The patch titled
mm: improve page_lock_anon_vma() comment
has been added to the -mm tree. Its filename is
mm-improve-page_lock_anon_vma-comment.patch
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------------------------------------------------------
Subject: mm: improve page_lock_anon_vma() comment
From: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
A slightly more verbose comment to go along with the trickery in
page_lock_anon_vma().
Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
Acked-by: Mel Gorman <mel@xxxxxxxxx>
Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
Cc: David Miller <davem@xxxxxxxxxxxxx>
Cc: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
Cc: Russell King <rmk@xxxxxxxxxxxxxxxx>
Cc: Paul Mundt <lethal@xxxxxxxxxxxx>
Cc: Jeff Dike <jdike@xxxxxxxxxxx>
Cc: Richard Weinberger <richard@xxxxxx>
Cc: Tony Luck <tony.luck@xxxxxxxxx>
Cc: Nick Piggin <npiggin@xxxxxxxxx>
Cc: Namhyung Kim <namhyung@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/rmap.c | 18 ++++++++++++++++--
1 file changed, 16 insertions(+), 2 deletions(-)
diff -puN mm/rmap.c~mm-improve-page_lock_anon_vma-comment mm/rmap.c
--- a/mm/rmap.c~mm-improve-page_lock_anon_vma-comment
+++ a/mm/rmap.c
@@ -320,8 +320,22 @@ void __init anon_vma_init(void)
}
/*
- * Getting a lock on a stable anon_vma from a page off the LRU is
- * tricky: page_lock_anon_vma rely on RCU to guard against the races.
+ * Getting a lock on a stable anon_vma from a page off the LRU is tricky!
+ *
+ * Since there is no serialization what so ever against page_remove_rmap()
+ * the best this function can do is return a locked anon_vma that might
+ * have been relevant to this page.
+ *
+ * The page might have been remapped to a different anon_vma or the anon_vma
+ * returned may already be freed (and even reused).
+ *
+ * All users of this function must be very careful when walking the anon_vma
+ * chain and verify that the page in question is indeed mapped in it
+ * [ something equivalent to page_mapped_in_vma() ].
+ *
+ * Since anon_vma's slab is DESTROY_BY_RCU and we know from page_remove_rmap()
+ * that the anon_vma pointer from page->mapping is valid if there is a
+ * mapcount, we can dereference the anon_vma after observing those.
*/
struct anon_vma *page_lock_anon_vma(struct page *page)
{
_
Patches currently in -mm which might be from a.p.zijlstra@xxxxxxxxx are
linux-next.patch
net-convert-%p-usage-to-%pk.patch
mm-mmu_gather-rework.patch
mm-mmu_gather-rework-fix.patch
powerpc-mmu_gather-rework.patch
sparc-mmu_gather-rework.patch
s390-mmu_gather-rework.patch
arm-mmu_gather-rework.patch
sh-mmu_gather-rework.patch
ia64-mmu_gather-rework.patch
um-mmu_gather-rework.patch
mm-now-that-all-old-mmu_gather-code-is-gone-remove-the-storage.patch
mm-powerpc-move-the-rcu-page-table-freeing-into-generic-code.patch
mm-extended-batches-for-generic-mmu_gather.patch
lockdep-mutex-provide-mutex_lock_nest_lock.patch
mm-remove-i_mmap_lock-lockbreak.patch
mm-convert-i_mmap_lock-to-a-mutex.patch
mm-revert-page_lock_anon_vma-lock-annotation.patch
mm-improve-page_lock_anon_vma-comment.patch
mm-use-refcounts-for-page_lock_anon_vma.patch
mm-convert-anon_vma-lock-to-a-mutex.patch
mm-optimize-page_lock_anon_vma-fast-path.patch
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