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. 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.

Manual migration is very 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. This allows automatic
control over locality of a process. 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 (as is the case today).

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() and then
a low level description of how the low level details work.

A. 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() or __isolate_lru_page().
   Calling isolate_lru_page increases the references to the page
   so that it cannot vanish under us.

2. Generate a list of newly allocates page to move the contents
   of the first list to.

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()!

B. Operation of migrate_pages()
--------------------------------

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.

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.

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 we are moving references.

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

6. The radix tree lock is taken

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.

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 reference has now
    been removed.

13. The radix tree lock is dropped.

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 remove them again.

19. The locks are dropped from the old and new page.

20. The new page is moved to the LRU.

Christoph Lameter, December 19, 2005.