Re: [Resend PATCH 1/6] mm/memcg: warning on !memcg after readahead page charged

Michal Hocko
2020-08-25T07:25:38Z

On Tue 25-08-20 09:25:01, Alex Shi wrote:
> reproduce using our linux-mm random bug collection on NUMA systems.
> >>
> >> OK, I must have missed that this was on ppc. The order makes more sense
> >> now. I will have a look at this next week.
> >
> > OK, so I've had a look and I know what's going on there. The
> > move_pages12 is migrating hugetlb pages. Those are not charged to any
> > memcg. We have completely missed this case. There are two ways going
> > around that. Drop the warning and update the comment so that we do not
> > forget about that or special case hugetlb pages.
> >
> > I think the first option is better.
> >
>
>
> Hi Michal,
>
> Compare to ignore the warning which is designed to give, seems addressing
> the hugetlb out of charge issue is a better solution, otherwise the memcg
> memory usage is out of control on hugetlb, is that right?

Hugetlb memory is out of memcg scope deliberately. This is not a
reclaimable memory and something that can easily get out of control. The
memory is preallocated and overcommit is strictly controlled as well. We
have a dedicated hugetlb cgroup controller to offer a better control of
the preallocated pool distribution.

Anyway this just shows that there are more subtle cases where a page
with no memcg can hit some common paths so the patch is clearly not
ready.

I should have realized that when giving my ack but same as you I got
misled by the existing comment.
--
Michal Hocko
SUSE Labs