Re: [PATCH v2 02/11] memcg: Reclaim when more than one page needed.

[Kamezawa Hiroyuki]

(2012/08/11 2:28), Michal Hocko wrote:
> On Sat 11-08-12 01:49:25, KAMEZAWA Hiroyuki wrote:
> > (2012/08/11 0:42), Michal Hocko wrote:
> > > On Thu 09-08-12 17:01:10, Glauber Costa wrote:
> > > [...]
> > > > @@ -2317,18 +2318,18 @@ static int mem_cgroup_do_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
> > > >   	} else
> > > >   		mem_over_limit = mem_cgroup_from_res_counter(fail_res, res);
> > > >   	/*
> > > > -	 * nr_pages can be either a huge page (HPAGE_PMD_NR), a batch
> > > > -	 * of regular pages (CHARGE_BATCH), or a single regular page (1).
> > > > -	 *
> > > >   	 * Never reclaim on behalf of optional batching, retry with a
> > > >   	 * single page instead.
> > > >   	 */
> > > > -	if (nr_pages == CHARGE_BATCH)
> > > > +	if (nr_pages > min_pages)
> > > >   		return CHARGE_RETRY;
> > >
> > > This is dangerous because THP charges will be retried now while they
> > > previously failed with CHARGE_NOMEM which means that we will keep
> > > attempting potentially endlessly.
> >
> > with THP, I thought nr_pages == min_pages, and no retry.
>
> right you are.
>
> > > Why cannot we simply do if (nr_pages < CHARGE_BATCH) and get rid of the
> > > min_pages altogether?
> >
> > Hm, I think a slab can be larger than CHARGE_BATCH.
> >
> > > Also the comment doesn't seem to be valid anymore.
> > I agree it's not clean. Because our assumption on nr_pages are changed,
> > I think this behavior should not depend on nr_pages value..
> > Shouldn't we have a flag to indicate "trial-for-batched charge" ?
>
> dunno, it would require a new parameter anyway (because abusing gfp
> doesn't seem great idea).
ok, agreed.

-Kame


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