1.. _admin_guide_memory_hotplug:
2
3==============
4Memory Hotplug
5==============
6
7:Created:							Jul 28 2007
8:Updated: Add some details about locking internals:		Aug 20 2018
9
10This document is about memory hotplug including how-to-use and current status.
11Because Memory Hotplug is still under development, contents of this text will
12be changed often.
13
14.. contents:: :local:
15
16.. note::
17
18    (1) x86_64's has special implementation for memory hotplug.
19        This text does not describe it.
20    (2) This text assumes that sysfs is mounted at ``/sys``.
21
22
23Introduction
24============
25
26Purpose of memory hotplug
27-------------------------
28
29Memory Hotplug allows users to increase/decrease the amount of memory.
30Generally, there are two purposes.
31
32(A) For changing the amount of memory.
33    This is to allow a feature like capacity on demand.
34(B) For installing/removing DIMMs or NUMA-nodes physically.
35    This is to exchange DIMMs/NUMA-nodes, reduce power consumption, etc.
36
37(A) is required by highly virtualized environments and (B) is required by
38hardware which supports memory power management.
39
40Linux memory hotplug is designed for both purpose.
41
42Phases of memory hotplug
43------------------------
44
45There are 2 phases in Memory Hotplug:
46
47  1) Physical Memory Hotplug phase
48  2) Logical Memory Hotplug phase.
49
50The First phase is to communicate hardware/firmware and make/erase
51environment for hotplugged memory. Basically, this phase is necessary
52for the purpose (B), but this is good phase for communication between
53highly virtualized environments too.
54
55When memory is hotplugged, the kernel recognizes new memory, makes new memory
56management tables, and makes sysfs files for new memory's operation.
57
58If firmware supports notification of connection of new memory to OS,
59this phase is triggered automatically. ACPI can notify this event. If not,
60"probe" operation by system administration is used instead.
61(see :ref:`memory_hotplug_physical_mem`).
62
63Logical Memory Hotplug phase is to change memory state into
64available/unavailable for users. Amount of memory from user's view is
65changed by this phase. The kernel makes all memory in it as free pages
66when a memory range is available.
67
68In this document, this phase is described as online/offline.
69
70Logical Memory Hotplug phase is triggered by write of sysfs file by system
71administrator. For the hot-add case, it must be executed after Physical Hotplug
72phase by hand.
73(However, if you writes udev's hotplug scripts for memory hotplug, these
74phases can be execute in seamless way.)
75
76Unit of Memory online/offline operation
77---------------------------------------
78
79Memory hotplug uses SPARSEMEM memory model which allows memory to be divided
80into chunks of the same size. These chunks are called "sections". The size of
81a memory section is architecture dependent. For example, power uses 16MiB, ia64
82uses 1GiB.
83
84Memory sections are combined into chunks referred to as "memory blocks". The
85size of a memory block is architecture dependent and represents the logical
86unit upon which memory online/offline operations are to be performed. The
87default size of a memory block is the same as memory section size unless an
88architecture specifies otherwise. (see :ref:`memory_hotplug_sysfs_files`.)
89
90To determine the size (in bytes) of a memory block please read this file::
91
92  /sys/devices/system/memory/block_size_bytes
93
94Kernel Configuration
95====================
96
97To use memory hotplug feature, kernel must be compiled with following
98config options.
99
100- For all memory hotplug:
101    - Memory model -> Sparse Memory  (``CONFIG_SPARSEMEM``)
102    - Allow for memory hot-add       (``CONFIG_MEMORY_HOTPLUG``)
103
104- To enable memory removal, the following are also necessary:
105    - Allow for memory hot remove    (``CONFIG_MEMORY_HOTREMOVE``)
106    - Page Migration                 (``CONFIG_MIGRATION``)
107
108- For ACPI memory hotplug, the following are also necessary:
109    - Memory hotplug (under ACPI Support menu) (``CONFIG_ACPI_HOTPLUG_MEMORY``)
110    - This option can be kernel module.
111
112- As a related configuration, if your box has a feature of NUMA-node hotplug
113  via ACPI, then this option is necessary too.
