Documentation/vm/page_migration

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. At some point in the future we may have
some mechanism in the scheduler that will automatically move the pages.

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. Generate a list of newly allocates page. These pages will contain the
   contents of the pages from the first list after page migration is
   complete.

3. The migrate_pages() function is called which attempts
   to do the migration. It returns the moved pages in the
   list specified as the third parameter and the failed
   migrations in the fourth parameter. The first parameter
   will contain the pages that could still be retried.

4. The leftover pages of various types are returned
   to the LRU using putback_to_lru_pages() or otherwise
   disposed of. The pages will still have the refcount as
   increased by isolate_lru_pages() if putback_to_lru_pages() is not
   used! The kernel may want to handle the various cases of failures in
   different ways.

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. Make sure that the page has assigned swap cache entry if
   it is an anonyous page. The swap cache reference is necessary
   to preserve the information contain in the page table maps while
   page migration occurs.

4. 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.

5. All the page table references to the page are either dropped (file
   backed pages) or converted to swap references (anonymous pages).
   This should decrease the reference count.

6. The radix tree lock is taken. This will cause all processes trying
   to reestablish a pte 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 mapping first.

9. The mapping is checked. If the mapping is gone then a truncate action may
   be in progress and we back out.

10. The new page is prepped with some settings from the old page so that
   accesses to the new page will be discovered to have the correct settings.

11. The radix tree is changed to point to the new page.

12. The reference count of the old page is dropped because the radix tree
    reference is gone.

13. The radix tree lock is dropped. With that lookups become possible again
    and other processes will move from spinning on the tree lock to sleeping on
    the locked new page.

14. The page contents are copied to the new page.

15. The remaining page flags are copied to the new page.

16. The old page flags are cleared to indicate that the page does
    not use any information anymore.

17. Queued up writeback on the new page is triggered.

18. If swap pte's were generated for the page then replace them with real
    ptes. This will reenable access for processes not blocked by the page lock.

19. The page locks are dropped from the old and new page.
    Processes waiting on the page lock can continue.

20. The new page is moved to the LRU and can be scanned by the swapper
    etc again.

TODO list
---------

- Page migration requires the use of swap handles to preserve the
  information of the anonymous page table entries. This means that swap
  space is reserved but never used. The maximum number of swap handles used
  is determined by CHUNK_SIZE (see mm/mempolicy.c) per ongoing migration.
  Reservation of pages could be avoided by having a special type of swap
  handle that does not require swap space and that would only track the page
  references. Something like that was proposed by Marcelo Tosatti in the
  past (search for migration cache on lkml or linux-mm@kvack.org).

- Page migration unmaps ptes for file backed pages and requires page
  faults to reestablish these ptes. This could be optimized by somehow
  recording the references before migration and then reestablish them later.
  However, there are several locking challenges that have to be overcome
  before this is possible.

- Page migration generates read ptes for anonymous pages. Dirty page
  faults are required to make the pages writable again. It may be possible
  to generate a pte marked dirty if it is known that the page is dirty and
  that this process has the only reference to that page.

Christoph Lameter, March 8, 2006.