[PATCH] page migration: Update documentation
Signed-off-by: Christoph Lameter <clameter@sgi.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
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@ -62,15 +62,15 @@ A. In kernel use of migrate_pages()
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It also prevents the swapper or other scans to encounter
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the page.
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2. Generate a list of newly allocates page. These pages will contain the
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2. Generate a list of newly allocates pages. These pages will contain the
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contents of the pages from the first list after page migration is
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complete.
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3. The migrate_pages() function is called which attempts
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to do the migration. It returns the moved pages in the
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list specified as the third parameter and the failed
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migrations in the fourth parameter. The first parameter
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will contain the pages that could still be retried.
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migrations in the fourth parameter. When the function
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returns the first list will contain the pages that could still be retried.
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4. The leftover pages of various types are returned
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to the LRU using putback_to_lru_pages() or otherwise
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@ -93,83 +93,58 @@ Steps:
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2. Insure that writeback is complete.
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3. Make sure that the page has assigned swap cache entry if
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it is an anonyous page. The swap cache reference is necessary
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to preserve the information contain in the page table maps while
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page migration occurs.
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4. Prep the new page that we want to move to. It is locked
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3. Prep the new page that we want to move to. It is locked
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and set to not being uptodate so that all accesses to the new
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page immediately lock while the move is in progress.
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5. All the page table references to the page are either dropped (file
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backed pages) or converted to swap references (anonymous pages).
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This should decrease the reference count.
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4. The new page is prepped with some settings from the old page so that
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accesses to the new page will discover a page with the correct settings.
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5. All the page table references to the page are converted
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to migration entries or dropped (nonlinear vmas).
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This decrease the mapcount of a page. If the resulting
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mapcount is not zero then we do not migrate the page.
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All user space processes that attempt to access the page
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will now wait on the page lock.
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6. The radix tree lock is taken. This will cause all processes trying
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to reestablish a pte to block on the radix tree spinlock.
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to access the page via the mapping to block on the radix tree spinlock.
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7. The refcount of the page is examined and we back out if references remain
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otherwise we know that we are the only one referencing this page.
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8. The radix tree is checked and if it does not contain the pointer to this
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page then we back out because someone else modified the mapping first.
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page then we back out because someone else modified the radix tree.
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9. The mapping is checked. If the mapping is gone then a truncate action may
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be in progress and we back out.
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9. The radix tree is changed to point to the new page.
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10. The new page is prepped with some settings from the old page so that
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accesses to the new page will be discovered to have the correct settings.
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10. The reference count of the old page is dropped because the radix tree
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reference is gone. A reference to the new page is established because
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the new page is referenced to by the radix tree.
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11. The radix tree is changed to point to the new page.
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11. The radix tree lock is dropped. With that lookups in the mapping
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become possible again. Processes will move from spinning on the tree_lock
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to sleeping on the locked new page.
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12. The reference count of the old page is dropped because the radix tree
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reference is gone.
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12. The page contents are copied to the new page.
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13. The radix tree lock is dropped. With that lookups become possible again
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and other processes will move from spinning on the tree lock to sleeping on
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the locked new page.
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13. The remaining page flags are copied to the new page.
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14. The page contents are copied to the new page.
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14. The old page flags are cleared to indicate that the page does
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not provide any information anymore.
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15. The remaining page flags are copied to the new page.
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15. Queued up writeback on the new page is triggered.
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16. The old page flags are cleared to indicate that the page does
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not use any information anymore.
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17. Queued up writeback on the new page is triggered.
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18. If swap pte's were generated for the page then replace them with real
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ptes. This will reenable access for processes not blocked by the page lock.
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16. If migration entries were page then replace them with real ptes. Doing
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so will enable access for user space processes not already waiting for
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the page lock.
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19. The page locks are dropped from the old and new page.
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Processes waiting on the page lock can continue.
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Processes waiting on the page lock will redo their page faults
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and will reach the new page.
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20. The new page is moved to the LRU and can be scanned by the swapper
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etc again.
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TODO list
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---------
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- Page migration requires the use of swap handles to preserve the
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information of the anonymous page table entries. This means that swap
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space is reserved but never used. The maximum number of swap handles used
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is determined by CHUNK_SIZE (see mm/mempolicy.c) per ongoing migration.
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Reservation of pages could be avoided by having a special type of swap
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handle that does not require swap space and that would only track the page
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references. Something like that was proposed by Marcelo Tosatti in the
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past (search for migration cache on lkml or linux-mm@kvack.org).
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- Page migration unmaps ptes for file backed pages and requires page
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faults to reestablish these ptes. This could be optimized by somehow
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recording the references before migration and then reestablish them later.
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However, there are several locking challenges that have to be overcome
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before this is possible.
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- Page migration generates read ptes for anonymous pages. Dirty page
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faults are required to make the pages writable again. It may be possible
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to generate a pte marked dirty if it is known that the page is dirty and
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that this process has the only reference to that page.
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Christoph Lameter, March 8, 2006.
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Christoph Lameter, May 8, 2006.
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