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    Subject[PATCH 03/21] mm: Improve page_lock_anon_vma() comment
    A slightly more verbose comment to go along with the trickery in
    page_lock_anon_vma().

    Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
    Acked-by: Mel Gorman <mel@csn.ul.ie>
    Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    LKML-Reference: <1271158226.4807.1107.camel@twins>
    ---
    mm/rmap.c | 18 ++++++++++++++++--
    1 file changed, 16 insertions(+), 2 deletions(-)

    Index: linux-2.6/mm/rmap.c
    ===================================================================
    --- linux-2.6.orig/mm/rmap.c
    +++ linux-2.6/mm/rmap.c
    @@ -311,8 +311,22 @@ 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)
    {



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