[PATCH -V4 1/2] Fix sub-block zeroing for buffered writes into unwritten extents

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

We need to mark the  buffer_head mapping prealloc space
as new during write_begin. Otherwise we don't zero out the
page cache content properly for a partial write. This will
cause file corruption with preallocation.

Also use block number -1 as the fake block number so that
unmap_underlying_metadata doesn't drop wrong buffer_head

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>

---
 fs/ext4/inode.c |   10 ++++++++++
 1 files changed, 10 insertions(+), 0 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index e91f978..12dcfab 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -2323,6 +2323,16 @@ static int ext4_da_get_block_prep(struct inode *inode, sector_t iblock,
 		set_buffer_delay(bh_result);
 	} else if (ret > 0) {
 		bh_result->b_size = (ret << inode->i_blkbits);
+		/*
+		 * With sub-block writes into unwritten extents
+		 * we also need to mark the buffer as new so that
+		 * the unwritten parts of the buffer gets correctly zeroed.
+		 */
+		if (buffer_unwritten(bh_result)) {
+			bh_result->b_bdev = inode->i_sb->s_bdev;
+			set_buffer_new(bh_result);
+			bh_result->b_blocknr = -1;
+		}
 		ret = 0;
 	}
 
-- 
tg: (56a50ad..) preallocate_corruption_quickfix (depends on: master)