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

[Kamezawa Hiroyuki]

(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.


> 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" ?


Thanks,
-Kame




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