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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 EST.

1	==============
2	Memory Hotplug
3	==============
4	
5	:Created:							Jul 28 2007
6	:Updated: 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	.. CONTENTS
13	
14	  1. Introduction
15	    1.1 purpose of memory hotplug
16	    1.2. Phases of memory hotplug
17	    1.3. Unit of Memory online/offline operation
18	  2. Kernel Configuration
19	  3. sysfs files for memory hotplug
20	  4. Physical memory hot-add phase
21	    4.1 Hardware(Firmware) Support
22	    4.2 Notify memory hot-add event by hand
23	  5. Logical Memory hot-add phase
24	    5.1. State of memory
25	    5.2. How to online memory
26	  6. Logical memory remove
27	    6.1 Memory offline and ZONE_MOVABLE
28	    6.2. How to offline memory
29	  7. Physical memory remove
30	  8. Memory hotplug event notifier
31	  9. Future Work List
32	
33	
34	.. note::
35	
36	    (1) x86_64's has special implementation for memory hotplug.
37	        This text does not describe it.
38	    (2) This text assumes that sysfs is mounted at /sys.
39	
40	
41	Introduction
42	============
43	
44	purpose of memory hotplug
45	-------------------------
46	
47	Memory Hotplug allows users to increase/decrease the amount of memory.
48	Generally, there are two purposes.
49	
50	(A) For changing the amount of memory.
51	    This is to allow a feature like capacity on demand.
52	(B) For installing/removing DIMMs or NUMA-nodes physically.
53	    This is to exchange DIMMs/NUMA-nodes, reduce power consumption, etc.
54	
55	(A) is required by highly virtualized environments and (B) is required by
56	hardware which supports memory power management.
57	
58	Linux memory hotplug is designed for both purpose.
59	
60	
61	Phases of memory hotplug
62	------------------------
63	
64	There are 2 phases in Memory Hotplug:
65	
66	  1) Physical Memory Hotplug phase
67	  2) Logical Memory Hotplug phase.
68	
69	The First phase is to communicate hardware/firmware and make/erase
70	environment for hotplugged memory. Basically, this phase is necessary
71	for the purpose (B), but this is good phase for communication between
72	highly virtualized environments too.
73	
74	When memory is hotplugged, the kernel recognizes new memory, makes new memory
75	management tables, and makes sysfs files for new memory's operation.
76	
77	If firmware supports notification of connection of new memory to OS,
78	this phase is triggered automatically. ACPI can notify this event. If not,
79	"probe" operation by system administration is used instead.
80	(see :ref:`memory_hotplug_physical_mem`).
81	
82	Logical Memory Hotplug phase is to change memory state into
83	available/unavailable for users. Amount of memory from user's view is
84	changed by this phase. The kernel makes all memory in it as free pages
85	when a memory range is available.
86	
87	In this document, this phase is described as online/offline.
88	
89	Logical Memory Hotplug phase is triggered by write of sysfs file by system
90	administrator. For the hot-add case, it must be executed after Physical Hotplug
91	phase by hand.
92	(However, if you writes udev's hotplug scripts for memory hotplug, these
93	phases can be execute in seamless way.)
94	
95	
96	Unit of Memory online/offline operation
97	---------------------------------------
98	
99	Memory hotplug uses SPARSEMEM memory model which allows memory to be divided
100	into chunks of the same size. These chunks are called "sections". The size of
101	a memory section is architecture dependent. For example, power uses 16MiB, ia64
102	uses 1GiB.
103	
104	Memory sections are combined into chunks referred to as "memory blocks". The
105	size of a memory block is architecture dependent and represents the logical
106	unit upon which memory online/offline operations are to be performed. The
107	default size of a memory block is the same as memory section size unless an
108	architecture specifies otherwise. (see :ref:`memory_hotplug_sysfs_files`.)
109	
110	To determine the size (in bytes) of a memory block please read this file:
111	
112	/sys/devices/system/memory/block_size_bytes
113	
114	
115	Kernel Configuration
116	====================
117	
118	To use memory hotplug feature, kernel must be compiled with following
119	config options.
