mm: page migration avoid touching newpage until no going back
We have had trouble in the past from the way in which page migration's
newpage is initialized in dribs and drabs - see commit 8bdd63809160 ("mm:
fix direct reclaim writeback regression") which proposed a cleanup.
We have no actual problem now, but I think the procedure would be clearer
(and alternative get_new_page pools safer to implement) if we assert that
newpage is not touched until we are sure that it's going to be used -
except for taking the trylock on it in __unmap_and_move().
So shift the early initializations from move_to_new_page() into
migrate_page_move_mapping(), mapping and NULL-mapping paths. Similarly
migrate_huge_page_move_mapping(), but its NULL-mapping path can just be
deleted: you cannot reach hugetlbfs_migrate_page() with a NULL mapping.
Adjust stages 3 to 8 in the Documentation file accordingly.
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Sasha Levin <sasha.levin@oracle.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/Documentation/vm/page_migration b/Documentation/vm/page_migration
index 479c3d0..fea5c08 100644
--- a/Documentation/vm/page_migration
+++ b/Documentation/vm/page_migration
@@ -92,27 +92,26 @@
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.
+3. Lock the new page that we want to move to. It is locked so that accesses to
+ this (not yet uptodate) 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
+4. All the page table references to the page are converted to migration
entries. This decreases 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
+5. 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
+6. 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
+7. 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.
+8. 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.
+
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