ext4: fix i_blocks/quota accounting when extent insertion fails

The current implementation of ext4_free_blocks() always calls
dquot_free_block This looks quite sensible in the most cases: blocks
to be freed are associated with inode and were accounted in quota and
i_blocks some time ago.

However, there is a case when blocks to free were not accounted by the
time calling ext4_free_blocks() yet:

1. delalloc is on, write_begin pre-allocated some space in quota
2. write-back happens, ext4 allocates some blocks in ext4_ext_map_blocks()
3. then ext4_ext_map_blocks() gets an error (e.g.  ENOSPC) from
   ext4_ext_insert_extent() and calls ext4_free_blocks().

In this scenario, ext4_free_blocks() calls dquot_free_block() who, in
turn, decrements i_blocks for blocks which were not accounted yet (due
to delalloc) After clean umount, e2fsck reports something like:

> Inode 21, i_blocks is 5080, should be 5128.  Fix<y>?
because i_blocks was erroneously decremented as explained above.

The patch fixes the problem by passing the new flag
EXT4_FREE_BLOCKS_NO_QUOT_UPDATE to ext4_free_blocks(), to request
that the dquot_free_block() call be skipped.
Signed-off-by: Maxim Patlasov <maxim.patlasov@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: stable@kernel.org

fs/ext4/ext4.h

@@ -526,6 +526,7 @@ struct ext4_new_group_data {
 #define EXT4_FREE_BLOCKS_METADATA	0x0001
 #define EXT4_FREE_BLOCKS_FORGET		0x0002
 #define EXT4_FREE_BLOCKS_VALIDATED	0x0004
+#define EXT4_FREE_BLOCKS_NO_QUOT_UPDATE	0x0008
 
 /*
  * ioctl commands

fs/ext4/extents.c

@@ -3565,12 +3565,14 @@ int ext4_ext_map_blocks(handle_t *handle, struct inode *inode,
 
 	err = ext4_ext_insert_extent(handle, inode, path, &newex, flags);
 	if (err) {
+		int fb_flags = flags & EXT4_GET_BLOCKS_DELALLOC_RESERVE ?
+			EXT4_FREE_BLOCKS_NO_QUOT_UPDATE : 0;
 		/* free data blocks we just allocated */
 		/* not a good idea to call discard here directly,
 		 * but otherwise we'd need to call it every free() */
 		ext4_discard_preallocations(inode);
 		ext4_free_blocks(handle, inode, NULL, ext4_ext_pblock(&newex),
-				 ext4_ext_get_actual_len(&newex), 0);
+				 ext4_ext_get_actual_len(&newex), fb_flags);
 		goto out2;
 	}
 

fs/ext4/mballoc.c

@@ -4637,7 +4637,7 @@ void ext4_free_blocks(handle_t *handle, struct inode *inode,
 	}
 	ext4_mark_super_dirty(sb);
 error_return:
-	if (freed)
+	if (freed && !(flags & EXT4_FREE_BLOCKS_NO_QUOT_UPDATE))
 		dquot_free_block(inode, freed);
 	brelse(bitmap_bh);
 	ext4_std_error(sb, err);