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Date:   Tue, 31 Jul 2018 16:39:08 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Vladimir Davydov <vdavydov.dev@gmail.com>,
        Michal Hocko <mhocko@kernel.org>,
        Kirill Tkhai <ktkhai@virtuozzo.com>, cgroups@vger.kernel.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] memcg: Remove memcg_cgroup::id from IDR on
 mem_cgroup_css_alloc() failure
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On Mon, 30 Jul 2018 11:31:13 -0400 Johannes Weiner <hannes@cmpxchg.org> wrote:

> 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);