Messages in this thread Patch in this message | | | Date | Fri, 26 Nov 2010 15:38:46 +0100 | From | Peter Zijlstra <> | Subject | [PATCH 03/21] mm: Improve page_lock_anon_vma() comment |
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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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