120	
121	- For all memory hotplug:
122	    - Memory model -> Sparse Memory  (CONFIG_SPARSEMEM)
123	    - Allow for memory hot-add       (CONFIG_MEMORY_HOTPLUG)
124	
125	- To enable memory removal, the following are also necessary:
126	    - Allow for memory hot remove    (CONFIG_MEMORY_HOTREMOVE)
127	    - Page Migration                 (CONFIG_MIGRATION)
128	
129	- For ACPI memory hotplug, the following are also necessary:
130	    - Memory hotplug (under ACPI Support menu) (CONFIG_ACPI_HOTPLUG_MEMORY)
131	    - This option can be kernel module.
132	
133	- As a related configuration, if your box has a feature of NUMA-node hotplug
134	  via ACPI, then this option is necessary too.
135	
136	    - ACPI0004,PNP0A05 and PNP0A06 Container Driver (under ACPI Support menu)
137	      (CONFIG_ACPI_CONTAINER).
138	
139	     This option can be kernel module too.
140	
141	
142	.. _memory_hotplug_sysfs_files:
143	
144	sysfs files for memory hotplug
145	==============================
146	
147	All memory blocks have their device information in sysfs.  Each memory block
148	is described under /sys/devices/system/memory as:
149	
150		/sys/devices/system/memory/memoryXXX
151		(XXX is the memory block id.)
152	
153	For the memory block covered by the sysfs directory.  It is expected that all
154	memory sections in this range are present and no memory holes exist in the
155	range. Currently there is no way to determine if there is a memory hole, but
156	the existence of one should not affect the hotplug capabilities of the memory
157	block.
158	
159	For example, assume 1GiB memory block size. A device for a memory starting at
160	0x100000000 is /sys/device/system/memory/memory4::
161	
162		(0x100000000 / 1Gib = 4)
163	
164	This device covers address range [0x100000000 ... 0x140000000)
165	
166	Under each memory block, you can see 5 files:
167	
168	- /sys/devices/system/memory/memoryXXX/phys_index
169	- /sys/devices/system/memory/memoryXXX/phys_device
170	- /sys/devices/system/memory/memoryXXX/state
171	- /sys/devices/system/memory/memoryXXX/removable
172	- /sys/devices/system/memory/memoryXXX/valid_zones
173	
174	=================== ============================================================
175	``phys_index``      read-only and contains memory block id, same as XXX.
176	``state``           read-write
177	
178	                    - at read:  contains online/offline state of memory.
179	                    - at write: user can specify "online_kernel",
180	
181	                    "online_movable", "online", "offline" command
182	                    which will be performed on all sections in the block.
183	``phys_device``     read-only: designed to show the name of physical memory
184	                    device.  This is not well implemented now.
185	``removable``       read-only: contains an integer value indicating
186	                    whether the memory block is removable or not
187	                    removable.  A value of 1 indicates that the memory
188	                    block is removable and a value of 0 indicates that
189	                    it is not removable. A memory block is removable only if
190	                    every section in the block is removable.
191	``valid_zones``     read-only: designed to show which zones this memory block
192			    can be onlined to.
193	
194			    The first column shows it`s default zone.
195	
196			    "memory6/valid_zones: Normal Movable" shows this memoryblock
197			    can be onlined to ZONE_NORMAL by default and to ZONE_MOVABLE
198			    by online_movable.
199	
200			    "memory7/valid_zones: Movable Normal" shows this memoryblock
201			    can be onlined to ZONE_MOVABLE by default and to ZONE_NORMAL
202			    by online_kernel.
203	=================== ============================================================
204	
205	.. note::
206	
207	  These directories/files appear after physical memory hotplug phase.
208	
209	If CONFIG_NUMA is enabled the memoryXXX/ directories can also be accessed
210	via symbolic links located in the /sys/devices/system/node/node* directories.
