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