linux-kernel - Re: [PATCH] mm: memcontrol: fix memsw uncharge for root_mem

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Date:   Wed, 24 Mar 2021 09:33:14 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     Roman Gushchin <guro@...com>, Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: [PATCH] mm: memcontrol: fix memsw uncharge for root_mem_cgroup

On Wed 24-03-21 12:11:35, Muchun Song wrote:
> On Tue, Mar 23, 2021 at 11:04 PM Muchun Song <songmuchun@...edance.com> wrote:
> >
> > The pages aren't accounted at the root level, so we cannot uncharge the
> > page to the memsw counter for the root memcg. Fix this.
> >
> > Fixes: 1f47b61fb407 ("mm: memcontrol: fix swap counter leak on swapout from offline cgroup")
> > Signed-off-by: Muchun Song <songmuchun@...edance.com>
> 
> I am very sorry. I should repent. I suddenly realise the fix is totally
> wrong. Because the @memcg cannot be root memcg when
> @memcg != @swap_memcg.

I am probably blind but I do not see why this would be the case.
We have memcg != swap_memcg in this branch but we do not know the
neither of the two is root_mem_cgroup, no? If we did knot that we
wouldn't have to check for swap_memcg != root_mem_cgroup. Or do I miss
something?
-- 
Michal Hocko
SUSE Labs