211	
212	For example:
213	/sys/devices/system/node/node0/memory9 -> ../../memory/memory9
214	
215	A backlink will also be created:
216	/sys/devices/system/memory/memory9/node0 -> ../../node/node0
217	
218	.. _memory_hotplug_physical_mem:
219	
220	Physical memory hot-add phase
221	=============================
222	
223	Hardware(Firmware) Support
224	--------------------------
225	
226	On x86_64/ia64 platform, memory hotplug by ACPI is supported.
227	
228	In general, the firmware (ACPI) which supports memory hotplug defines
229	memory class object of _HID "PNP0C80". When a notify is asserted to PNP0C80,
230	Linux's ACPI handler does hot-add memory to the system and calls a hotplug udev
231	script. This will be done automatically.
232	
233	But scripts for memory hotplug are not contained in generic udev package(now).
234	You may have to write it by yourself or online/offline memory by hand.
235	Please see :ref:`memory_hotplug_how_to_online_memory` and
236	:ref:`memory_hotplug_how_to_offline_memory`.
237	
238	If firmware supports NUMA-node hotplug, and defines an object _HID "ACPI0004",
239	"PNP0A05", or "PNP0A06", notification is asserted to it, and ACPI handler
240	calls hotplug code for all of objects which are defined in it.
241	If memory device is found, memory hotplug code will be called.
242	
243	
244	Notify memory hot-add event by hand
245	-----------------------------------
246	
247	On some architectures, the firmware may not notify the kernel of a memory
248	hotplug event.  Therefore, the memory "probe" interface is supported to
249	explicitly notify the kernel.  This interface depends on
250	CONFIG_ARCH_MEMORY_PROBE and can be configured on powerpc, sh, and x86
251	if hotplug is supported, although for x86 this should be handled by ACPI
252	notification.
253	
254	Probe interface is located at
255	/sys/devices/system/memory/probe
256	
257	You can tell the physical address of new memory to the kernel by::
258	
259		% echo start_address_of_new_memory > /sys/devices/system/memory/probe
260	
261	Then, [start_address_of_new_memory, start_address_of_new_memory +
262	memory_block_size] memory range is hot-added. In this case, hotplug script is
263	not called (in current implementation). You'll have to online memory by
264	yourself.  Please see :ref:`memory_hotplug_how_to_online_memory`.
265	
266	
267	Logical Memory hot-add phase
268	============================
269	
270	State of memory
271	---------------
272	
273	To see (online/offline) state of a memory block, read 'state' file::
274	
275		% cat /sys/device/system/memory/memoryXXX/state
276	
277	
278	- If the memory block is online, you'll read "online".
279	- If the memory block is offline, you'll read "offline".
280	
281	
282	.. _memory_hotplug_how_to_online_memory:
283	
284	How to online memory
285	--------------------
286	
287	When the memory is hot-added, the kernel decides whether or not to "online"
288	it according to the policy which can be read from "auto_online_blocks" file::
289	
290		% cat /sys/devices/system/memory/auto_online_blocks
291	
292	The default depends on the CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE kernel config
293	option. If it is disabled the default is "offline" which means the newly added
294	memory is not in a ready-to-use state and you have to "online" the newly added
295	memory blocks manually. Automatic onlining can be requested by writing "online"
296	to "auto_online_blocks" file::
297	
298		% echo online > /sys/devices/system/memory/auto_online_blocks
299	
300	This sets a global policy and impacts all memory blocks that will subsequently
301	be hotplugged. Currently offline blocks keep their state. It is possible, under
302	certain circumstances, that some memory blocks will be added but will fail to
303	online. User space tools can check their "state" files
304	(/sys/devices/system/memory/memoryXXX/state) and try to online them manually.
305	
306	If the automatic onlining wasn't requested, failed, or some memory block was
307	offlined it is possible to change the individual block's state by writing to the
308	"state" file::
309	
310		% echo online > /sys/devices/system/memory/memoryXXX/state
311	
312	This onlining will not change the ZONE type of the target memory block,
313	If the memory block doesn't belong to any zone an appropriate kernel zone
314	(usually ZONE_NORMAL) will be used unless movable_node kernel command line
315	option is specified when ZONE_MOVABLE will be used.
