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Commit 6111e4ca6829a0e8b092b8e5eeb6b5366091f29c

Authored by Peter Zijlstra
Committed by Linus Torvalds
1 parent 25aeeb046e
Exists in master and in 20 other branches dlt-processor-sdk-linux-03.00.00.04, newt-ti-linux-3.12.y, processor-sdk-linux-01.00.00, processor-sdk-linux-02.00.01, smarc-ti-linux-3.12.10, smarc-ti-linux-3.12.y, smarc-ti-linux-3.14.y, smarc-ti-linux-3.15.y, smarc-ti-lsk-linux-4.1.y, smarct3x-processor-sdk-04.01.00.06, smarct3x-processor-sdk-linux-02.00.01, smarct3x-processor-sdk-linux-03.00.00.04, smarct4x-800-processor-sdk-linux-02.00.01, smarct4x-processor-sdk-04.01.00.06, smarct4x-processor-sdk-linux-02.00.01, smarct4x-processor-sdk-linux-03.00.00.04, ti-linux-3.12.y, ti-linux-3.14.y, ti-linux-3.15.y, ti-lsk-linux-4.1.y

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>

Showing 1 changed file with 16 additions and 2 deletions Side-by-side Diff

... ... @@ -320,8 +320,22 @@
320 320 }
321 321  
322 322 /*
323   - * Getting a lock on a stable anon_vma from a page off the LRU is
324   - * tricky: page_lock_anon_vma rely on RCU to guard against the races.
  323 + * Getting a lock on a stable anon_vma from a page off the LRU is tricky!
  324 + *
  325 + * Since there is no serialization what so ever against page_remove_rmap()
  326 + * the best this function can do is return a locked anon_vma that might
  327 + * have been relevant to this page.
  328 + *
  329 + * The page might have been remapped to a different anon_vma or the anon_vma
  330 + * returned may already be freed (and even reused).
  331 + *
  332 + * All users of this function must be very careful when walking the anon_vma
  333 + * chain and verify that the page in question is indeed mapped in it
  334 + * [ something equivalent to page_mapped_in_vma() ].
  335 + *
  336 + * Since anon_vma's slab is DESTROY_BY_RCU and we know from page_remove_rmap()
  337 + * that the anon_vma pointer from page->mapping is valid if there is a
  338 + * mapcount, we can dereference the anon_vma after observing those.
325 339 */
326 340 struct anon_vma *page_lock_anon_vma(struct page *page)
327 341 {