[50/90] ext4: fix a BUG_ON crash by checking that page has buffers attached to it

[Greg KH]

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

------------------
(cherry picked from commit 1f94533d9cd75f6d2826018d54a971b9cc085992)

In ext4_num_dirty_pages() we were calling page_buffers() before
checking to see if the page actually had pages attached to it; this
would cause a BUG check crash in the inline function page_buffers().

Thanks to Markus Trippelsdorf for reporting this bug.

Signed-off-by: "Theodore Ts'o" <tytso@xxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>
---
 fs/ext4/inode.c |   22 +++++++++++-----------
 1 file changed, 11 insertions(+), 11 deletions(-)

--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -1147,8 +1147,8 @@ static int check_block_validity(struct i
 }
 
 /*
- * Return the number of dirty pages in the given inode starting at
- * page frame idx.
+ * Return the number of contiguous dirty pages in a given inode
+ * starting at page frame idx.
  */
 static pgoff_t ext4_num_dirty_pages(struct inode *inode, pgoff_t idx,
 				    unsigned int max_pages)
@@ -1182,15 +1182,15 @@ static pgoff_t ext4_num_dirty_pages(stru
 				unlock_page(page);
 				break;
 			}
-			head = page_buffers(page);
-			bh = head;
-			do {
-				if (!buffer_delay(bh) &&
-				    !buffer_unwritten(bh)) {
-					done = 1;
-					break;
-				}
-			} while ((bh = bh->b_this_page) != head);
+			if (page_has_buffers(page)) {
+				bh = head = page_buffers(page);
+				do {
+					if (!buffer_delay(bh) &&
+					    !buffer_unwritten(bh))
+						done = 1;
+					bh = bh->b_this_page;
+				} while (!done && (bh != head));
+			}
 			unlock_page(page);
 			if (done)
 				break;


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