Re: [PATCH] memcg: Remove memcg_cgroup::id from IDR on mem_cgroup_css_alloc() failure

On Mon, 30 Jul 2018 11:31:13 -0400 Johannes Weiner [off-list ref] wrote:

quoted

Subject: [PATCH] mm: memcontrol: simplify memcg idr allocation and error
 unwinding

The memcg ID is allocated early in the multi-step memcg creation
process, which needs 2-step ID allocation and IDR publishing, as well
as two separate IDR cleanup/unwind sites on error.

Defer the IDR allocation until the last second during onlining to
eliminate all this complexity. There is no requirement to have the ID
and IDR entry earlier than that. And the root reference to the ID is
put in the offline path, so this matches nicely.

This patch isn't aware of Kirill's later "mm, memcg: assign memcg-aware
shrinkers bitmap to memcg", which altered mem_cgroup_css_online():

@@ -4356,6 +4470,11 @@ static int mem_cgroup_css_online(struct
 {
 	struct mem_cgroup *memcg = mem_cgroup_from_css(css);
 
+	if (memcg_alloc_shrinker_maps(memcg)) {
+		mem_cgroup_id_remove(memcg);
+		return -ENOMEM;
+	}
+
 	/* Online state pins memcg ID, memcg ID pins CSS */
 	atomic_set(&memcg->id.ref, 1);
 	css_get(css);