btrfs: fix squota compressed stats leak

The following workload on a squota enabled fs:

  btrfs subvol create mnt/subvol

  # ensure subvol extents get accounted
  sync
  btrfs qgroup create 1/1 mnt
  btrfs qgroup assign mnt/subvol 1/1 mnt
  btrfs qgroup delete mnt/subvol

  # make the cleaner thread run
  btrfs filesystem sync mnt
  sleep 1
  btrfs filesystem sync mnt
  btrfs qgroup destroy 1/1 mnt

will fail with EBUSY. The reason is that 1/1 does the quick accounting
when we assign subvol to it, gaining its exclusive usage as excl and
excl_cmpr. But then when we delete subvol, the decrement happens via
record_squota_delta() which does not update excl_cmpr, as squotas does
not make any distinction between compressed and normal extents. Thus,
we increment excl_cmpr but never decrement it, and are unable to delete
1/1. The two possible fixes are to make squota always mirror excl and
excl_cmpr or to make the fast accounting separately track the plain and
cmpr numbers. The latter felt cleaner to me so that is what I opted for.

Fixes: 1e0e9d5 ("btrfs: add helper for recording simple quota deltas")
CC: [email protected] # 6.12+
Reviewed-by: Qu Wenruo <[email protected]>
Signed-off-by: Boris Burkov <[email protected]>
Signed-off-by: David Sterba <[email protected]>

Author: boryas

fs/btrfs/qgroup.c

@@ -1455,6 +1455,7 @@ static int __qgroup_excl_accounting(struct btrfs_fs_info *fs_info, u64 ref_root,
 	struct btrfs_qgroup *qgroup;
 	LIST_HEAD(qgroup_list);
 	u64 num_bytes = src->excl;
+	u64 num_bytes_cmpr = src->excl_cmpr;
 	int ret = 0;
 
 	qgroup = find_qgroup_rb(fs_info, ref_root);
@@ -1466,11 +1467,12 @@ static int __qgroup_excl_accounting(struct btrfs_fs_info *fs_info, u64 ref_root,
 		struct btrfs_qgroup_list *glist;
 
 		qgroup->rfer += sign * num_bytes;
-		qgroup->rfer_cmpr += sign * num_bytes;
+		qgroup->rfer_cmpr += sign * num_bytes_cmpr;
 
 		WARN_ON(sign < 0 && qgroup->excl < num_bytes);
+		WARN_ON(sign < 0 && qgroup->excl_cmpr < num_bytes_cmpr);
 		qgroup->excl += sign * num_bytes;
-		qgroup->excl_cmpr += sign * num_bytes;
+		qgroup->excl_cmpr += sign * num_bytes_cmpr;
 
 		if (sign > 0)
 			qgroup_rsv_add_by_qgroup(fs_info, qgroup, src);