114
115    - ACPI0004,PNP0A05 and PNP0A06 Container Driver (under ACPI Support menu)
116      (``CONFIG_ACPI_CONTAINER``).
117
118     This option can be kernel module too.
119
120
121.. _memory_hotplug_sysfs_files:
122
123sysfs files for memory hotplug
124==============================
125
126All memory blocks have their device information in sysfs.  Each memory block
127is described under ``/sys/devices/system/memory`` as::
128
129	/sys/devices/system/memory/memoryXXX
130
131where XXX is the memory block id.
132
133For the memory block covered by the sysfs directory.  It is expected that all
134memory sections in this range are present and no memory holes exist in the
135range. Currently there is no way to determine if there is a memory hole, but
136the existence of one should not affect the hotplug capabilities of the memory
137block.
138
139For example, assume 1GiB memory block size. A device for a memory starting at
1400x100000000 is ``/sys/device/system/memory/memory4``::
141
142	(0x100000000 / 1Gib = 4)
143
144This device covers address range [0x100000000 ... 0x140000000)
145
146Under each memory block, you can see 5 files:
147
148- ``/sys/devices/system/memory/memoryXXX/phys_index``
149- ``/sys/devices/system/memory/memoryXXX/phys_device``
150- ``/sys/devices/system/memory/memoryXXX/state``
151- ``/sys/devices/system/memory/memoryXXX/removable``
152- ``/sys/devices/system/memory/memoryXXX/valid_zones``
153
154=================== ============================================================
155``phys_index``      read-only and contains memory block id, same as XXX.
156``state``           read-write
157
158                    - at read:  contains online/offline state of memory.
159                    - at write: user can specify "online_kernel",
160
161                    "online_movable", "online", "offline" command
162                    which will be performed on all sections in the block.
163``phys_device``     read-only: designed to show the name of physical memory
164                    device.  This is not well implemented now.
165``removable``       read-only: contains an integer value indicating
166                    whether the memory block is removable or not
167                    removable.  A value of 1 indicates that the memory
168                    block is removable and a value of 0 indicates that
169                    it is not removable. A memory block is removable only if
170                    every section in the block is removable.
171``valid_zones``     read-only: designed to show which zones this memory block
172		    can be onlined to.
173
174		    The first column shows it`s default zone.
175
176		    "memory6/valid_zones: Normal Movable" shows this memoryblock
177		    can be onlined to ZONE_NORMAL by default and to ZONE_MOVABLE
178		    by online_movable.
179
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
185.. note::
186
187  These directories/files appear after physical memory hotplug phase.
188
189If CONFIG_NUMA is enabled the memoryXXX/ directories can also be accessed
190via symbolic links located in the ``/sys/devices/system/node/node*`` directories.
191
192For example::
193
194	/sys/devices/system/node/node0/memory9 -> ../../memory/memory9
195
196A backlink will also be created::
197
198	/sys/devices/system/memory/memory9/node0 -> ../../node/node0
199
200.. _memory_hotplug_physical_mem:
201
202Physical memory hot-add phase
203=============================
204
205Hardware(Firmware) Support
206--------------------------
207
208On x86_64/ia64 platform, memory hotplug by ACPI is supported.
209
210In general, the firmware (ACPI) which supports memory hotplug defines
211memory class object of _HID "PNP0C80". When a notify is asserted to PNP0C80,
212Linux's ACPI handler does hot-add memory to the system and calls a hotplug udev
213script. This will be done automatically.
214
215But scripts for memory hotplug are not contained in generic udev package(now).
216You may have to write it by yourself or online/offline memory by hand.
217Please see :ref:`memory_hotplug_how_to_online_memory` and
218:ref:`memory_hotplug_how_to_offline_memory`.
219
220If firmware supports NUMA-node hotplug, and defines an object _HID "ACPI0004",
221"PNP0A05", or "PNP0A06", notification is asserted to it, and ACPI handler
222calls hotplug code for all of objects which are defined in it.
223If memory device is found, memory hotplug code will be called.
