7cdb0d25bc
Documentation/memory-hotplug.txt incorrectly states that the memory driver "probe" interface is only supported on powerpc and is vague about its application on x86. Clarify the platforms that make this interface available if memory hotplug is enabled. Signed-off-by: David Rientjes <rientjes@google.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
417 lines
16 KiB
Plaintext
417 lines
16 KiB
Plaintext
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Memory Hotplug
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==============
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Created: Jul 28 2007
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Add description of notifier of memory hotplug Oct 11 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. Memory hotplug event notifier
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9. 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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available/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 triggered 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 which allows memory to be divided
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into chunks of the same size. These chunks are called "sections". The size of
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a memory section is architecture dependent. For example, power uses 16MiB, ia64
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uses 1GiB.
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Memory sections are combined into chunks referred to as "memory blocks". The
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size of a memory block is architecture dependent and represents the logical
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unit upon which memory online/offline operations are to be performed. The
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default size of a memory block is the same as memory section size unless an
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architecture specifies otherwise. (see Section 3.)
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To determine the size (in bytes) of a memory block please read this file:
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/sys/devices/system/memory/block_size_bytes
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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 memory blocks have their device information in sysfs. Each memory block
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is described under /sys/devices/system/memory as
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/sys/devices/system/memory/memoryXXX
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(XXX is the memory block id.)
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For the memory block covered by the sysfs directory. It is expected that all
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memory sections in this range are present and no memory holes exist in the
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range. Currently there is no way to determine if there is a memory hole, but
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the existence of one should not affect the hotplug capabilities of the memory
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block.
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For example, assume 1GiB memory block 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 memory block, you can see 4 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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/sys/devices/system/memory/memoryXXX/removable
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'phys_index' : read-only and contains memory block 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_kernel",
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"online_movable", "online", "offline" command
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which will be performed on all sections in the block.
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'phys_device' : read-only: designed to show the name of physical memory
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device. This is not well implemented now.
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'removable' : read-only: contains an integer value indicating
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whether the memory block is removable or not
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removable. A value of 1 indicates that the memory
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block is removable and a value of 0 indicates that
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it is not removable. A memory block is removable only if
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every section in the block is removable.
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NOTE:
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These directories/files appear after physical memory hotplug phase.
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If CONFIG_NUMA is enabled the memoryXXX/ directories can also be accessed
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via symbolic links located in the /sys/devices/system/node/node* directories.
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For example:
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/sys/devices/system/node/node0/memory9 -> ../../memory/memory9
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A backlink will also be created:
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/sys/devices/system/memory/memory9/node0 -> ../../node/node0
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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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On some architectures, the firmware may not notify the kernel of a memory
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hotplug event. Therefore, the memory "probe" interface is supported to
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explicitly notify the kernel. This interface depends on
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CONFIG_ARCH_MEMORY_PROBE and can be configured on powerpc, sh, and x86
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if hotplug is supported, although for x86 this should be handled by ACPI
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notification.
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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 +
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memory_block_size] memory range is hot-added. In this case, hotplug script is
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not called (in current implementation). You'll have to online memory by
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yourself. 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 a memory block, read 'state' file.
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% cat /sys/device/system/memory/memoryXXX/state
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If the memory block is online, you'll read "online".
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If the memory block 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 block.
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For onlining, you have to write "online" to the memory block's state file as:
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% echo online > /sys/devices/system/memory/memoryXXX/state
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This onlining will not change the ZONE type of the target memory block,
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If the memory block is in ZONE_NORMAL, you can change it to ZONE_MOVABLE:
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% echo online_movable > /sys/devices/system/memory/memoryXXX/state
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(NOTE: current limit: this memory block must be adjacent to ZONE_MOVABLE)
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And if the memory block is in ZONE_MOVABLE, you can change it to ZONE_NORMAL:
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% echo online_kernel > /sys/devices/system/memory/memoryXXX/state
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(NOTE: current limit: this memory block must be adjacent to ZONE_NORMAL)
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After this, memory block XXX'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 block be unused, memory offline can fail if
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the memory block 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 memory block.
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(2) migrate all pages in the memory block.
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In the current implementation, Linux's memory offline uses method (2), freeing
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all pages in the memory block 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 memory block by migration, the kernel has to
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guarantee that the memory block contains only migratable pages.
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Now, a boot option for making a memory block which consists of migratable pages
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is 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 memory block 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 memory block by using the same sysfs interface that was used
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in 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 block 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 memory block does not belong to ZONE_MOVABLE, you can try to offline
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it. If it doesn't contain 'unmovable' memory, you'll get success.
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A memory block under ZONE_MOVABLE is considered to be able to be offlined
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easily. But under some busy state, it may return -EBUSY. Even if a memory
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block cannot be offlined due to -EBUSY, you can retry offlining it and may be
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able to offline it (or not). (For example, a page is referred to by some kernel
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internal call and released 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. Memory hotplug event notifier
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--------------------------------
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Memory hotplug has event notifier. There are 6 types of notification.
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MEMORY_GOING_ONLINE
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Generated before new memory becomes available in order to be able to
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prepare subsystems to handle memory. The page allocator is still unable
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to allocate from the new memory.
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MEMORY_CANCEL_ONLINE
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Generated if MEMORY_GOING_ONLINE fails.
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MEMORY_ONLINE
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Generated when memory has successfully brought online. The callback may
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allocate pages from the new memory.
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MEMORY_GOING_OFFLINE
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Generated to begin the process of offlining memory. Allocations are no
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longer possible from the memory but some of the memory to be offlined
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is still in use. The callback can be used to free memory known to a
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subsystem from the indicated memory block.
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MEMORY_CANCEL_OFFLINE
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Generated if MEMORY_GOING_OFFLINE fails. Memory is available again from
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the memory block that we attempted to offline.
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MEMORY_OFFLINE
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Generated after offlining memory is complete.
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A callback routine can be registered by
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hotplug_memory_notifier(callback_func, priority)
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The second argument of callback function (action) is event types of above.
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The third argument is passed by pointer of struct memory_notify.
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struct memory_notify {
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unsigned long start_pfn;
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unsigned long nr_pages;
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int status_change_nid_normal;
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int status_change_nid_high;
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int status_change_nid;
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}
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start_pfn is start_pfn of online/offline memory.
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nr_pages is # of pages of online/offline memory.
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status_change_nid_normal is set node id when N_NORMAL_MEMORY of nodemask
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is (will be) set/clear, if this is -1, then nodemask status is not changed.
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status_change_nid_high is set node id when N_HIGH_MEMORY of nodemask
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is (will be) set/clear, if this is -1, then nodemask status is not changed.
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status_change_nid is set node id when N_MEMORY of nodemask is (will be)
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set/clear. It means a new(memoryless) node gets new memory by online and a
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node loses all memory. If this is -1, then nodemask status is not changed.
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If status_changed_nid* >= 0, callback should create/discard structures for the
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node if necessary.
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--------------
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9. 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 block and physical device relationship.
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- showing memory block 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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