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linux/Documentation/vm/page_migration
Christoph Lameter 742755a1d8 [PATCH] page migration: sys_move_pages(): support moving of individual pages 
move_pages() is used to move individual pages of a process. The function can
be used to determine the location of pages and to move them onto the desired
node. move_pages() returns status information for each page.

long move_pages(pid, number_of_pages_to_move,
		addresses_of_pages[],
		nodes[] or NULL,
		status[],
		flags);

The addresses of pages is an array of void * pointing to the
pages to be moved.

The nodes array contains the node numbers that the pages should be moved
to. If a NULL is passed instead of an array then no pages are moved but
the status array is updated. The status request may be used to determine
the page state before issuing another move_pages() to move pages.

The status array will contain the state of all individual page migration
attempts when the function terminates. The status array is only valid if
move_pages() completed successfullly.

Possible page states in status[]:

0..MAX_NUMNODES	The page is now on the indicated node.

-ENOENT		Page is not present

-EACCES		Page is mapped by multiple processes and can only
		be moved if MPOL_MF_MOVE_ALL is specified.

-EPERM		The page has been mlocked by a process/driver and
		cannot be moved.

-EBUSY		Page is busy and cannot be moved. Try again later.

-EFAULT		Invalid address (no VMA or zero page).

-ENOMEM		Unable to allocate memory on target node.

-EIO		Unable to write back page. The page must be written
		back in order to move it since the page is dirty and the
		filesystem does not provide a migration function that
		would allow the moving of dirty pages.

-EINVAL		A dirty page cannot be moved. The filesystem does not provide
		a migration function and has no ability to write back pages.

The flags parameter indicates what types of pages to move:

MPOL_MF_MOVE	Move pages that are only mapped by the process.

MPOL_MF_MOVE_ALL Also move pages that are mapped by multiple processes.
		Requires sufficient capabilities.

Possible return codes from move_pages()

-ENOENT		No pages found that would require moving. All pages
		are either already on the target node, not present, had an
		invalid address or could not be moved because they were
		mapped by multiple processes.

-EINVAL		Flags other than MPOL_MF_MOVE(_ALL) specified or an attempt
		to migrate pages in a kernel thread.

-EPERM		MPOL_MF_MOVE_ALL specified without sufficient priviledges.
		or an attempt to move a process belonging to another user.

-EACCES		One of the target nodes is not allowed by the current cpuset.

-ENODEV		One of the target nodes is not online.

-ESRCH		Process does not exist.

-E2BIG		Too many pages to move.

-ENOMEM		Not enough memory to allocate control array.

-EFAULT		Parameters could not be accessed.

A test program for move_pages() may be found with the patches
on ftp.kernel.org:/pub/linux/kernel/people/christoph/pmig/patches-2.6.17-rc4-mm3

From: Christoph Lameter <clameter@sgi.com>

  Detailed results for sys_move_pages()

  Pass a pointer to an integer to get_new_page() that may be used to
  indicate where the completion status of a migration operation should be
  placed.  This allows sys_move_pags() to report back exactly what happened to
  each page.

  Wish there would be a better way to do this. Looks a bit hacky.

Signed-off-by: Christoph Lameter <clameter@sgi.com>
Cc: Hugh Dickins <hugh@veritas.com>
Cc: Jes Sorensen <jes@trained-monkey.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Lee Schermerhorn <lee.schermerhorn@hp.com>
Cc: Andi Kleen <ak@muc.de>
Cc: Michael Kerrisk <mtk-manpages@gmx.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
2006-06-23 07:42:53 -07:00

