==============
  Memory Hotplug
  ==============

  Created:					Jul 28 2007
  Add description of notifier of memory hotplug	Oct 11 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. Memory hotplug event notifier
  9. 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

  available/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 triggered 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 which allows memory to be divided
  into chunks of the same size. These chunks are called "sections". The size of
  a memory section is architecture dependent. For example, power uses 16MiB, ia64
  uses 1GiB.

  Memory sections are combined into chunks referred to as "memory blocks". The
  size of a memory block is architecture dependent and represents the logical
  unit upon which memory online/offline operations are to be performed. The
  default size of a memory block is the same as memory section size unless an
  architecture specifies otherwise. (see Section 3.)
  
  To determine the size (in bytes) of a memory block please read this file:

  
  /sys/devices/system/memory/block_size_bytes

  
  -----------------------
  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 memory blocks have their device information in sysfs.  Each memory block
  is described under /sys/devices/system/memory as

  
  /sys/devices/system/memory/memoryXXX

  (XXX is the memory block id.)

  For the memory block covered by the sysfs directory.  It is expected that all

  memory sections in this range are present and no memory holes exist in the
  range. Currently there is no way to determine if there is a memory hole, but
  the existence of one should not affect the hotplug capabilities of the memory
  block.

  For example, assume 1GiB memory block 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 memory block, you can see 5 files:

  /sys/devices/system/memory/memoryXXX/phys_index

  /sys/devices/system/memory/memoryXXX/phys_device
  /sys/devices/system/memory/memoryXXX/state

  /sys/devices/system/memory/memoryXXX/removable

  /sys/devices/system/memory/memoryXXX/valid_zones

  'phys_index'      : read-only and contains memory block id, same as XXX.

  'state'           : read-write
                      at read:  contains online/offline state of memory.

                      at write: user can specify "online_kernel",
                      "online_movable", "online", "offline" command

                      which will be performed on all sections in the block.

  'phys_device'     : read-only: designed to show the name of physical memory
                      device.  This is not well implemented now.
  'removable'       : read-only: contains an integer value indicating
                      whether the memory block is removable or not
                      removable.  A value of 1 indicates that the memory
                      block is removable and a value of 0 indicates that
                      it is not removable. A memory block is removable only if
                      every section in the block is removable.

  'valid_zones'     : read-only: designed to show which zones this memory block
  		    can be onlined to.
  		    The first column shows it's default zone.
  		    "memory6/valid_zones: Normal Movable" shows this memoryblock
  		    can be onlined to ZONE_NORMAL by default and to ZONE_MOVABLE
  		    by online_movable.
  		    "memory7/valid_zones: Movable Normal" shows this memoryblock
  		    can be onlined to ZONE_MOVABLE by default and to ZONE_NORMAL
  		    by online_kernel.

  
  NOTE:
    These directories/files appear after physical memory hotplug phase.

  If CONFIG_NUMA is enabled the memoryXXX/ directories can also be accessed
  via symbolic links located in the /sys/devices/system/node/node* directories.
  
  For example:

  /sys/devices/system/node/node0/memory9 -> ../../memory/memory9

  A backlink will also be created:
  /sys/devices/system/memory/memory9/node0 -> ../../node/node0

  --------------------------------
  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
  ------------

  On some architectures, the firmware may not notify the kernel of a memory
  hotplug event.  Therefore, the memory "probe" interface is supported to
  explicitly notify the kernel.  This interface depends on
  CONFIG_ARCH_MEMORY_PROBE and can be configured on powerpc, sh, and x86
  if hotplug is supported, although for x86 this should be handled by ACPI
  notification.

  
  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 +
  memory_block_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 a memory block, read 'state' file.

  
  % cat /sys/device/system/memory/memoryXXX/state

  If the memory block is online, you'll read "online".
  If the memory block 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 block.

  For onlining, you have to write "online" to the memory block's state file as:

  
  % echo online > /sys/devices/system/memory/memoryXXX/state

  This onlining will not change the ZONE type of the target memory block,
  If the memory block is in ZONE_NORMAL, you can change it to ZONE_MOVABLE:

  
  % echo online_movable > /sys/devices/system/memory/memoryXXX/state

  (NOTE: current limit: this memory block must be adjacent to ZONE_MOVABLE)

  And if the memory block is in ZONE_MOVABLE, you can change it to ZONE_NORMAL:

  
  % echo online_kernel > /sys/devices/system/memory/memoryXXX/state

  (NOTE: current limit: this memory block must be adjacent to ZONE_NORMAL)

  After this, memory block XXX'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 block be unused, memory offline can fail if
  the memory block includes memory which cannot be freed.

