[PATCH -v3] ext4: Take page lock before looking at attached buffer_heads flags

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

Inorder 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@linux.vnet.ibm.com>
---
 fs/ext4/inode.c |   13 +++++++++++--
 1 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index f9c642b..2feb2d5 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -5281,12 +5281,21 @@ int ext4_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
 	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
-- 
1.6.4.2.253.g0b1fac