316	
317	You can explicitly request to associate it with ZONE_MOVABLE by::
318	
319		% echo online_movable > /sys/devices/system/memory/memoryXXX/state
320	
321	.. note:: current limit: this memory block must be adjacent to ZONE_MOVABLE
322	
323	Or you can explicitly request a kernel zone (usually ZONE_NORMAL) by::
324	
325		% echo online_kernel > /sys/devices/system/memory/memoryXXX/state
326	
327	.. note:: current limit: this memory block must be adjacent to ZONE_NORMAL
328	
329	An explicit zone onlining can fail (e.g. when the range is already within
330	and existing and incompatible zone already).
331	
332	After this, memory block XXX's state will be 'online' and the amount of
333	available memory will be increased.
334	
335	This may be changed in future.
336	
337	
338	
339	Logical memory remove
340	=====================
341	
342	Memory offline and ZONE_MOVABLE
343	-------------------------------
344	
345	Memory offlining is more complicated than memory online. Because memory offline
346	has to make the whole memory block be unused, memory offline can fail if
347	the memory block includes memory which cannot be freed.
348	
349	In general, memory offline can use 2 techniques.
350	
351	(1) reclaim and free all memory in the memory block.
352	(2) migrate all pages in the memory block.
353	
354	In the current implementation, Linux's memory offline uses method (2), freeing
355	all  pages in the memory block by page migration. But not all pages are
356	migratable. Under current Linux, migratable pages are anonymous pages and
357	page caches. For offlining a memory block by migration, the kernel has to
358	guarantee that the memory block contains only migratable pages.
359	
360	Now, a boot option for making a memory block which consists of migratable pages
361	is supported. By specifying "kernelcore=" or "movablecore=" boot option, you can
362	create ZONE_MOVABLE...a zone which is just used for movable pages.
363	(See also Documentation/admin-guide/kernel-parameters.rst)
364	
365	Assume the system has "TOTAL" amount of memory at boot time, this boot option
366	creates ZONE_MOVABLE as following.
367	
368	1) When kernelcore=YYYY boot option is used,
369	   Size of memory not for movable pages (not for offline) is YYYY.
370	   Size of memory for movable pages (for offline) is TOTAL-YYYY.
371	
372	2) When movablecore=ZZZZ boot option is used,
373	   Size of memory not for movable pages (not for offline) is TOTAL - ZZZZ.
374	   Size of memory for movable pages (for offline) is ZZZZ.
375	
376	.. note::
377	
378	   Unfortunately, there is no information to show which memory block belongs
379	   to ZONE_MOVABLE. This is TBD.
380	
381	.. _memory_hotplug_how_to_offline_memory:
382	
383	How to offline memory
384	---------------------
385	
386	You can offline a memory block by using the same sysfs interface that was used
387	in memory onlining::
388	
389		% echo offline > /sys/devices/system/memory/memoryXXX/state
390	
391	If offline succeeds, the state of the memory block is changed to be "offline".
392	If it fails, some error core (like -EBUSY) will be returned by the kernel.
393	Even if a memory block does not belong to ZONE_MOVABLE, you can try to offline
394	it.  If it doesn't contain 'unmovable' memory, you'll get success.
395	
396	A memory block under ZONE_MOVABLE is considered to be able to be offlined
397	easily.  But under some busy state, it may return -EBUSY. Even if a memory
398	block cannot be offlined due to -EBUSY, you can retry offlining it and may be
399	able to offline it (or not). (For example, a page is referred to by some kernel
400	internal call and released soon.)
401	
402	Consideration:
403	  Memory hotplug's design direction is to make the possibility of memory
404	  offlining higher and to guarantee unplugging memory under any situation. But
405	  it needs more work. Returning -EBUSY under some situation may be good because
406	  the user can decide to retry more or not by himself. Currently, memory
407	  offlining code does some amount of retry with 120 seconds timeout.
