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diff --git a/Documentation/memory-hotplug.txt b/Documentation/memory-hotplug.txt new file mode 100644 index 000000000000..5fbcc22c98e9 --- /dev/null +++ b/Documentation/memory-hotplug.txt @@ -0,0 +1,322 @@ +============== +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. + |