memory-hotplug: update documentation to hide information about SECTIONS and remove end_phys_index

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.)

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.

To determine the size of sections, please read this file:

Memory sections are combined into chunks referred to as "memory blocks". The

size of a memory block is architecture dependent and represents the logical

unit upon which memory online/offline operations are to be performed. The

default size of a memory block is the same as memory section size unless an

architecture specifies otherwise. (see Section 3.)

To determine the size (in bytes) of a memory block please read this file:

/sys/devices/system/memory/block_size_bytes

This file shows the size of sections in byte.

-----------------------

2. Kernel Configuration

    (CONFIG_ACPI_CONTAINER).

    This option can be kernel module too.

--------------------------------

4 sysfs files for memory hotplug

3 sysfs files for memory hotplug

--------------------------------

All sections have their device information in sysfs.  Each section is part of

a memory block under /sys/devices/system/memory as

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 section id.)

(XXX is the memory block id.)

Now, XXX is defined as (start_address_of_section / section_size) of the first

section contained in the memory block.  The files 'phys_index' and

'end_phys_index' under each directory report the beginning and end section id's

for the memory block covered by the sysfs directory.  It is expected that all

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 section size. A device for a memory starting at

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 section, you can see 4 or 5 files, the end_phys_index file being

a recent addition and not present on older kernels.

Under each memory block, you can see 4 files:

/sys/devices/system/memory/memoryXXX/start_phys_index

/sys/devices/system/memory/memoryXXX/end_phys_index

/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

'phys_index'      : read-only and contains section id of the first section

		    in the memory block, same as XXX.

'end_phys_index'  : read-only and contains section id of the last section

		    in the memory block.

'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",

A backlink will also be created:

/sys/devices/system/memory/memory9/node0 -> ../../node/node0

--------------------------------

4. Physical memory hot-add phase

--------------------------------

% 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.

Then, [start_address_of_new_memory, start_address_of_new_memory +

memory_block_size] memory range is hot-added. In this case, hotplug script is

not called (in current implementation). You'll have to online memory by

yourself.  Please see "How to online memory" in this text.

------------------------------

5.1. State of memory

------------

To see (online/offline) state of memory section, read 'state' file.

To see (online/offline) state of a memory block, 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".

If the memory block is online, you'll read "online".

If the memory block is offline, you'll read "offline".

5.2. How to online memory

------------

Even if the memory is hot-added, it is not at ready-to-use state.

For using newly added memory, you have to "online" the memory section.

For using newly added memory, you have to "online" the memory block.

For onlining, you have to write "online" to the section's state file as:

For onlining, you have to write "online" to the memory block's state file as:

% echo online > /sys/devices/system/memory/memoryXXX/state

This onlining will not change the ZONE type of the target memory section,

If the memory section is in ZONE_NORMAL, you can change it to ZONE_MOVABLE:

This onlining will not change the ZONE type of the target memory block,

If the memory block is in ZONE_NORMAL, you can change it to ZONE_MOVABLE:

% echo online_movable > /sys/devices/system/memory/memoryXXX/state

(NOTE: current limit: this memory section must be adjacent to ZONE_MOVABLE)

(NOTE: current limit: this memory block must be adjacent to ZONE_MOVABLE)

And if the memory section is in ZONE_MOVABLE, you can change it to ZONE_NORMAL:

And if the memory block is in ZONE_MOVABLE, you can change it to ZONE_NORMAL:

% echo online_kernel > /sys/devices/system/memory/memoryXXX/state

(NOTE: current limit: this memory section must be adjacent to ZONE_NORMAL)

(NOTE: current limit: this memory block must be adjacent to ZONE_NORMAL)

After this, section memoryXXX's state will be 'online' and the amount of

After this, memory block XXX's state will be 'online' and the amount of

available memory will be increased.

Currently, newly added memory is added as ZONE_NORMAL (for powerpc, ZONE_DMA).

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.

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 section.

(2) migrate all pages in the section.

(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 section by page migration. But not all pages are

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 section by migration, the kernel has to guarantee

that the section contains only migratable pages.

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 section which consists of migratable pages is

supported. By specifying "kernelcore=" or "movablecore=" boot option, you can

Now, a boot option for making a memory block which consists of migratable pages

is supported. By specifying "kernelcore=" or "movablecore=" boot option, you can

create ZONE_MOVABLE...a zone which is just used for movable pages.

(See also Documentation/kernel-parameters.txt)

  Size of memory for movable pages (for offline) is ZZZZ.

Note) Unfortunately, there is no information to show which section belongs

Note: Unfortunately, there is no information to show which memory block belongs

to ZONE_MOVABLE. This is TBD.

6.2. How to offline memory

------------

You can offline a section by using the same sysfs interface that was used in

memory onlining.

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 section is changed to be "offline".

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 section does not belong to ZONE_MOVABLE, you can try to offline it.

If it doesn't contain 'unmovable' memory, you'll get success.

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 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.)

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

  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 section.

  subsystem from the indicated memory block.

MEMORY_CANCEL_OFFLINE

  Generated if MEMORY_GOING_OFFLINE fails. Memory is available again from

  the section that we attempted to offline.

  the memory block that we attempted to offline.

MEMORY_OFFLINE

  Generated after offlining memory is complete.

--------------

  - 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 is under ZONE_MOVABLE or not

  - showing memory block and physical device relationship.

  - showing memory block is under ZONE_MOVABLE or not

  - test and make it better memory offlining.

  - support HugeTLB page migration and offlining.

  - memmap removing at memory offline.

	return sprintf(buf, "%08lx\n", phys_index);

}

static ssize_t show_mem_end_phys_index(struct device *dev,

			struct device_attribute *attr, char *buf)

{

	struct memory_block *mem = to_memory_block(dev);

	unsigned long phys_index;

	phys_index = mem->end_section_nr / sections_per_block;

	return sprintf(buf, "%08lx\n", phys_index);

}

/*

 * Show whether the section of memory is likely to be hot-removable

 */

}

static DEVICE_ATTR(phys_index, 0444, show_mem_start_phys_index, NULL);

static DEVICE_ATTR(end_phys_index, 0444, show_mem_end_phys_index, NULL);

static DEVICE_ATTR(state, 0644, show_mem_state, store_mem_state);

static DEVICE_ATTR(phys_device, 0444, show_phys_device, NULL);

static DEVICE_ATTR(removable, 0444, show_mem_removable, NULL);

static struct attribute *memory_memblk_attrs[] = {

	&dev_attr_phys_index.attr,

	&dev_attr_end_phys_index.attr,

	&dev_attr_state.attr,

	&dev_attr_phys_device.attr,

	&dev_attr_removable.attr,