[20/90] ext4: Take page lock before looking at attached buffer_heads flags

[Greg KH]

2.6.31-stable review patch.  If anyone has any objections, please let us know.

------------------
(cherry picked from commit a827eaffff07c7d58a4cb32158cbeb4849f4e33a)

In order to check whether the buffer_heads are mapped we need to hold
page lock. Otherwise a reclaim can cleanup the attached buffer_heads.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
Signed-off-by: "Theodore Ts'o" <tytso@xxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>
---
 fs/ext4/inode.c |   13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -5298,12 +5298,21 @@ int ext4_page_mkwrite(struct vm_area_str
 	else
 		len = PAGE_CACHE_SIZE;
 
+	lock_page(page);
+	/*
+	 * return if we have all the buffers mapped. This avoid
+	 * the need to call write_begin/write_end which does a
+	 * journal_start/journal_stop which can block and take
+	 * long time
+	 */
 	if (page_has_buffers(page)) {
-		/* return if we have all the buffers mapped */
 		if (!walk_page_buffers(NULL, page_buffers(page), 0, len, NULL,
-				       ext4_bh_unmapped))
+					ext4_bh_unmapped)) {
+			unlock_page(page);
 			goto out_unlock;
+		}
 	}
+	unlock_page(page);
 	/*
 	 * OK, we need to fill the hole... Do write_begin write_end
 	 * to do block allocation/reservation.We are not holding


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