btrfs: qgroup: do quick checks if quotas are enabled before starting ioctls

The ioctls that add relations, create qgroups or set limits start/join
transaction. When quotas are not enabled this is not necessary, there
will be errors reported back anyway but this could be also misleading
and we should really report that quotas are not enabled. For that use
-ENOTCONN.

The helper is meant to do a quick check before any other standard ioctl
checks are done. If quota is disabled meanwhile we still rely on proper
locking inside any active operation changing the qgroup structures.
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ioctl.c

@@ -3807,6 +3807,22 @@ static long btrfs_ioctl_quota_ctl(struct file *file, void __user *arg)
 	return ret;
 }
 
+/*
+ * Quick check for ioctl handlers if quotas are enabled. Proper locking must be
+ * done before any operations.
+ */
+static bool qgroup_enabled(struct btrfs_fs_info *fs_info)
+{
+	bool ret = true;
+
+	mutex_lock(&fs_info->qgroup_ioctl_lock);
+	if (!fs_info->quota_root)
+		ret = false;
+	mutex_unlock(&fs_info->qgroup_ioctl_lock);
+
+	return ret;
+}
+
 static long btrfs_ioctl_qgroup_assign(struct file *file, void __user *arg)
 {
 	struct inode *inode = file_inode(file);
@@ -3820,6 +3836,9 @@ static long btrfs_ioctl_qgroup_assign(struct file *file, void __user *arg)
 	if (!capable(CAP_SYS_ADMIN))
 		return -EPERM;
 
+	if (!qgroup_enabled(root->fs_info))
+		return -ENOTCONN;
+
 	ret = mnt_want_write_file(file);
 	if (ret)
 		return ret;
@@ -3872,6 +3891,9 @@ static long btrfs_ioctl_qgroup_create(struct file *file, void __user *arg)
 	if (!capable(CAP_SYS_ADMIN))
 		return -EPERM;
 
+	if (!qgroup_enabled(root->fs_info))
+		return -ENOTCONN;
+
 	ret = mnt_want_write_file(file);
 	if (ret)
 		return ret;
@@ -3928,6 +3950,9 @@ static long btrfs_ioctl_qgroup_limit(struct file *file, void __user *arg)
 	if (!capable(CAP_SYS_ADMIN))
 		return -EPERM;
 
+	if (!qgroup_enabled(root->fs_info))
+		return -ENOTCONN;
+
 	ret = mnt_want_write_file(file);
 	if (ret)
 		return ret;
@@ -3973,6 +3998,9 @@ static long btrfs_ioctl_quota_rescan(struct file *file, void __user *arg)
 	if (!capable(CAP_SYS_ADMIN))
 		return -EPERM;
 
+	if (!qgroup_enabled(fs_info))
+		return -ENOTCONN;
+
 	ret = mnt_want_write_file(file);
 	if (ret)
 		return ret;