--- /dev/null
+==============
+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.
+
Code Review / kvmfornfv.git / blobdiff