224
225Notify memory hot-add event by hand
226-----------------------------------
227
228On some architectures, the firmware may not notify the kernel of a memory
229hotplug event.  Therefore, the memory "probe" interface is supported to
230explicitly notify the kernel.  This interface depends on
231CONFIG_ARCH_MEMORY_PROBE and can be configured on powerpc, sh, and x86
232if hotplug is supported, although for x86 this should be handled by ACPI
233notification.
234
235Probe interface is located at::
236
237	/sys/devices/system/memory/probe
238
239You can tell the physical address of new memory to the kernel by::
240
241	% echo start_address_of_new_memory > /sys/devices/system/memory/probe
242
243Then, [start_address_of_new_memory, start_address_of_new_memory +
244memory_block_size] memory range is hot-added. In this case, hotplug script is
245not called (in current implementation). You'll have to online memory by
246yourself.  Please see :ref:`memory_hotplug_how_to_online_memory`.
247
248Logical Memory hot-add phase
249============================
250
251State of memory
252---------------
253
254To see (online/offline) state of a memory block, read 'state' file::
255
256	% cat /sys/device/system/memory/memoryXXX/state
257
258
259- If the memory block is online, you'll read "online".
260- If the memory block is offline, you'll read "offline".
261
262
263.. _memory_hotplug_how_to_online_memory:
264
265How to online memory
266--------------------
267
268When the memory is hot-added, the kernel decides whether or not to "online"
269it according to the policy which can be read from "auto_online_blocks" file::
270
271	% cat /sys/devices/system/memory/auto_online_blocks
272
273The default depends on the CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE kernel config
274option. If it is disabled the default is "offline" which means the newly added
275memory is not in a ready-to-use state and you have to "online" the newly added
276memory blocks manually. Automatic onlining can be requested by writing "online"
277to "auto_online_blocks" file::
278
279	% echo online > /sys/devices/system/memory/auto_online_blocks
280
281This sets a global policy and impacts all memory blocks that will subsequently
282be hotplugged. Currently offline blocks keep their state. It is possible, under
283certain circumstances, that some memory blocks will be added but will fail to
284online. User space tools can check their "state" files
285(``/sys/devices/system/memory/memoryXXX/state``) and try to online them manually.
286
287If the automatic onlining wasn't requested, failed, or some memory block was
288offlined it is possible to change the individual block's state by writing to the
289"state" file::
290
291	% echo online > /sys/devices/system/memory/memoryXXX/state
292
293This onlining will not change the ZONE type of the target memory block,
294If the memory block doesn't belong to any zone an appropriate kernel zone
295(usually ZONE_NORMAL) will be used unless movable_node kernel command line
296option is specified when ZONE_MOVABLE will be used.
297
298You can explicitly request to associate it with ZONE_MOVABLE by::
299
300	% echo online_movable > /sys/devices/system/memory/memoryXXX/state
301
302.. note:: current limit: this memory block must be adjacent to ZONE_MOVABLE
303
304Or you can explicitly request a kernel zone (usually ZONE_NORMAL) by::
305
306	% echo online_kernel > /sys/devices/system/memory/memoryXXX/state
307
308.. note:: current limit: this memory block must be adjacent to ZONE_NORMAL
309
310An explicit zone onlining can fail (e.g. when the range is already within
311and existing and incompatible zone already).
312
313After this, memory block XXX's state will be 'online' and the amount of
314available memory will be increased.
315
316This may be changed in future.
317
318Logical memory remove
319=====================
320
321Memory offline and ZONE_MOVABLE
322-------------------------------
323
324Memory offlining is more complicated than memory online. Because memory offline
325has to make the whole memory block be unused, memory offline can fail if
326the memory block includes memory which cannot be freed.
327
328In general, memory offline can use 2 techniques.
329
330(1) reclaim and free all memory in the memory block.
331(2) migrate all pages in the memory block.
332
333In the current implementation, Linux's memory offline uses method (2), freeing
334all  pages in the memory block by page migration. But not all pages are
335migratable. Under current Linux, migratable pages are anonymous pages and
336page caches. For offlining a memory block by migration, the kernel has to
337guarantee that the memory block contains only migratable pages.
338
339Now, a boot option for making a memory block which consists of migratable pages
340is supported. By specifying "kernelcore=" or "movablecore=" boot option, you can
341create ZONE_MOVABLE...a zone which is just used for movable pages.
