Memory hotplug document

This is add a document for memory hotplug to describe "How to use" and
"Current status".
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Yasunori Goto <y-goto@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/memory-hotplug.txt

+==============
+Memory Hotplug
+==============
+
+Last Updated: Jul 28 2007
+
+This document is about memory hotplug including how-to-use and current status.
+Because Memory Hotplug is still under development, contents of this text will
+be changed often.
+
+1. Introduction
+  1.1 purpose of memory hotplug
+  1.2. Phases of memory hotplug
+  1.3. Unit of Memory online/offline operation
+2. Kernel Configuration
+3. sysfs files for memory hotplug
+4. Physical memory hot-add phase
+  4.1 Hardware(Firmware) Support
+  4.2 Notify memory hot-add event by hand
+5. Logical Memory hot-add phase
+  5.1. State of memory
+  5.2. How to online memory
+6. Logical memory remove
+  6.1 Memory offline and ZONE_MOVABLE
+  6.2. How to offline memory
+7. Physical memory remove
+8. Future Work List
+
+Note(1): x86_64's has special implementation for memory hotplug.
+         This text does not describe it.
+Note(2): This text assumes that sysfs is mounted at /sys.
+
+
+---------------
+1. Introduction
+---------------
+
+1.1 purpose of memory hotplug
+------------
+Memory Hotplug allows users to increase/decrease the amount of memory.
+Generally, there are two purposes.
+
+(A) For changing the amount of memory.
+    This is to allow a feature like capacity on demand.
+(B) For installing/removing DIMMs or NUMA-nodes physically.
+    This is to exchange DIMMs/NUMA-nodes, reduce power consumption, etc.
+
+(A) is required by highly virtualized environments and (B) is required by
+hardware which supports memory power management.
+
+Linux memory hotplug is designed for both purpose.
+
+
+1.2. Phases of memory hotplug
+---------------
+There are 2 phases in Memory Hotplug.
+  1) Physical Memory Hotplug phase
+  2) Logical Memory Hotplug phase.
+
+The First phase is to communicate hardware/firmware and make/erase
+environment for hotplugged memory. Basically, this phase is necessary
+for the purpose (B), but this is good phase for communication between
+highly virtualized environments too.
+
+When memory is hotplugged, the kernel recognizes new memory, makes new memory
+management tables, and makes sysfs files for new memory's operation.
+
+If firmware supports notification of connection of new memory to OS,
+this phase is triggered automatically. ACPI can notify this event. If not,
+"probe" operation by system administration is used instead.
+(see Section 4.).
+
+Logical Memory Hotplug phase is to change memory state into
+avaiable/unavailable for users. Amount of memory from user's view is
+changed by this phase. The kernel makes all memory in it as free pages
+when a memory range is available.
+
+In this document, this phase is described as online/offline.
+
+Logical Memory Hotplug phase is triggred by write of sysfs file by system
+administrator. For the hot-add case, it must be executed after Physical Hotplug
+phase by hand.
+(However, if you writes udev's hotplug scripts for memory hotplug, these
+ phases can be execute in seamless way.)
+
+
+1.3. Unit of Memory online/offline operation
+------------
+Memory hotplug uses SPARSEMEM memory model. SPARSEMEM divides the whole memory
+into chunks of the same size. The chunk is called a "section". The size of
+a section is architecture dependent. For example, power uses 16MiB, ia64 uses
+1GiB. The unit of online/offline operation is "one section". (see Section 3.)
+
+To determine the size of sections, please read this file:
+
+/sys/devices/system/memory/block_size_bytes
+
+This file shows the size of sections in byte.
+
+-----------------------
+2. Kernel Configuration
+-----------------------
+To use memory hotplug feature, kernel must be compiled with following
+config options.
