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Commit 6111e4ca authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Linus Torvalds
Browse files

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: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
Reviewed-by: default avatarKOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Reviewed-by: default avatarKAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: default avatarMel Gorman <mel@csn.ul.ie>
Acked-by: default avatarHugh 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: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 25aeeb04

mm/rmap.c

+16 −2
Original line number Diff line number Diff line
@@ -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)

{