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[PATCH] page migration: Update documentation

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Signed-off-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
This commit is contained in:
Christoph Lameter 2006-06-23 02:03:39 -07:00 committed by Linus Torvalds
parent 04e62a29bf
commit 8d3c138b77
1 changed files with 33 additions and 58 deletions
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91
Documentation/vm/page_migration
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@ -62,15 +62,15 @@ A. In kernel use of migrate_pages()
It also prevents the swapper or other scans to encounter It also prevents the swapper or other scans to encounter
the page. the page.
2. Generate a list of newly allocates page. These pages will contain the 2. Generate a list of newly allocates pages. These pages will contain the
contents of the pages from the first list after page migration is contents of the pages from the first list after page migration is
complete. complete.
3. The migrate_pages() function is called which attempts 3. The migrate_pages() function is called which attempts
to do the migration. It returns the moved pages in the to do the migration. It returns the moved pages in the
list specified as the third parameter and the failed list specified as the third parameter and the failed
migrations in the fourth parameter. The first parameter migrations in the fourth parameter. When the function
will contain the pages that could still be retried. returns the first list will contain the pages that could still be retried.
4. The leftover pages of various types are returned 4. The leftover pages of various types are returned
to the LRU using putback_to_lru_pages() or otherwise to the LRU using putback_to_lru_pages() or otherwise
@ -93,83 +93,58 @@ Steps:
2. Insure that writeback is complete. 2. Insure that writeback is complete.
3. Make sure that the page has assigned swap cache entry if 3. Prep the new page that we want to move to. It is locked
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 and set to not being uptodate so that all accesses to the new
page immediately lock while the move is in progress. page immediately lock while the move is in progress.
5. All the page table references to the page are either dropped (file 4. The new page is prepped with some settings from the old page so that
backed pages) or converted to swap references (anonymous pages). accesses to the new page will discover a page with the correct settings.
This should decrease the reference count.
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 6. The radix tree lock is taken. This will cause all processes trying
to reestablish a pte to block on the radix tree spinlock. 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 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. 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 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. page then we back out because someone else modified the radix tree.
9. The mapping is checked. If the mapping is gone then a truncate action may 9. The radix tree is changed to point to the new page.
be in progress and we back out.
10. The new page is prepped with some settings from the old page so that 10. The reference count of the old page is dropped because the radix tree
accesses to the new page will be discovered to have the correct settings. 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 is changed to point to the new page. 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 reference count of the old page is dropped because the radix tree 12. The page contents are copied to the new page.
reference is gone.
13. The radix tree lock is dropped. With that lookups become possible again 13. The remaining page flags are copied to the new page.
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. 14. The old page flags are cleared to indicate that the page does
not provide any information anymore.
15. The remaining page flags are copied to the new page. 15. Queued up writeback on the new page is triggered.
16. The old page flags are cleared to indicate that the page does 16. If migration entries were page then replace them with real ptes. Doing
not use any information anymore. so will enable access for user space processes not already waiting for
the page lock.
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. 19. The page locks are dropped from the old and new page.
Processes waiting on the page lock can continue. 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 20. The new page is moved to the LRU and can be scanned by the swapper
etc again. etc again.
TODO list Christoph Lameter, May 8, 2006.
---------
- 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.