==============Memory Hotplug============== :Created: Jul 28 2007:Updated: 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 willbe changed often. .. CONTENTS 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. (2) This text assumes that sysfs is mounted at /sys. Introduction============ 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 byhardware which supports memory power management. Linux memory hotplug is designed for both purpose. 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/eraseenvironment for hotplugged memory. Basically, this phase is necessaryfor the purpose (B), but this is good phase for communication betweenhighly virtualized environments too. When memory is hotplugged, the kernel recognizes new memory, makes new memorymanagement 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 :ref:`memory_hotplug_physical_mem`). Logical Memory Hotplug phase is to change memory state intoavailable/unavailable for users. Amount of memory from user's view ischanged by this phase. The kernel makes all memory in it as free pageswhen 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 systemadministrator. For the hot-add case, it must be executed after Physical Hotplugphase by hand.(However, if you writes udev's hotplug scripts for memory hotplug, thesephases can be execute in seamless way.) Unit of Memory online/offline operation--------------------------------------- Memory hotplug uses SPARSEMEM memory model which allows memory to be dividedinto chunks of the same size. These chunks are called "sections". The size ofa memory section is architecture dependent. For example, power uses 16MiB, ia64uses 1GiB. Memory sections are combined into chunks referred to as "memory blocks". Thesize of a memory block is architecture dependent and represents the logicalunit upon which memory online/offline operations are to be performed. Thedefault size of a memory block is the same as memory section size unless anarchitecture specifies otherwise. (see :ref:`memory_hotplug_sysfs_files`.) To determine the size (in bytes) of a memory block please read this file: /sys/devices/system/memory/block_size_bytes Kernel Configuration==================== To use memory hotplug feature, kernel must be compiled with followingconfig options. - For all memory hotplug: - Memory model -> Sparse Memory (CONFIG_SPARSEMEM) - Allow for memory hot-add (CONFIG_MEMORY_HOTPLUG) - To enable memory removal, the following are also necessary: - Allow for memory hot remove (CONFIG_MEMORY_HOTREMOVE) - Page Migration (CONFIG_MIGRATION) - For ACPI memory hotplug, the following 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. .. _memory_hotplug_sysfs_files: sysfs files for memory hotplug============================== All memory blocks have their device information in sysfs. Each memory blockis 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 allmemory sections in this range are present and no memory holes exist in therange. Currently there is no way to determine if there is a memory hole, butthe existence of one should not affect the hotplug capabilities of the memoryblock. For example, assume 1GiB memory block size. A device for a memory starting at0x100000000 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 accessedvia 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 .. _memory_hotplug_physical_mem: Physical memory hot-add phase============================= Hardware(Firmware) Support-------------------------- On x86_64/ia64 platform, memory hotplug by ACPI is supported. In general, the firmware (ACPI) which supports memory hotplug definesmemory 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 udevscript. 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 :ref:`memory_hotplug_how_to_online_memory` and:ref:`memory_hotplug_how_to_offline_memory`. If firmware supports NUMA-node hotplug, and defines an object _HID "ACPI0004","PNP0A05", or "PNP0A06", notification is asserted to it, and ACPI handlercalls hotplug code for all of objects which are defined in it.If memory device is found, memory hotplug code will be called. Notify memory hot-add event by hand----------------------------------- On some architectures, the firmware may not notify the kernel of a memoryhotplug event. Therefore, the memory "probe" interface is supported toexplicitly notify the kernel. This interface depends onCONFIG_ARCH_MEMORY_PROBE and can be configured on powerpc, sh, and x86if hotplug is supported, although for x86 this should be handled by ACPInotification. 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 isnot called (in current implementation). You'll have to online memory byyourself. Please see :ref:`memory_hotplug_how_to_online_memory`. Logical Memory hot-add phase============================ 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". .. _memory_hotplug_how_to_online_memory: How to online memory-------------------- When the memory is hot-added, the kernel decides whether or not to "online"it according to the policy which can be read from "auto_online_blocks" file:: % cat /sys/devices/system/memory/auto_online_blocks The default depends on the CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE kernel configoption. If it is disabled the default is "offline" which means the newly addedmemory is not in a ready-to-use state and you have to "online" the newly addedmemory blocks manually. Automatic onlining can be requested by writing "online"to "auto_online_blocks" file:: % echo online > /sys/devices/system/memory/auto_online_blocks This sets a global policy and impacts all memory blocks that will subsequentlybe hotplugged. Currently offline blocks keep their state. It is possible, undercertain circumstances, that some memory blocks will be added but will fail toonline. User space tools can check their "state" files(/sys/devices/system/memory/memoryXXX/state) and try to online them manually. If the automatic onlining wasn't requested, failed, or some memory block wasofflined it is possible to change the individual block's state by writing to the"state" file:: % 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 doesn't belong to any zone an appropriate kernel zone(usually ZONE_NORMAL) will be used unless movable_node kernel command lineoption is specified when ZONE_MOVABLE will be used. You can explicitly request to associate it with ZONE_MOVABLE by:: % echo online_movable > /sys/devices/system/memory/memoryXXX/state .. note:: current limit: this memory block must be adjacent to ZONE_MOVABLE Or you can explicitly request a kernel zone (usually ZONE_NORMAL) by:: % echo online_kernel > /sys/devices/system/memory/memoryXXX/state .. note:: current limit: this memory block must be adjacent to ZONE_NORMAL An explicit zone onlining can fail (e.g. when the range is