btrfs: sysfs: expose quota mode via sysfs

Add a new sysfs file /sys/fs/btrfs/<uuid>/qgroups/mode
which prints out the mode qgroups is running in. The possible modes are
qgroup, and squota.

If quotas are not enabled, then the qgroups directory will not exist,
so don't handle that mode.

Signed-off-by: Boris Burkov <boris@bur.io>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/sysfs.c

@@ -2089,6 +2089,33 @@ static ssize_t qgroup_enabled_show(struct kobject *qgroups_kobj,
 }
 BTRFS_ATTR(qgroups, enabled, qgroup_enabled_show);
 
+static ssize_t qgroup_mode_show(struct kobject *qgroups_kobj,
+				struct kobj_attribute *a,
+				char *buf)
+{
+	struct btrfs_fs_info *fs_info = to_fs_info(qgroups_kobj->parent);
+	ssize_t ret = 0;
+
+	spin_lock(&fs_info->qgroup_lock);
+	ASSERT(btrfs_qgroup_enabled(fs_info));
+	switch (btrfs_qgroup_mode(fs_info)) {
+	case BTRFS_QGROUP_MODE_FULL:
+		ret = sysfs_emit(buf, "qgroup\n");
+		break;
+	case BTRFS_QGROUP_MODE_SIMPLE:
+		ret = sysfs_emit(buf, "squota\n");
+		break;
+	default:
+		btrfs_warn(fs_info, "unexpected qgroup mode %d\n",
+			   btrfs_qgroup_mode(fs_info));
+		break;
+	}
+	spin_unlock(&fs_info->qgroup_lock);
+
+	return ret;
+}
+BTRFS_ATTR(qgroups, mode, qgroup_mode_show);
+
 static ssize_t qgroup_inconsistent_show(struct kobject *qgroups_kobj,
 					struct kobj_attribute *a,
 					char *buf)
@@ -2151,6 +2178,7 @@ static struct attribute *qgroups_attrs[] = {
 	BTRFS_ATTR_PTR(qgroups, enabled),
 	BTRFS_ATTR_PTR(qgroups, inconsistent),
 	BTRFS_ATTR_PTR(qgroups, drop_subtree_threshold),
+	BTRFS_ATTR_PTR(qgroups, mode),
 	NULL
 };
 ATTRIBUTE_GROUPS(qgroups);