Re: [PATCH v4 07/25] memcg: Reclaim when more than one page needed.

[Glauber Costa]

On 06/20/2012 05:47 PM, Michal Hocko wrote:
> On Mon 18-06-12 14:28:00, Glauber Costa wrote:
> > From: Suleiman Souhlal <ssouhlal@xxxxxxxxxxx>
> >
> > mem_cgroup_do_charge() was written before slab accounting, and expects
> > three cases: being called for 1 page, being called for a stock of 32 pages,
> > or being called for a hugepage.  If we call for 2 or 3 pages (and several
> > slabs used in process creation are such, at least with the debug options I
> > had), it assumed it's being called for stock and just retried without reclaiming.
> >
> > Fix that by passing down a minsize argument in addition to the csize.
> >
> > And what to do about that (csize == PAGE_SIZE && ret) retry?  If it's
> > needed at all (and presumably is since it's there, perhaps to handle
> > races), then it should be extended to more than PAGE_SIZE, yet how far?
> > And should there be a retry count limit, of what?  For now retry up to
> > COSTLY_ORDER (as page_alloc.c does), stay safe with a cond_resched(),
> > and make sure not to do it if __GFP_NORETRY.
> >
> > [v4: fixed nr pages calculation pointed out by Christoph Lameter ]
> >
> > Signed-off-by: Suleiman Souhlal <suleiman@xxxxxxxxxx>
> > Signed-off-by: Glauber Costa <glommer@xxxxxxxxxxxxx>
> > Reviewed-by: Kamezawa Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
>
> I think this is not ready to be merged yet.
Fair Enough

> Two comments below.
>
> [...]
> > @@ -2210,18 +2211,18 @@ 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;
> >
> >   	if (!(gfp_mask & __GFP_WAIT))
> >   		return CHARGE_WOULDBLOCK;
> >
> > +	if (gfp_mask & __GFP_NORETRY)
> > +		return CHARGE_NOMEM;
>
> This is kmem specific and should be preparated out in case this should
> be merged before the rest.
ok.

> Btw. I assume that oom==false when called from kmem...

What prevents the oom killer to be called for a reclaimable kmem 
allocation that can be satisfied ?

> > +
> >   	ret = mem_cgroup_reclaim(mem_over_limit, gfp_mask, flags);
> >   	if (mem_cgroup_margin(mem_over_limit) >= nr_pages)
> >   		return CHARGE_RETRY;
> > @@ -2234,8 +2235,10 @@ static int mem_cgroup_do_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
> >   	 * unlikely to succeed so close to the limit, and we fall back
> >   	 * to regular pages anyway in case of failure.
> >   	 */
> > -	if (nr_pages == 1 && ret)
> > +	if (nr_pages <= (1 << PAGE_ALLOC_COSTLY_ORDER) && ret) {
> > +		cond_resched();
> >   		return CHARGE_RETRY;
> > +	}
>
> What prevents us from looping for unbounded amount of time here?
> Maybe you need to consider the number of reclaimed pages here.

Why would we even loop here? It will just return CHARGE_RETRY, it is up 
to the caller to decide whether or not it will retry.
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