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Subject[20/90] ext4: Take page lock before looking at attached buffer_heads flags
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@linux.vnet.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
---
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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