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Page migration
--------------
Page migration allows the moving of the physical location of pages between
nodes in a numa system while the process is running. This means that the
virtual addresses that the process sees do not change. However, the
system rearranges the physical location of those pages.
The main intend of page migration is to reduce the latency of memory access
by moving pages near to the processor where the process accessing that memory
is running.
Page migration allows a process to manually relocate the node on which its
pages are located through the MF_MOVE and MF_MOVE_ALL options while setting
a new memory policy via mbind(). The pages of process can also be relocated
from another process using the sys_migrate_pages() function call. The
migrate_pages function call takes two sets of nodes and moves pages of a
process that are located on the from nodes to the destination nodes.
Page migration functions are provided by the numactl package by Andi Kleen
(a version later than 0.9.3 is required. Get it from
ftp://ftp.suse.com/pub/people/ak). numactl provided libnuma which
provides an interface similar to other numa functionality for page migration.
cat /proc/<pid>/numa_maps allows an easy review of where the pages of
a process are located. See also the numa_maps manpage in the numactl package.
Manual migration is useful if for example the scheduler has relocated
a process to a processor on a distant node. A batch scheduler or an
administrator may detect the situation and move the pages of the process
nearer to the new processor. The kernel itself does only provide
manual page migration support. Automatic page migration may be implemented
through user space processes that move pages. A special function call
"move_pages" allows the moving of individual pages within a process.
A NUMA profiler may f.e. obtain a log showing frequent off node
accesses and may use the result to move pages to more advantageous
locations.
Larger installations usually partition the system using cpusets into
sections of nodes. Paul Jackson has equipped cpusets with the ability to
move pages when a task is moved to another cpuset (See ../cpusets.txt).
Cpusets allows the automation of process locality. If a task is moved to
a new cpuset then also all its pages are moved with it so that the
performance of the process does not sink dramatically. Also the pages
of processes in a cpuset are moved if the allowed memory nodes of a
cpuset are changed.
Page migration allows the preservation of the relative location of pages
within a group of nodes for all migration techniques which will preserve a
particular memory allocation pattern generated even after migrating a
process. This is necessary in order to preserve the memory latencies.
Processes will run with similar performance after migration.
Page migration occurs in several steps. First a high level
description for those trying to use migrate_pages() from the kernel
(for userspace usage see the Andi Kleen's numactl package mentioned above)
and then a low level description of how the low level details work.
A. In kernel use of migrate_pages()
-----------------------------------
1. Remove pages from the LRU.
Lists of pages to be migrated are generated by scanning over
pages and moving them into lists. This is done by
calling isolate_lru_page().
Calling isolate_lru_page increases the references to the page
so that it cannot vanish while the page migration occurs.
It also prevents the swapper or other scans to encounter
the page.
2. We need to have a function of type new_page_t that can be
passed to migrate_pages(). This function should figure out
how to allocate the correct new page given the old page.
3. The migrate_pages() function is called which attempts
to do the migration. It will call the function to allocate
the new page for each page that is considered for
moving.
B. How migrate_pages() works
----------------------------
migrate_pages() does several passes over its list of pages. A page is moved
if all references to a page are removable at the time. The page has
already been removed from the LRU via isolate_lru_page() and the refcount
is increased so that the page cannot be freed while page migration occurs.
Steps:
1. Lock the page to be migrated
2. Insure that writeback is complete.
3. Prep the new page that we want to move to. It is locked
and set to not being uptodate so that all accesses to the new
page immediately lock while the move is in progress.
4. The new page is prepped with some settings from the old page so that
accesses to the new page will discover a page with the correct settings.
5. All the page table references to the page are converted
to migration entries or dropped (nonlinear vmas).
This decrease the mapcount of a page. If the resulting
mapcount is not zero then we do not migrate the page.
All user space processes that attempt to access the page
will now wait on the page lock.
6. The radix tree lock is taken. This will cause all processes trying
to access the page via the mapping to block on the radix tree spinlock.
7. The refcount of the page is examined and we back out if references remain
otherwise we know that we are the only one referencing this page.
8. The radix tree is checked and if it does not contain the pointer to this
page then we back out because someone else modified the radix tree.
9. The radix tree is changed to point to the new page.
10. The reference count of the old page is dropped because the radix tree
reference is gone. A reference to the new page is established because
the new page is referenced to by the radix tree.
11. The radix tree lock is dropped. With that lookups in the mapping
become possible again. Processes will move from spinning on the tree_lock
to sleeping on the locked new page.
12. The page contents are copied to the new page.
13. The remaining page flags are copied to the new page.
14. The old page flags are cleared to indicate that the page does
not provide any information anymore.
15. Queued up writeback on the new page is triggered.
16. If migration entries were page then replace them with real ptes. Doing
so will enable access for user space processes not already waiting for
the page lock.
19. The page locks are dropped from the old and new page.
Processes waiting on the page lock will redo their page faults
and will reach the new page.
20. The new page is moved to the LRU and can be scanned by the swapper
etc again.
Christoph Lameter, May 8, 2006.
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