ext4: disable merging of uninitialized extents

Derived from Jan's patch:http://permalink.gmane.org/gmane.comp.file-systems.ext4/36470

Merging of uninitialized extents creates all sorts of interesting race
possibilities when writeback / DIO races with fallocate. Thus
ext4_convert_unwritten_extents_endio() has to deal with a case where
extent to be converted needs to be split out first. That isn't nice
for two reasons:

1) It may need allocation of extent tree block so ENOSPC is possible.
2) It complicates end_io handling code

So we disable merging of uninitialized extents which allows us to simplify
the code. Extents will get merged after they are converted to initialized
ones.

Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>

fs/ext4/extents.c

@@ -1584,10 +1584,12 @@ ext4_can_extents_be_merged(struct inode *inode, struct ext4_extent *ex1,
 	unsigned short ext1_ee_len, ext2_ee_len, max_len;
 
 	/*
-	 * Make sure that either both extents are uninitialized, or
-	 * both are _not_.
+	 * Make sure that both extents are initialized. We don't merge
+	 * uninitialized extents so that we can be sure that end_io code has
+	 * the extent that was written properly split out and conversion to
+	 * initialized is trivial.
 	 */
-	if (ext4_ext_is_uninitialized(ex1) ^ ext4_ext_is_uninitialized(ex2))
+	if (ext4_ext_is_uninitialized(ex1) || ext4_ext_is_uninitialized(ex2))
 		return 0;
 
 	if (ext4_ext_is_uninitialized(ex1))