btrfs: qgroup: Fix qgroup reserved space underflow caused by buffered write and quotas being enabled

Qu Wenruo · kdave · commit a12b877b557d · 2017-06-29T20:17:02.000+02:00
[BUG] Under the following case, we can underflow qgroup reserved space. Task A | Task B --------------------------------------------------------------- Quota disabled | Buffered write | |- btrfs_check_data_free_space() | | *NO* qgroup space is reserved | | since quota is *DISABLED* | |- All pages are copied to page | cache | | Enable quota | Quota scan finished | | Sync_fs | |- run_delalloc_range | |- Write pages | |- btrfs_finish_ordered_io | |- insert_reserved_file_extent | |- btrfs_qgroup_release_data() | Since no qgroup space is reserved in Task A, we underflow qgroup reserved space This can be detected by fstest btrfs/104. [CAUSE] In insert_reserved_file_extent() we tell qgroup to release the @ram_bytes size of qgroup reserved_space in all cases. And btrfs_qgroup_release_data() will check if quotas are enabled. However in the above case, the buffered write happens before quota is enabled, so we don't have the reserved space for that range. [FIX] In insert_reserved_file_extent(), we tell qgroup to release the acctual byte number it released. In the above case, since we don't have the reserved space, we tell qgroups to release 0 byte, so the problem can be fixed. And thanks to the @reserved parameter introduced by the qgroup rework, and previous patch to return released bytes, the fix can be as small as 10 lines. Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com> [ changelog updates ] Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
@@ -2145,6 +2145,7 @@ static int insert_reserved_file_extent(struct btrfs_trans_handle *trans,
 	struct btrfs_path *path;
 	struct extent_buffer *leaf;
 	struct btrfs_key ins;
+	u64 qg_released;
 	int extent_inserted = 0;
 	int ret;
 
@@ -2200,13 +2201,17 @@ static int insert_reserved_file_extent(struct btrfs_trans_handle *trans,
 	ins.objectid = disk_bytenr;
 	ins.offset = disk_num_bytes;
 	ins.type = BTRFS_EXTENT_ITEM_KEY;
-	ret = btrfs_alloc_reserved_file_extent(trans, root->root_key.objectid,
-			btrfs_ino(BTRFS_I(inode)), file_pos, ram_bytes, &ins);
+
 	/*
 	 * Release the reserved range from inode dirty range map, as it is
 	 * already moved into delayed_ref_head
 	 */
-	btrfs_qgroup_release_data(inode, file_pos, ram_bytes);
+	ret = btrfs_qgroup_release_data(inode, file_pos, ram_bytes);
+	if (ret < 0)
+		goto out;
+	qg_released = ret;
+	ret = btrfs_alloc_reserved_file_extent(trans, root->root_key.objectid,
+			btrfs_ino(BTRFS_I(inode)), file_pos, qg_released, &ins);
 out:
 	btrfs_free_path(path);
 
