cgroup: reserve ID 0 for dummy_root and 1 for unified hierarchy

Before 1a574231 ("cgroup: make hierarchy_id use cyclic idr"),
hierarchy IDs were allocated from 0.  As the dummy hierarchy was
always the one first initialized, it got assigned 0 and all other
hierarchies from 1.  The patch accidentally changed the minimum
useable ID to 2.

Let's restore ID 0 for dummy_root and while at it reserve 1 for
unified hierarchy.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Cc: stable@vger.kernel.org

kernel/cgroup.c

@@ -1425,14 +1425,15 @@ static void init_cgroup_root(struct cgroupfs_root *root)
 	init_cgroup_housekeeping(cgrp);
 }
 
-static int cgroup_init_root_id(struct cgroupfs_root *root)
+static int cgroup_init_root_id(struct cgroupfs_root *root, int start, int end)
 {
 	int id;
 
 	lockdep_assert_held(&cgroup_mutex);
 	lockdep_assert_held(&cgroup_root_mutex);
 
-	id = idr_alloc_cyclic(&cgroup_hierarchy_idr, root, 2, 0, GFP_KERNEL);
+	id = idr_alloc_cyclic(&cgroup_hierarchy_idr, root, start, end,
+			      GFP_KERNEL);
 	if (id < 0)
 		return id;
 
@@ -1635,7 +1636,8 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
 		if (ret)
 			goto unlock_drop;
 
-		ret = cgroup_init_root_id(root);
+		/* ID 0 is reserved for dummy root, 1 for unified hierarchy */
+		ret = cgroup_init_root_id(root, 2, 0);
 		if (ret)
 			goto unlock_drop;
 
@@ -4898,7 +4900,7 @@ int __init cgroup_init(void)
 	key = css_set_hash(init_css_set.subsys);
 	hash_add(css_set_table, &init_css_set.hlist, key);
 
-	BUG_ON(cgroup_init_root_id(&cgroup_dummy_root));
+	BUG_ON(cgroup_init_root_id(&cgroup_dummy_root, 0, 1));
 
 	mutex_unlock(&cgroup_root_mutex);
 	mutex_unlock(&cgroup_mutex);