linux-kernel - Re: [PATCH 1/3] mm: page_counter: remove unneeded atomic ops for low/min

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Message-ID: <YwShA6npuK0ZsjPI@dhcp22.suse.cz>
Date:   Tue, 23 Aug 2022 11:42:27 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Muchun Song <songmuchun@...edance.com>,
        Michal Koutný <mkoutny@...e.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Feng Tang <feng.tang@...el.com>,
        Oliver Sang <oliver.sang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>, lkp@...ts.01.org,
        Cgroups <cgroups@...r.kernel.org>, Linux MM <linux-mm@...ck.org>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] mm: page_counter: remove unneeded atomic ops for
 low/min

On Mon 22-08-22 17:20:02, Michal Hocko wrote:
> On Mon 22-08-22 07:55:58, Shakeel Butt wrote:
> > On Mon, Aug 22, 2022 at 3:18 AM Michal Hocko <mhocko@...e.com> wrote:
[...]
> > > Unless I have missed anything this shouldn't break the correctness but I
> > > still have to think about the proportional distribution of the
> > > protection because that adds to the complexity here.
> > 
> > The patch is not changing any semantics. It is just removing an
> > unnecessary atomic xchg() for a specific scenario (min > 0 && min <
> > usage). I don't think there will be any change related to proportional
> > distribution of the protection.
> 
> Yes, I suspect you are right. I just remembered previous fixes
> like 503970e42325 ("mm: memcontrol: fix memory.low proportional
> distribution") which just made me nervous that this is a tricky area.
> 
> I will have another look tomorrow with a fresh brain and send an ack.

I cannot spot any problem. But I guess it would be good to have a little
comment to explain that races on the min_usage update (mentioned by Roman)
are acceptable and savings from atomic update are preferred.

The worst case I can imagine would be something like uncharge 4kB racing
with charge 2MB. The first reduces the protection (min_usage) while the other one
misses that update and doesn't increase it. But even then the effect
shouldn't be really large. At least I have hard time imagine this would
throw things off too much.
-- 
Michal Hocko
SUSE Labs