  
  In general, memory offline can use 2 techniques.

  (1) reclaim and free all memory in the memory block.
  (2) migrate all pages in the memory block.

  
  In the current implementation, Linux's memory offline uses method (2), freeing

  all  pages in the memory block by page migration. But not all pages are

  migratable. Under current Linux, migratable pages are anonymous pages and

  page caches. For offlining a memory block by migration, the kernel has to
  guarantee that the memory block contains only migratable pages.

  Now, a boot option for making a memory block 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 memory block belongs

  to ZONE_MOVABLE. This is TBD.
  
  
  6.2. How to offline memory
  ------------

  You can offline a memory block 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 block is changed to be "offline".

  If it fails, some error core (like -EBUSY) will be returned by the kernel.

  Even if a memory block does not belong to ZONE_MOVABLE, you can try to offline
  it.  If it doesn't contain 'unmovable' memory, you'll get success.

  A memory block 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
  block 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. Memory hotplug event notifier
  --------------------------------

  Hotplugging events are sent to a notification queue.

  There are six types of notification defined in include/linux/memory.h:
  
  MEM_GOING_ONLINE

    Generated before new memory becomes available in order to be able to
    prepare subsystems to handle memory. The page allocator is still unable
    to allocate from the new memory.

  MEM_CANCEL_ONLINE

    Generated if MEMORY_GOING_ONLINE fails.

  MEM_ONLINE

    Generated when memory has successfully brought online. The callback may

    allocate pages from the new memory.

  MEM_GOING_OFFLINE

    Generated to begin the process of offlining memory. Allocations are no
    longer possible from the memory but some of the memory to be offlined
    is still in use. The callback can be used to free memory known to a

    subsystem from the indicated memory block.

  MEM_CANCEL_OFFLINE

    Generated if MEMORY_GOING_OFFLINE fails. Memory is available again from

    the memory block that we attempted to offline.

  MEM_OFFLINE

    Generated after offlining memory is complete.

  A callback routine can be registered by calling

    hotplug_memory_notifier(callback_func, priority)

  Callback functions with higher values of priority are called before callback
  functions with lower values.
  
  A callback function must have the following prototype:
  
    int callback_func(
      struct notifier_block *self, unsigned long action, void *arg);
  
  The first argument of the callback function (self) is a pointer to the block
  of the notifier chain that points to the callback function itself.
  The second argument (action) is one of the event types described above.
  The third argument (arg) passes a pointer of struct memory_notify.

  
  struct memory_notify {
         unsigned long start_pfn;
         unsigned long nr_pages;

         int status_change_nid_normal;

         int status_change_nid_high;

         int status_change_nid;

  }
  
  start_pfn is start_pfn of online/offline memory.
  nr_pages is # of pages of online/offline memory.

  status_change_nid_normal is set node id when N_NORMAL_MEMORY of nodemask
  is (will be) set/clear, if this is -1, then nodemask status is not changed.

  status_change_nid_high is set node id when N_HIGH_MEMORY of nodemask
  is (will be) set/clear, if this is -1, then nodemask status is not changed.
  status_change_nid is set node id when N_MEMORY of nodemask is (will be)

  set/clear. It means a new(memoryless) node gets new memory by online and a
  node loses all memory. If this is -1, then nodemask status is not changed.

  If status_changed_nid* >= 0, callback should create/discard structures for the

  node if necessary.

  The callback routine shall return one of the values
  NOTIFY_DONE, NOTIFY_OK, NOTIFY_BAD, NOTIFY_STOP
  defined in include/linux/notifier.h
  
  NOTIFY_DONE and NOTIFY_OK have no effect on the further processing.
  
  NOTIFY_BAD is used as response to the MEM_GOING_ONLINE, MEM_GOING_OFFLINE,
  MEM_ONLINE, or MEM_OFFLINE action to cancel hotplugging. It stops
  further processing of the notification queue.
  
  NOTIFY_STOP stops further processing of the notification queue.

  --------------

  9. Future Work

  --------------
    - allowing memory hot-add to ZONE_MOVABLE. maybe we need some switch like
      sysctl or new control file.

    - showing memory block and physical device relationship.

    - test and make it better memory offlining.
    - support HugeTLB page migration and offlining.
    - memmap removing at memory offline.
    - physical remove memory.