[PATCH 1/2] btrfs: slightly loose the requirement for qgroup removal

[Qu Wenruo <wqu@suse.com>]

[BUG]
Currently if one is utilizing "qgroups/drop_subtree_threshold" sysfs,
and a snapshot with level higher than that value is dropped, btrfs will
not be able to delete the qgroup until next qgroup rescan:

  uuid=ffffffff-eeee-dddd-cccc-000000000000

  wipefs -fa $dev
  mkfs.btrfs -f $dev -O quota -s 4k -n 4k -U $uuid
  mount $dev $mnt

  btrfs subvolume create $mnt/subv1/
  for (( i = 0; i < 1024; i++ )); do
  	xfs_io -f -c "pwrite 0 2k" $mnt/subv1/file_$i > /dev/null
  done
  sync
  btrfs subv snapshot $mnt/subv1 $mnt/snapshot
  btrfs quota enable $mnt
  btrfs quota rescan -w $mnt
  sync
  echo 1 > /sys/fs/btrfs/$uuid/qgroups/drop_subtree_threshold
  btrfs subvolume delete $mnt/snapshot
  btrfs subv sync $mnt
  btrfs qgroup show -prce --sync $mnt
  btrfs qgroup destroy 0/256 $mnt
  umount $mnt

The final qgroup removal would fail with the following error:

  ERROR: unable to destroy quota group: Device or resource busy

[CAUSE]
The above script would generate a subvolume of level 2, then snapshot
it, enable qgroup, set the drop_subtree_threshold, then drop the
snapshot.

Since the subvolume drop would meet the threshold, qgroup would be
marked inconsistent and skip accounting to avoid hanging the system at
transaction commit.

But currently we do not allow a qgroup with any rfer/excl numbers to be
dropped, and this is not really compatible with the new
drop_subtree_threshold behavior.

[FIX]
Instead of a strong requirement for zero rfer/excl numbers, just check
if there is any child for higher level qgroup, and for subvolume qgroups
check if there is a corresponding subvolume for it.

For rsv values, just do a WARN_ON() in case.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/qgroup.c | 48 ++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 41 insertions(+), 7 deletions(-)

diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
index 9a9f84c4d3b8..e6fcce4372a4 100644
--- a/fs/btrfs/qgroup.c
+++ b/fs/btrfs/qgroup.c
@@ -1736,13 +1736,38 @@ int btrfs_create_qgroup(struct btrfs_trans_handle *trans, u64 qgroupid)
 	return ret;
 }
 
-static bool qgroup_has_usage(struct btrfs_qgroup *qgroup)
+static bool can_delete_qgroup(struct btrfs_fs_info *fs_info,
+			      struct btrfs_qgroup *qgroup)
 {
-	return (qgroup->rfer > 0 || qgroup->rfer_cmpr > 0 ||
-		qgroup->excl > 0 || qgroup->excl_cmpr > 0 ||
-		qgroup->rsv.values[BTRFS_QGROUP_RSV_DATA] > 0 ||
-		qgroup->rsv.values[BTRFS_QGROUP_RSV_META_PREALLOC] > 0 ||
-		qgroup->rsv.values[BTRFS_QGROUP_RSV_META_PERTRANS] > 0);
+	struct btrfs_key key;
+	struct btrfs_path *path;
+	int ret;
+
+	/* For higher level qgroup, we can only delete it if it has no child. */
+	if (btrfs_qgroup_level(qgroup->qgroupid)) {
+		if (!list_empty(&qgroup->members))
+			return false;
+		return true;
+	}
+
+	/*
+	 * For level-0 qgroups, we can only delete it if it has no subvolume
+	 * for it.
+	 * This means even a subvolume is unlinked but not yet fully dropped,
+	 * we can not delete the qgroup.
+	 */
+	key.objectid = qgroup->qgroupid;
+	key.type = BTRFS_ROOT_ITEM_KEY;
+	key.offset = -1ULL;
+	path = btrfs_alloc_path();
+	if (!path)
+		return false;
+
+	ret = btrfs_find_root(fs_info->tree_root, &key, path, NULL, NULL);
+	btrfs_free_path(path);
+	if (ret > 0)
+		return true;
+	return false;
 }
 
 int btrfs_remove_qgroup(struct btrfs_trans_handle *trans, u64 qgroupid)
@@ -1764,7 +1789,7 @@ int btrfs_remove_qgroup(struct btrfs_trans_handle *trans, u64 qgroupid)
 		goto out;
 	}
 
-	if (is_fstree(qgroupid) && qgroup_has_usage(qgroup)) {
+	if (!can_delete_qgroup(fs_info, qgroup)) {
 		ret = -EBUSY;
 		goto out;
 	}
@@ -1775,6 +1800,15 @@ int btrfs_remove_qgroup(struct btrfs_trans_handle *trans, u64 qgroupid)
 		goto out;
 	}
 
+	/*
+	 * Warn on reserved space. The subvolume should has no child nor
+	 * corresponding subvolume.
+	 * Thus its reserved space should all be zero.
+	 */
+	WARN_ON(qgroup->rsv.values[BTRFS_QGROUP_RSV_DATA] ||
+		qgroup->rsv.values[BTRFS_QGROUP_RSV_META_PREALLOC] ||
+		qgroup->rsv.values[BTRFS_QGROUP_RSV_META_PERTRANS]);
+
 	ret = del_qgroup_item(trans, qgroupid);
 	if (ret && ret != -ENOENT)
 		goto out;
-- 
2.44.0