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Date:   Tue, 31 Aug 2021 11:59:05 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Rik van Riel <riel@surriel.com>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, kernel-team@fb.com,
        stable@kernel.org, Chris Down <chris@chrisdown.name>
Subject: Re: [PATCH] mm,vmscan: fix divide by zero in get_scan_count
Message-ID: <YS39aci6yhjIplLx@dhcp22.suse.cz>
References: <20210826220149.058089c6@imladris.surriel.com>
 <YS1EA3U4XXH7X0qz@cmpxchg.org>
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On Mon 30-08-21 16:48:03, Johannes Weiner wrote:
> On Thu, Aug 26, 2021 at 10:01:49PM -0400, Rik van Riel wrote:
[...]
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index eeae2f6bc532..f1782b816c98 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -2592,7 +2592,7 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
> >  			cgroup_size = max(cgroup_size, protection);
> >  
> >  			scan = lruvec_size - lruvec_size * protection /
> > -				cgroup_size;
> > +				(cgroup_size + 1);
> 
> I have no overly strong preferences, but if Michal prefers max(), how about:
> 
> 	cgroup_size = max3(cgroup_size, protection, 1);

Yes this is better.

> Or go back to not taking the branch in the first place when there is
> no protection in effect...
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 6247f6f4469a..9c200bb3ae51 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2547,7 +2547,7 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
>  		mem_cgroup_protection(sc->target_mem_cgroup, memcg,
>  				      &min, &low);
>  
> -		if (min || low) {
> +		if (min || (!sc->memcg_low_reclaim && low)) {
>  			/*
>  			 * Scale a cgroup's reclaim pressure by proportioning
>  			 * its current usage to its memory.low or memory.min

This is slightly more complex to read but it is also better than +1
trick.

Either of the two work for me.

-- 
Michal Hocko
SUSE Labs