[PATCH 2/2] btrfs: qgroup: Fix qgroup reserved space underflow caused by buffered write and quota enable

[Qu Wenruo <quwenruo@cn.fujitsu.com>]

[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 info qgroup to release the @ram_bytes
size of qgroup reserved_space under all case.
And btrfs_qgroup_release_data() will check if qgroup is enabled.

However in above case, the buffered write happens before quota is
enabled, so we don't havee reserved space for that range.

[FIX]
In insert_reserved_file_extent(), we info qgroup to release the acctual
byte number it released.
In above case, since we don't have reserved space, we info qgroup to
release 0 byte, so the problem can be fixed.

And thanks to the @reserved parameter introduced by qgroup rework, and
previous patch to return release bytes, the fix can be as small as less
than 10 lines.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
---
To David:
These 2 patches, with updated extra WARN_ON() patch(V5), btrfs on x86_64 is
good for current qgroup test group.
But the bug reported by Chadan still exists, and the fix for that will be
delayed for a while, as the fix involves large interface change
(add struct extent_changeset for every qgroup reserve caller).

I'll run extra tests for that fix to ensure they are OK and won't cause
extra bugs.

Thanks,
Qu
---
 fs/btrfs/inode.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 242dc7e..3db58d9 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -2256,6 +2256,7 @@ 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;
 
@@ -2311,15 +2312,19 @@ 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->root_key.objectid,
-					btrfs_ino(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->root_key.objectid,
+					btrfs_ino(inode), file_pos,
+					qg_released, &ins);
 out:
 	btrfs_free_path(path);
 
-- 
2.10.2