cgroup: warn about broken hierarchies only after css_online

If everything goes right, it shouldn't really matter if we are spitting
this warning after css_alloc or css_online. If we fail between then,
there are some ill cases where we would previously see the message and
now we won't (like if the files fail to be created).

I believe it really shouldn't matter: this message is intended in spirit
to be shown when creation succeeds, but with insane settings.
Signed-off-by: NGlauber Costa <glommer@parallels.com>
Signed-off-by: NTejun Heo <tj@kernel.org>

kernel/cgroup.c

@@ -4151,15 +4151,6 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
 			if (err)
 				goto err_free_all;
 		}
-
-		if (ss->broken_hierarchy && !ss->warned_broken_hierarchy &&
-		    parent->parent) {
-			pr_warning("cgroup: %s (%d) created nested cgroup for controller \"%s\" which has incomplete hierarchy support. Nested cgroups may change behavior in the future.\n",
-				   current->comm, current->pid, ss->name);
-			if (!strcmp(ss->name, "memory"))
-				pr_warning("cgroup: \"memory\" requires setting use_hierarchy to 1 on the root.\n");
-			ss->warned_broken_hierarchy = true;
-		}
 	}
 
 	/*
@@ -4188,6 +4179,15 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
 		err = online_css(ss, cgrp);
 		if (err)
 			goto err_destroy;
+
+		if (ss->broken_hierarchy && !ss->warned_broken_hierarchy &&
+		    parent->parent) {
+			pr_warning("cgroup: %s (%d) created nested cgroup for controller \"%s\" which has incomplete hierarchy support. Nested cgroups may change behavior in the future.\n",
+				   current->comm, current->pid, ss->name);
+			if (!strcmp(ss->name, "memory"))
+				pr_warning("cgroup: \"memory\" requires setting use_hierarchy to 1 on the root.\n");
+			ss->warned_broken_hierarchy = true;
+		}
 	}
 
 	err = cgroup_populate_dir(cgrp, true, root->subsys_mask);