408	
409	Physical memory remove
410	======================
411	
412	Need more implementation yet....
413	 - Notification completion of remove works by OS to firmware.
414	 - Guard from remove if not yet.
415	
416	Memory hotplug event notifier
417	=============================
418	
419	Hotplugging events are sent to a notification queue.
420	
421	There are six types of notification defined in include/linux/memory.h:
422	
423	MEM_GOING_ONLINE
424	  Generated before new memory becomes available in order to be able to
425	  prepare subsystems to handle memory. The page allocator is still unable
426	  to allocate from the new memory.
427	
428	MEM_CANCEL_ONLINE
429	  Generated if MEMORY_GOING_ONLINE fails.
430	
431	MEM_ONLINE
432	  Generated when memory has successfully brought online. The callback may
433	  allocate pages from the new memory.
434	
435	MEM_GOING_OFFLINE
436	  Generated to begin the process of offlining memory. Allocations are no
437	  longer possible from the memory but some of the memory to be offlined
438	  is still in use. The callback can be used to free memory known to a
439	  subsystem from the indicated memory block.
440	
441	MEM_CANCEL_OFFLINE
442	  Generated if MEMORY_GOING_OFFLINE fails. Memory is available again from
443	  the memory block that we attempted to offline.
444	
445	MEM_OFFLINE
446	  Generated after offlining memory is complete.
447	
448	A callback routine can be registered by calling::
449	
450	  hotplug_memory_notifier(callback_func, priority)
451	
452	Callback functions with higher values of priority are called before callback
453	functions with lower values.
454	
455	A callback function must have the following prototype::
456	
457	  int callback_func(
458	    struct notifier_block *self, unsigned long action, void *arg);
459	
460	The first argument of the callback function (self) is a pointer to the block
461	of the notifier chain that points to the callback function itself.
462	The second argument (action) is one of the event types described above.
463	The third argument (arg) passes a pointer of struct memory_notify::
464	
465		struct memory_notify {
466			unsigned long start_pfn;
467			unsigned long nr_pages;
468			int status_change_nid_normal;
469			int status_change_nid_high;
470			int status_change_nid;
471		}
472	
473	- start_pfn is start_pfn of online/offline memory.
474	- nr_pages is # of pages of online/offline memory.
475	- status_change_nid_normal is set node id when N_NORMAL_MEMORY of nodemask
476	  is (will be) set/clear, if this is -1, then nodemask status is not changed.
477	- status_change_nid_high is set node id when N_HIGH_MEMORY of nodemask
478	  is (will be) set/clear, if this is -1, then nodemask status is not changed.
479	- status_change_nid is set node id when N_MEMORY of nodemask is (will be)
480	  set/clear. It means a new(memoryless) node gets new memory by online and a
481	  node loses all memory. If this is -1, then nodemask status is not changed.
482	
483	  If status_changed_nid* >= 0, callback should create/discard structures for the
484	  node if necessary.
485	
486	The callback routine shall return one of the values
487	NOTIFY_DONE, NOTIFY_OK, NOTIFY_BAD, NOTIFY_STOP
488	defined in include/linux/notifier.h
489	
490	NOTIFY_DONE and NOTIFY_OK have no effect on the further processing.
491	
492	NOTIFY_BAD is used as response to the MEM_GOING_ONLINE, MEM_GOING_OFFLINE,
493	MEM_ONLINE, or MEM_OFFLINE action to cancel hotplugging. It stops
494	further processing of the notification queue.
495	
496	NOTIFY_STOP stops further processing of the notification queue.
497	
498	Future Work
499	===========
500	
501	  - allowing memory hot-add to ZONE_MOVABLE. maybe we need some switch like
502	    sysctl or new control file.
503	  - showing memory block and physical device relationship.
504	  - test and make it better memory offlining.
505	  - support HugeTLB page migration and offlining.
506	  - memmap removing at memory offline.
507	  - physical remove memory.
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