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yaml --- r: 250897 b: refs/heads/master c: 6111e4c h: refs/heads/master i: 250895: 3e161ab v: v3
git-mirror · May 25, 2011 · 38e1844 · 38e1844
1 parent 3e5d67d
commit 38e1844
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Showing 2 changed files with 17 additions and 3 deletions.
diff --git a/[refs] b/[refs]
@@ -1,2 +1,2 @@
 ---
-refs/heads/master: 25aeeb046e695c3093a86aa9386128ffb3b1bc32
+refs/heads/master: 6111e4ca6829a0e8b092b8e5eeb6b5366091f29c
diff --git a/trunk/mm/rmap.c b/trunk/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)
 {