ext4: Clear the unwritten buffer_head flag after the extent is initialized

The BH_Unwritten flag indicates that the buffer is allocated on disk
but has not been written; that is, the disk was part of a persistent
preallocation area.  That flag should only be set when a get_blocks()
function is looking up a inode's logical to physical block mapping.

When ext4_get_blocks_wrap() is called with create=1, the uninitialized
extent is converted into an initialized one, so the BH_Unwritten flag
is no longer appropriate.  Hence, we need to make sure the
BH_Unwritten is not left set, since the combination of BH_Mapped and
BH_Unwritten is not allowed; among other things, it will result ext4's
get_block() to be called over and over again during the write_begin
phase of write(2).
Signed-off-by: NAneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

fs/ext4/inode.c

@@ -1149,6 +1149,7 @@ int ext4_get_blocks_wrap(handle_t *handle, struct inode *inode, sector_t block,
 	int retval;
 
 	clear_buffer_mapped(bh);
+	clear_buffer_unwritten(bh);
 
 	/*
 	 * Try to see if we can get  the block without requesting
@@ -1178,6 +1179,18 @@ int ext4_get_blocks_wrap(handle_t *handle, struct inode *inode, sector_t block,
 	if (retval > 0 && buffer_mapped(bh))
 		return retval;
 
+	/*
+	 * When we call get_blocks without the create flag, the
+	 * BH_Unwritten flag could have gotten set if the blocks
+	 * requested were part of a uninitialized extent.  We need to
+	 * clear this flag now that we are committed to convert all or
+	 * part of the uninitialized extent to be an initialized
+	 * extent.  This is because we need to avoid the combination
+	 * of BH_Unwritten and BH_Mapped flags being simultaneously
+	 * set on the buffer_head.
+	 */
+	clear_buffer_unwritten(bh);
+
 	/*
 	 * New blocks allocate and/or writing to uninitialized extent
 	 * will possibly result in updating i_data, so we take