already withinand existing and incompatible zone already). After this, memory block XXX's state will be 'online' and the amount ofavailable memory will be increased. This may be changed in future. Logical memory remove===================== Memory offline and ZONE_MOVABLE------------------------------- Memory offlining is more complicated than memory online. Because memory offlinehas to make the whole memory block be unused, memory offline can fail ifthe 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), freeingall pages in the memory block by page migration. But not all pages aremigratable. Under current Linux, migratable pages are anonymous pages andpage caches. For offlining a memory block by migration, the kernel has toguarantee that the memory block contains only migratable pages. Now, a boot option for making a memory block which consists of migratable pagesis supported. By specifying "kernelcore=" or "movablecore=" boot option, you cancreate ZONE_MOVABLE...a zone which is just used for movable pages.(See also Documentation/admin-guide/kernel-parameters.rst) Assume the system has "TOTAL" amount of memory at boot time, this boot optioncreates 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. .. _memory_hotplug_how_to_offline_memory: How to offline memory--------------------- You can offline a memory block by using the same sysfs interface that was usedin 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 offlineit. If it doesn't contain 'unmovable' memory, you'll get success. A memory block under ZONE_MOVABLE is considered to be able to be offlinedeasily. But under some busy state, it may return -EBUSY. Even if a memoryblock cannot be offlined due to -EBUSY, you can retry offlining it and may beable to offline it (or not). (For example, a page is referred to by some kernelinternal 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. Physical memory remove====================== Need more implementation yet.... - Notification completion of remove works by OS to firmware. - Guard from remove if not yet. 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 callbackfunctions 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 blockof 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 valuesNOTIFY_DONE, NOTIFY_OK, NOTIFY_BAD, NOTIFY_STOPdefined 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 stopsfurther processing of the notification queue. NOTIFY_STOP stops further processing of the notification queue. Future Work=========== Lines 1–500 of 507. Load more
============== Memory Hotplug ============== :Created: Jul 28 2007 :Updated: 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. .. CONTENTS 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. (2) This text assumes that sysfs is mounted at /sys. Introduction ============ 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. 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 :ref:`memory_hotplug_physical_mem`). 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.) 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 :ref:`memory_hotplug_sysfs_files`.) To determine the size (in bytes) of a memory block please read this file: /sys/devices/system/memory/block_size_bytes 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 following are also necessary: - Allow for memory hot remove (CONFIG_MEMORY_HOTREMOVE) - Page Migration (CONFIG_MIGRATION) - For ACPI memory hotplug, the following 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. .. _memory_hotplug_sysfs_files: 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 .. _memory_hotplug_physical_mem: Physical memory hot-add phase ============================= 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 :ref:`memory_hotplug_how_to_online_memory` and :ref:`memory_hotplug_how_to_offline_memory`. 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. 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 :ref:`memory_hotplug_how_to_online_memory`. Logical Memory hot-add phase ============================ 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". .. _memory_hotplug_how_to_online_memory: How to online memory -------------------- When the memory is hot-added, the kernel decides whether or not to "online" it according to the policy which can be read from "auto_online_blocks" file:: % cat /sys/devices/system/memory/auto_online_blocks The default depends on the CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE kernel config option. If it is disabled the default is "offline" which means the newly added memory is not in a ready-to-use state and you have to "online" the newly added memory blocks manually. Automatic onlining can be requested by writing "online" to "auto_online_blocks" file:: % echo online > /sys/devices/system/memory/auto_online_blocks This sets a global policy and impacts all memory blocks that will subsequently be hotplugged. Currently offline blocks keep their state. It is possible, under certain circumstances, that some memory blocks will be added but will fail to online. User space tools can check their "state" files (/sys/devices/system/memory/memoryXXX/state) and try to online them manually. If the automatic onlining wasn't requested, failed, or some memory block was offlined it is possible to change the individual block's state by writing to the "state" file:: % 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 doesn't belong to any zone an appropriate kernel zone (usually ZONE_NORMAL) will be used unless movable_node kernel command line option is specified when ZONE_MOVABLE will be used. You can explicitly request to associate it with ZONE_MOVABLE by:: % echo online_movable > /sys/devices/system/memory/memoryXXX/state .. note:: current limit: this memory block must be adjacent to ZONE_MOVABLE Or you can explicitly request a kernel zone (usually ZONE_NORMAL) by:: % echo online_kernel > /sys/devices/system/memory/memoryXXX/state .. note:: current limit: this memory block must be adjacent to ZONE_NORMAL An explicit zone onlining can fail (e.g. when the range is already within and existing and incompatible zone already). After this, memory block XXX's state will be 'online' and the amount of available memory will be increased. This may be changed in future. Logical memory remove ===================== 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/admin-guide/kernel-parameters.rst) 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. .. _memory_hotplug_how_to_offline_memory: 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. Physical memory remove ====================== Need more implementation yet.... - Notification completion of remove works by OS to firmware. - Guard from remove if not yet. 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. Future Work ===========