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From:   Shakeel Butt <shakeelb@google.com>
Date:   Thu, 31 Mar 2022 21:23:26 -0700
Message-ID: <CALvZod54A5mkqK3uLhy-ibQ0ioYhrARmJiFdh=2DWqcMuUt5cA@mail.gmail.com>
Subject: Re: [PATCH v2] mm/list_lru: Fix possible race in memcg_reparent_list_lru_node()
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Michal Hocko <mhocko@suse.com>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Waiman Long <longman@redhat.com>,
        Linux MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Muchun Song <songmuchun@bytedance.com>
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On Thu, Mar 31, 2022 at 6:11 PM Andrew Morton <akpm@linux-foundation.org> w=
rote:
>
> On Thu, 31 Mar 2022 09:46:52 +0200 Michal Hocko <mhocko@suse.com> wrote:
>
> > On Thu 31-03-22 06:39:56, Shakeel Butt wrote:
> > > On Wed, Mar 30, 2022 at 07:48:45PM -0700, Roman Gushchin wrote:
> > > >
> > > >
> > > [...]
> > > >
> > > >
> > > > But honestly, I=E2=80=99d drop the original optimization together w=
ith
> > > > the fix, if only there is no _real world_ data on the problem and
> > > > the improvement. It seems like it has started as a nice simple
> > > > improvement, but the race makes it complex and probably not worth
> > > > the added complexity and fragility.
> > >
> > > I agree with dropping the original optimization as it is not really
> > > fixing an observed issue which may justify adding some complexity.
> >
> > Completely agreed. The patch as it is proposed is not really acceptable
> > IMHO and I have to say I am worried that this is not the first time we
> > are in a situation when a follow up fixes or unrelated patches are
> > growing in complexity to fit on top of a performance optimizations whic=
h
> > do not refer to any actual numbers.
>
> Yup.  I did this:
>
> From: Andrew Morton <akpm@linux-foundation.org>
> Subject: mm/list_lru.c: revert "mm/list_lru: optimize memcg_reparent_list=
_lru_node()"
>
> 405cc51fc1049c73 ("mm/list_lru: optimize memcg_reparent_list_lru_node()")
> has subtle races which are proving ugly to fix.  Revert the original
> optimization.  If quantitative testing indicates that we have a
> significant problem here then other implementations can be looked at.
>
> Fixes: 405cc51fc1049c73 ("mm/list_lru: optimize memcg_reparent_list_lru_n=
ode()")
> Cc: Waiman Long <longman@redhat.com>
> Cc: Roman Gushchin <roman.gushchin@linux.dev>
> Cc: Muchun Song <songmuchun@bytedance.com>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Shakeel Butt <shakeelb@google.com>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Acked-by: Shakeel Butt <shakeelb@google.com>