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authorPeter Zijlstra <a.p.zijlstra@chello.nl>2011-05-24 17:12:08 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2011-05-25 08:39:18 -0700
commit6111e4ca6829a0e8b092b8e5eeb6b5366091f29c (patch)
tree12d0f6bf4dad72e6842b8d68b72f24fd23bb236c /mm/rmap.c
parent25aeeb046e695c3093a86aa9386128ffb3b1bc32 (diff)
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mm: improve page_lock_anon_vma() comment
A slightly more verbose comment to go along with the trickery in page_lock_anon_vma(). Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> Acked-by: Mel Gorman <mel@csn.ul.ie> Acked-by: Hugh Dickins <hughd@google.com> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org> Cc: David Miller <davem@davemloft.net> Cc: Martin Schwidefsky <schwidefsky@de.ibm.com> Cc: Russell King <rmk@arm.linux.org.uk> Cc: Paul Mundt <lethal@linux-sh.org> Cc: Jeff Dike <jdike@addtoit.com> Cc: Richard Weinberger <richard@nod.at> Cc: Tony Luck <tony.luck@intel.com> Cc: Nick Piggin <npiggin@kernel.dk> Cc: Namhyung Kim <namhyung@gmail.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'mm/rmap.c')
-rw-r--r--mm/rmap.c18
1 files changed, 16 insertions, 2 deletions
diff --git a/mm/rmap.c b/mm/rmap.c
index c6044761617e..cc140811af56 100644
--- a/mm/rmap.c
+++ b/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)
{