linux-kernel - Re: [PATCH v4 01/11] mm: memcontrol: prepare objcg API for non-kmem usage

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Message-ID: <Yo4jJjBksFVeJHxA@cmpxchg.org>
Date:   Wed, 25 May 2022 08:37:58 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     Roman Gushchin <roman.gushchin@...ux.dev>, mhocko@...nel.org,
        shakeelb@...gle.com, cgroups@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, duanxiongchun@...edance.com,
        longman@...hat.com
Subject: Re: [PATCH v4 01/11] mm: memcontrol: prepare objcg API for non-kmem
 usage

On Wed, May 25, 2022 at 03:57:17PM +0800, Muchun Song wrote:
> On Tue, May 24, 2022 at 07:36:24PM -0700, Roman Gushchin wrote:
> > On Tue, May 24, 2022 at 02:05:41PM +0800, Muchun Song wrote:
> > > -	memcg_reparent_list_lrus(memcg, parent);
> > > +	memcg_reparent_list_lrus(memcg, parent_mem_cgroup(memcg));
> > We effectively dropped this:
> > 	if (!parent)
> > 		parent = root_mem_cgroup;
> > Is it safe? (assuming v1 non-hierarchical mode, it's usually when all
> > is getting complicated)

Yes, it's correct. But it's a quiet, incidental cleanup, so I can see
why it's confusing. It might be better to split the dead code removal
into a separate patch - with the following in the changelog ;):

> Since no-hierarchy mode is deprecated after commit bef8620cd8e0
> ("mm: memcg: deprecate the non-hierarchical mode"), so
> parent_mem_cgroup() cannot return a NULL except root memcg,
> however, root memcg will not be offline, so it is safe. Right?