ext4: Fix race between migration and mmap write

Fail migrate if we allocated new blocks via mmap write.

If we write to holes in the file via mmap, we end up allocating
new blocks. This block allocation happens without taking inode->i_mutex.
Since migrate is protected by i_mutex and migrate expects that no
new blocks get allocated during migrate, fail migrate if new blocks
get allocated.

We can't take inode->i_mutex in the mmap write path because that
would result in a locking order violation between i_mutex and mmap_sem.
Also adding a separate rw_sempahore for protection is really high overhead
for a rare operation such as migrate.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Acked-by: Jan Kara <jack@suse.cz>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/linux/ext4_fs.h b/include/linux/ext4_fs.h
index 250032548597..105337ca9ed0 100644
--- a/include/linux/ext4_fs.h
+++ b/include/linux/ext4_fs.h
@@ -231,6 +231,7 @@ struct ext4_group_desc
 #define EXT4_TOPDIR_FL                  0x00020000 /* Top of directory hierarchies*/
 #define EXT4_HUGE_FILE_FL               0x00040000 /* Set to each huge file */
 #define EXT4_EXTENTS_FL                 0x00080000 /* Inode uses extents */
+#define EXT4_EXT_MIGRATE                0x00100000 /* Inode is migrating */
 #define EXT4_RESERVED_FL                0x80000000 /* reserved for ext4 lib */
 
 #define EXT4_FL_USER_VISIBLE            0x000BDFFF /* User visible flags */