+
+- For all memory hotplug
+    Memory model -> Sparse Memory  (CONFIG_SPARSEMEM)
+    Allow for memory hot-add       (CONFIG_MEMORY_HOTPLUG)
+
+- To enable memory removal, the followings are also necessary
+    Allow for memory hot remove    (CONFIG_MEMORY_HOTREMOVE)
+    Page Migration                 (CONFIG_MIGRATION)
+
+- For ACPI memory hotplug, the followings are also necessary
+    Memory hotplug (under ACPI Support menu) (CONFIG_ACPI_HOTPLUG_MEMORY)
+    This option can be kernel module.
+
+- As a related configuration, if your box has a feature of NUMA-node hotplug
+  via ACPI, then this option is necessary too.
+    ACPI0004,PNP0A05 and PNP0A06 Container Driver (under ACPI Support menu)
+    (CONFIG_ACPI_CONTAINER).
+    This option can be kernel module too.
+
+--------------------------------
+3 sysfs files for memory hotplug
+--------------------------------
+All sections have their device information under /sys/devices/system/memory as
+
+/sys/devices/system/memory/memoryXXX
+(XXX is section id.)
+
+Now, XXX is defined as start_address_of_section / section_size.
+
+For example, assume 1GiB section size. A device for a memory starting at
+0x100000000 is /sys/device/system/memory/memory4
+(0x100000000 / 1Gib = 4)
+This device covers address range [0x100000000 ... 0x140000000)
+
+Under each section, you can see 3 files.
+
+/sys/devices/system/memory/memoryXXX/phys_index
+/sys/devices/system/memory/memoryXXX/phys_device
+/sys/devices/system/memory/memoryXXX/state
+
+'phys_index' : read-only and contains section id, same as XXX.
+'state'      : read-write
+               at read:  contains online/offline state of memory.
+               at write: user can specify "online", "offline" command
+'phys_device': read-only: designed to show the name of physical memory device.
+               This is not well implemented now.
+
+NOTE:
+  These directories/files appear after physical memory hotplug phase.
+
+
+--------------------------------
+4. Physical memory hot-add phase
+--------------------------------
+
+4.1 Hardware(Firmware) Support
+------------
+On x86_64/ia64 platform, memory hotplug by ACPI is supported.
+
+In general, the firmware (ACPI) which supports memory hotplug defines
+memory class object of _HID "PNP0C80". When a notify is asserted to PNP0C80,
+Linux's ACPI handler does hot-add memory to the system and calls a hotplug udev
+script. This will be done automatically.
+
+But scripts for memory hotplug are not contained in generic udev package(now).
+You may have to write it by yourself or online/offline memory by hand.
+Please see "How to online memory", "How to offline memory" in this text.
+
+If firmware supports NUMA-node hotplug, and defines an object _HID "ACPI0004",
+"PNP0A05", or "PNP0A06", notification is asserted to it, and ACPI handler
+calls hotplug code for all of objects which are defined in it.
+If memory device is found, memory hotplug code will be called.
+
+
+4.2 Notify memory hot-add event by hand
+------------
+In some environments, especially virtualized environment, firmware will not
+notify memory hotplug event to the kernel. For such environment, "probe"
+interface is supported. This interface depends on CONFIG_ARCH_MEMORY_PROBE.
+
+Now, CONFIG_ARCH_MEMORY_PROBE is supported only by powerpc but it does not
+contain highly architecture codes. Please add config if you need "probe"
+interface.
+
+Probe interface is located at
+/sys/devices/system/memory/probe
+
+You can tell the physical address of new memory to the kernel by
+
+% echo start_address_of_new_memory > /sys/devices/system/memory/probe
+
+Then, [start_address_of_new_memory, start_address_of_new_memory + section_size)
+memory range is hot-added. In this case, hotplug script is not called (in
+current implementation). You'll have to online memory by yourself.
+Please see "How to online memory" in this text.
+
+
+
+------------------------------
+5. Logical Memory hot-add phase
+------------------------------
+
+5.1. State of memory
+------------
+To see (online/offline) state of memory section, read 'state' file.
+
+% cat /sys/device/system/memory/memoryXXX/state
+
+
+If the memory section is online, you'll read "online".
+If the memory section is offline, you'll read "offline".
