[PATCH 6/8] btrfs: Add code to check if a qgroup's subvol exists

[Sargun Dhillon <sargun@sargun.me>]

This patch is to prepare for following patches in this patchset. The
purpose is to make it so that we can prevent accidental removal of
qgroups that are actively in use.

Signed-off-by: Sargun Dhillon <sargun@sargun.me>
---
 fs/btrfs/ioctl.c  |  4 ++--
 fs/btrfs/qgroup.c | 50 +++++++++++++++++++++++++++++++++++++++++++++++++-
 fs/btrfs/qgroup.h |  3 ++-
 3 files changed, 53 insertions(+), 4 deletions(-)

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index b10d7bb..2b1a8c1 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -2554,7 +2554,7 @@ static noinline int btrfs_ioctl_snap_destroy(struct file *file,
 	 */
 	if (!btrfs_test_opt(fs_info, QGROUP_KEEP)) {
 		ret = btrfs_remove_qgroup(trans, fs_info,
-					  dest->root_key.objectid);
+					  dest->root_key.objectid, 0);
 		if (ret && ret != -ENOENT)
 			pr_info("Could not automatically delete qgroup: %d\n", ret);
 	}
@@ -4974,7 +4974,7 @@ static long btrfs_ioctl_qgroup_create(struct file *file, void __user *arg)
 	if (sa->create) {
 		ret = btrfs_create_qgroup(trans, fs_info, sa->qgroupid);
 	} else {
-		ret = btrfs_remove_qgroup(trans, fs_info, sa->qgroupid);
+		ret = btrfs_remove_qgroup(trans, fs_info, sa->qgroupid, 0);
 	}
 
 	err = btrfs_end_transaction(trans);
diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
index 588248b..a0699fd 100644
--- a/fs/btrfs/qgroup.c
+++ b/fs/btrfs/qgroup.c
@@ -1247,6 +1247,45 @@ int btrfs_del_qgroup_relation(struct btrfs_trans_handle *trans,
 	return ret;
 }
 
+/*
+ * Meant to only operate on level-0 qroupid.
+ *
+ * It returns 1 if a matching subvolume is found; 0 if none is found.
+ * < 0 if there is an error.
+ */
+static int btrfs_subvolume_exists(struct btrfs_fs_info *fs_info, u64 qgroupid)
+{
+	struct btrfs_path *path;
+	struct btrfs_key key;
+	int err, ret = 0;
+
+	path = btrfs_alloc_path();
+	if (!path)
+		return -ENOMEM;
+
+	key.objectid = qgroupid;
+	key.type = BTRFS_ROOT_BACKREF_KEY;
+	key.offset = 0;
+
+	err = btrfs_search_slot_for_read(fs_info->tree_root, &key, path, 1, 0);
+	if (err == 1)
+		goto out;
+
+	if (err) {
+		ret = err;
+		goto out;
+	}
+
+	btrfs_item_key_to_cpu(path->nodes[0], &key, path->slots[0]);
+	if (key.objectid != qgroupid || key.type != BTRFS_ROOT_BACKREF_KEY)
+		goto out;
+
+	ret = 1;
+out:
+	btrfs_free_path(path);
+	return ret;
+}
+
 /* Must be called with qgroup_ioctl_lock held */
 static int __btrfs_create_qgroup(struct btrfs_trans_handle *trans,
 				 struct btrfs_fs_info *fs_info, u64 qgroupid)
@@ -1333,10 +1372,19 @@ static int __btrfs_remove_qgroup(struct btrfs_trans_handle *trans,
 }
 
 int btrfs_remove_qgroup(struct btrfs_trans_handle *trans,
-			struct btrfs_fs_info *fs_info, u64 qgroupid)
+			struct btrfs_fs_info *fs_info, u64 qgroupid,
+			int check_in_use)
 {
 	int ret;
 
+	if (check_in_use && btrfs_qgroup_level(qgroupid) == 0) {
+		ret = btrfs_subvolume_exists(fs_info, qgroupid);
+		if (ret < 0)
+			return ret;
+		if (ret)
+			return -EBUSY;
+	}
+
 	mutex_lock(&fs_info->qgroup_ioctl_lock);
 	ret = __btrfs_remove_qgroup(trans, fs_info, qgroupid);
 	mutex_unlock(&fs_info->qgroup_ioctl_lock);
diff --git a/fs/btrfs/qgroup.h b/fs/btrfs/qgroup.h
index fb6c7da..fc08bdb 100644
--- a/fs/btrfs/qgroup.h
+++ b/fs/btrfs/qgroup.h
@@ -127,7 +127,8 @@ int btrfs_del_qgroup_relation(struct btrfs_trans_handle *trans,
 int btrfs_create_qgroup(struct btrfs_trans_handle *trans,
 			struct btrfs_fs_info *fs_info, u64 qgroupid);
 int btrfs_remove_qgroup(struct btrfs_trans_handle *trans,
-			struct btrfs_fs_info *fs_info, u64 qgroupid);
+			struct btrfs_fs_info *fs_info, u64 qgroupid,
+			int check_in_use);
 int btrfs_limit_qgroup(struct btrfs_trans_handle *trans,
 		       struct btrfs_fs_info *fs_info, u64 qgroupid,
 		       struct btrfs_qgroup_limit *limit);
-- 
2.9.3