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Date:   Fri, 13 Apr 2018 13:54:54 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Kirill Tkhai <ktkhai@virtuozzo.com>
Cc:     akpm@linux-foundation.org, hannes@cmpxchg.org,
        vdavydov.dev@gmail.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 Fri 13-04-18 14:49:32, Kirill Tkhai wrote:
> On 13.04.2018 14:38, Michal Hocko wrote:
> > On Fri 13-04-18 14:29:11, Kirill Tkhai wrote:
[...]
> >> mem_cgroup_id_put_many() unpins css, but this may be not the last reference to the css.
> >> Thus, we release ID earlier, then all references to css are freed.
> > 
> > Right and so what. If we have released the idr then we are not going to
> > do that again in css_free. That is why we have that memcg->id.id > 0
> > check before idr_remove and memcg->id.id = 0 for the last memcg ref.
> > count. So again, why cannot we do the clean up in mem_cgroup_free and
> > have a less confusing code? Or am I just not getting your point and
> > being dense here?
> 
> We can, but mem_cgroup_free() called from mem_cgroup_css_alloc() is unlikely case.
> The likely case is mem_cgroup_free() is called from mem_cgroup_css_free(), where
> this idr manipulations will be a noop. Noop in likely case looks more confusing
> for me.

Well, I would really prefer to have _free being symmetric to _alloc so
that you can rely that the full state is gone after _free is called.
This confused the hell out of me. Because I _did_ expect that
mem_cgroup_free would do that and so I was looking at completely
different place.
 
> Less confusing will be to move
> 
>         memcg->id.id = idr_alloc(&mem_cgroup_idr, NULL,
>                                  1, MEM_CGROUP_ID_MAX,
>                                  GFP_KERNEL);
> 
> into mem_cgroup_css_alloc(). How are you think about this?

I would have to double check. Maybe it can be done on top. But for the
actual fix and a stable backport potentially should be as clear as
possible. Your original patch would be just fine but if I would prefer 
mem_cgroup_free for the symmetry.
-- 
Michal Hocko
SUSE Labs