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

Andrew Morton
2018-07-31T23:39:13Z

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);