btrfs: qgroup: Fix qgroup reserved space underflow caused by buffered write and quota...

[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
index 86cac431864be77f31e6feb85efb295f77005e7f..b66ea03a3a1c051dd30634b3c46b01e4d973dea0 100644 (file)
--- 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);