeout.
369	
370	-------------------------
371	7. Physical memory remove
372	-------------------------
373	Need more implementation yet....
374	 - Notification completion of remove works by OS to firmware.
375	 - Guard from remove if not yet.
376	
377	--------------------------------
378	8. Memory hotplug event notifier
379	--------------------------------
380	Hotplugging events are sent to a notification queue.
381	
382	There are six types of notification defined in include/linux/memory.h:
383	
384	MEM_GOING_ONLINE
385	  Generated before new memory becomes available in order to be able to
386	  prepare subsystems to handle memory. The page allocator is still unable
387	  to allocate from the new memory.
388	
389	MEM_CANCEL_ONLINE
390	  Generated if MEMORY_GOING_ONLINE fails.
391	
392	MEM_ONLINE
393	  Generated when memory has successfully brought online. The callback may
394	  allocate pages from the new memory.
395	
396	MEM_GOING_OFFLINE
397	  Generated to begin the process of offlining memory. Allocations are no
398	  longer possible from the memory but some of the memory to be offlined
399	  is still in use. The callback can be used to free memory known to a
400	  subsystem from the indicated memory block.
401	
402	MEM_CANCEL_OFFLINE
403	  Generated if MEMORY_GOING_OFFLINE fails. Memory is available again from
404	  the memory block that we attempted to offline.
405	
406	MEM_OFFLINE
407	  Generated after offlining memory is complete.
408	
409	A callback routine can be registered by calling
410	
411	  hotplug_memory_notifier(callback_func, priority)
412	
413	Callback functions with higher values of priority are called before callback
414	functions with lower values.
415	
416	A callback function must have the following prototype:
417	
418	  int callback_func(
419	    struct notifier_block *self, unsigned long action, void *arg);
420	
421	The first argument of the callback function (self) is a pointer to the block
422	of the notifier chain that points to the callback function itself.
423	The second argument (action) is one of the event types described above.
424	The third argument (arg) passes a pointer of struct memory_notify.
425	
426	struct memory_notify {
427	       unsigned long start_pfn;
428	       unsigned long nr_pages;
429	       int status_change_nid_normal;
430	       int status_change_nid_high;
431	       int status_change_nid;
432	}
433	
434	start_pfn is start_pfn of online/offline memory.
435	nr_pages is # of pages of online/offline memory.
436	status_change_nid_normal is set node id when N_NORMAL_MEMORY of nodemask
437	is (will be) set/clear, if this is -1, then nodemask status is not changed.
438	status_change_nid_high is set node id when N_HIGH_MEMORY of nodemask
439	is (will be) set/clear, if this is -1, then nodemask status is not changed.
440	status_change_nid is set node id when N_MEMORY of nodemask is (will be)
441	set/clear. It means a new(memoryless) node gets new memory by online and a
442	node loses all memory. If this is -1, then nodemask status is not changed.
443	If status_changed_nid* >= 0, callback should create/discard structures for the
444	node if necessary.
445	
446	The callback routine shall return one of the values
447	NOTIFY_DONE, NOTIFY_OK, NOTIFY_BAD, NOTIFY_STOP
448	defined in include/linux/notifier.h
449	
450	NOTIFY_DONE and NOTIFY_OK have no effect on the further processing.
451	
452	NOTIFY_BAD is used as response to the MEM_GOING_ONLINE, MEM_GOING_OFFLINE,
453	MEM_ONLINE, or MEM_OFFLINE action to cancel hotplugging. It stops
454	further processing of the notification queue.
455	
456	NOTIFY_STOP stops further processing of the notification queue.
457	
458	--------------
459	9. Future Work
460	--------------
461	  - allowing memory hot-add to ZONE_MOVABLE. maybe we need some switch like
462	    sysctl or new control file.
463	  - showing memory block and physical device relationship.
464	  - test and make it better memory offlining.
465	  - support HugeTLB page migration and offlining.
466	  - memmap removing at memory offline.
467	  - physical remove memory.
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