+
+
+5.2. How to online memory
+------------
+Even if the memory is hot-added, it is not at ready-to-use state.
+For using newly added memory, you have to "online" the memory section.
+
+For onlining, you have to write "online" to the section's state file as:
+
+% echo online > /sys/devices/system/memory/memoryXXX/state
+
+After this, section memoryXXX's state will be 'online' and the amount of
+available memory will be increased.
+
+Currently, newly added memory is added as ZONE_NORMAL (for powerpc, ZONE_DMA).
+This may be changed in future.
+
+
+
+------------------------
+6. Logical memory remove
+------------------------
+
+6.1 Memory offline and ZONE_MOVABLE
+------------
+Memory offlining is more complicated than memory online. Because memory offline
+has to make the whole memory section be unused, memory offline can fail if
+the section includes memory which cannot be freed.
+
+In general, memory offline can use 2 techniques.
+
+(1) reclaim and free all memory in the section.
+(2) migrate all pages in the section.
+
+In the current implementation, Linux's memory offline uses method (2), freeing
+all  pages in the section by page migration. But not all pages are
+migratable. Under current Linux, migratable pages are anonymous pages and
+page caches. For offlining a section by migration, the kernel has to guarantee
+that the section contains only migratable pages.
+
+Now, a boot option for making a section which consists of migratable pages is
+supported. By specifying "kernelcore=" or "movablecore=" boot option, you can
+create ZONE_MOVABLE...a zone which is just used for movable pages.
+(See also Documentation/kernel-parameters.txt)
+
+Assume the system has "TOTAL" amount of memory at boot time, this boot option
+creates ZONE_MOVABLE as following.
+
+1) When kernelcore=YYYY boot option is used,
+  Size of memory not for movable pages (not for offline) is YYYY.
+  Size of memory for movable pages (for offline) is TOTAL-YYYY.
+
+2) When movablecore=ZZZZ boot option is used,
+  Size of memory not for movable pages (not for offline) is TOTAL - ZZZZ.
+  Size of memory for movable pages (for offline) is ZZZZ.
+
+
+Note) Unfortunately, there is no information to show which section belongs
+to ZONE_MOVABLE. This is TBD.
+
+
+6.2. How to offline memory
+------------
+You can offline a section by using the same sysfs interface that was used in
+memory onlining.
+
+% echo offline > /sys/devices/system/memory/memoryXXX/state
+
+If offline succeeds, the state of the memory section is changed to be "offline".
+If it fails, some error core (like -EBUSY) will be returned by the kernel.
+Even if a section does not belong to ZONE_MOVABLE, you can try to offline it.
+If it doesn't contain 'unmovable' memory, you'll get success.
+
+A section under ZONE_MOVABLE is considered to be able to be offlined easily.
+But under some busy state, it may return -EBUSY. Even if a memory section
+cannot be offlined due to -EBUSY, you can retry offlining it and may be able to
+offline it (or not).
+(For example, a page is referred to by some kernel internal call and released
+ soon.)
+
+Consideration:
+Memory hotplug's design direction is to make the possibility of memory offlining
+higher and to guarantee unplugging memory under any situation. But it needs
+more work. Returning -EBUSY under some situation may be good because the user
+can decide to retry more or not by himself. Currently, memory offlining code
+does some amount of retry with 120 seconds timeout.
+
+-------------------------
+7. Physical memory remove
+-------------------------
+Need more implementation yet....
+ - Notification completion of remove works by OS to firmware.
+ - Guard from remove if not yet.
+
+--------------
+8. Future Work
+--------------
+  - allowing memory hot-add to ZONE_MOVABLE. maybe we need some switch like
+    sysctl or new control file.
+  - showing memory section and physical device relationship.
+  - showing memory section and node relationship (maybe good for NUMA)
+  - showing memory section is under ZONE_MOVABLE or not
+  - test and make it better memory offlining.
+  - support HugeTLB page migration and offlining.
+  - memmap removing at memory offline.
+  - physical remove memory.
+