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, 12 insertions, 1 deletions

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 8fab233cb05f..24a2604dde7b 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -985,6 +985,16 @@ int ext4_get_blocks_wrap(handle_t *handle, struct inode *inode, sector_t block,
         } else {
                 retval = ext4_get_blocks_handle(handle, inode, block,
                                 max_blocks, bh, create, extend_disksize);
+
+                if (retval > 0 && buffer_new(bh)) {
+                        /*
+                         * We allocated new blocks which will result in
+                         * i_data's format changing.  Force the migrate
+                         * to fail by clearing migrate flags
+                         */
+                        EXT4_I(inode)->i_flags = EXT4_I(inode)->i_flags &
+                                                        ~EXT4_EXT_MIGRATE;
+                }
         }
         up_write((&EXT4_I(inode)->i_data_sem));
         return retval;
@@ -2976,7 +2986,8 @@ static int ext4_do_update_inode(handle_t *handle,
         if (ext4_inode_blocks_set(handle, raw_inode, ei))
                 goto out_brelse;
         raw_inode->i_dtime = cpu_to_le32(ei->i_dtime);
-        raw_inode->i_flags = cpu_to_le32(ei->i_flags);
+        /* clear the migrate flag in the raw_inode */
+        raw_inode->i_flags = cpu_to_le32(ei->i_flags & ~EXT4_EXT_MIGRATE);
         if (EXT4_SB(inode->i_sb)->s_es->s_creator_os !=
             cpu_to_le32(EXT4_OS_HURD))
                 raw_inode->i_file_acl_high =