From 547dc035291531ac91c522a019323e878d5f6d9b Mon Sep 17 00:00:00 2001
From: Hugh Dickins <hughd@google.com>
Date: Sat, 28 May 2011 13:20:21 -0700
Subject: [PATCH] --- yaml --- r: 252737 b: refs/heads/master c:
 eee0f252c6537da2e883f75d22cff1427515eaf3 h: refs/heads/master i:   252735:
 c72970212ecfaeafc6a1f3c824dea4f471a53a92 v: v3

---
 [refs]          |  2 +-
 trunk/mm/rmap.c | 13 ++++++++-----
 2 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/[refs] b/[refs]
index d9766a92f41c..573da0f8abad 100644
--- a/[refs]
+++ b/[refs]
@@ -1,2 +1,2 @@
 ---
-refs/heads/master: 5dbe0af47f8a8f968bac2991c3ec974c6e3eaabc
+refs/heads/master: eee0f252c6537da2e883f75d22cff1427515eaf3
diff --git a/trunk/mm/rmap.c b/trunk/mm/rmap.c
index ba58ca36fc90..6bada99cd61c 100644
--- a/trunk/mm/rmap.c
+++ b/trunk/mm/rmap.c
@@ -405,6 +405,7 @@ struct anon_vma *page_get_anon_vma(struct page *page)
 struct anon_vma *page_lock_anon_vma(struct page *page)
 {
 	struct anon_vma *anon_vma = NULL;
+	struct anon_vma *root_anon_vma;
 	unsigned long anon_mapping;
 
 	rcu_read_lock();
@@ -415,13 +416,15 @@ struct anon_vma *page_lock_anon_vma(struct page *page)
 		goto out;
 
 	anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
-	if (mutex_trylock(&anon_vma->root->mutex)) {
+	root_anon_vma = ACCESS_ONCE(anon_vma->root);
+	if (mutex_trylock(&root_anon_vma->mutex)) {
 		/*
-		 * If we observe a !0 refcount, then holding the lock ensures
-		 * the anon_vma will not go away, see __put_anon_vma().
+		 * If the page is still mapped, then this anon_vma is still
+		 * its anon_vma, and holding the mutex ensures that it will
+		 * not go away, see __put_anon_vma().
 		 */
-		if (!atomic_read(&anon_vma->refcount)) {
-			anon_vma_unlock(anon_vma);
+		if (!page_mapped(page)) {
+			mutex_unlock(&root_anon_vma->mutex);
 			anon_vma = NULL;
 		}
 		goto out;