342(See also Documentation/admin-guide/kernel-parameters.rst)
343
344Assume the system has "TOTAL" amount of memory at boot time, this boot option
345creates ZONE_MOVABLE as following.
346
3471) When kernelcore=YYYY boot option is used,
348   Size of memory not for movable pages (not for offline) is YYYY.
349   Size of memory for movable pages (for offline) is TOTAL-YYYY.
350
3512) When movablecore=ZZZZ boot option is used,
352   Size of memory not for movable pages (not for offline) is TOTAL - ZZZZ.
353   Size of memory for movable pages (for offline) is ZZZZ.
354
355.. note::
356
357   Unfortunately, there is no information to show which memory block belongs
358   to ZONE_MOVABLE. This is TBD.
359
360.. _memory_hotplug_how_to_offline_memory:
361
362How to offline memory
363---------------------
364
365You can offline a memory block by using the same sysfs interface that was used
366in memory onlining::
367
368	% echo offline > /sys/devices/system/memory/memoryXXX/state
369
370If offline succeeds, the state of the memory block is changed to be "offline".
371If it fails, some error core (like -EBUSY) will be returned by the kernel.
372Even if a memory block does not belong to ZONE_MOVABLE, you can try to offline
373it.  If it doesn't contain 'unmovable' memory, you'll get success.
374
375A memory block under ZONE_MOVABLE is considered to be able to be offlined
376easily.  But under some busy state, it may return -EBUSY. Even if a memory
377block cannot be offlined due to -EBUSY, you can retry offlining it and may be
378able to offline it (or not). (For example, a page is referred to by some kernel
379internal call and released soon.)
380
381Consideration:
382  Memory hotplug's design direction is to make the possibility of memory
383  offlining higher and to guarantee unplugging memory under any situation. But
384  it needs more work. Returning -EBUSY under some situation may be good because
385  the user can decide to retry more or not by himself. Currently, memory
386  offlining code does some amount of retry with 120 seconds timeout.
387
388Physical memory remove
389======================
390
391Need more implementation yet....
392 - Notification completion of remove works by OS to firmware.
393 - Guard from remove if not yet.
394
395
396Locking Internals
397=================
398
399When adding/removing memory that uses memory block devices (i.e. ordinary RAM),
400the device_hotplug_lock should be held to:
401
402- synchronize against online/offline requests (e.g. via sysfs). This way, memory
403  block devices can only be accessed (.online/.state attributes) by user
404  space once memory has been fully added. And when removing memory, we
405  know nobody is in critical sections.
406- synchronize against CPU hotplug and similar (e.g. relevant for ACPI and PPC)
407
408Especially, there is a possible lock inversion that is avoided using
409device_hotplug_lock when adding memory and user space tries to online that
410memory faster than expected:
411
412- device_online() will first take the device_lock(), followed by
413  mem_hotplug_lock
414- add_memory_resource() will first take the mem_hotplug_lock, followed by
415  the device_lock() (while creating the devices, during bus_add_device()).
416
417As the device is visible to user space before taking the device_lock(), this
418can result in a lock inversion.
419
420onlining/offlining of memory should be done via device_online()/
421device_offline() - to make sure it is properly synchronized to actions
422via sysfs. Holding device_hotplug_lock is advised (to e.g. protect online_type)
423
424When adding/removing/onlining/offlining memory or adding/removing
425heterogeneous/device memory, we should always hold the mem_hotplug_lock in
426write mode to serialise memory hotplug (e.g. access to global/zone
427variables).
428
429In addition, mem_hotplug_lock (in contrast to device_hotplug_lock) in read
430mode allows for a quite efficient get_online_mems/put_online_mems
431implementation, so code accessing memory can protect from that memory
432vanishing.
433
434
435Future Work
436===========
437
438  - allowing memory hot-add to ZONE_MOVABLE. maybe we need some switch like
439    sysctl or new control file.
440  - showing memory block and physical device relationship.
441  - test and make it better memory offlining.
442  - support HugeTLB page migration and offlining.
443  - memmap removing at memory offline.
444  - physical remove memory.
445