Re: [PATCH] mm,vmscan: fix divide by zero in get_scan_count

Michal Hocko
2021-08-31T09:59:13Z

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