Re: [PATCH 2/2] mm, oom: do not enfore OOM killer for __GFP_NOFAIL automatically

[Michal Hocko <mhocko@kernel.org>]

On Fri 16-12-16 12:31:51, Johannes Weiner wrote:
> On Fri, Dec 16, 2016 at 04:58:08PM +0100, Michal Hocko wrote:
> > @@ -1013,7 +1013,7 @@ bool out_of_memory(struct oom_control *oc)
> >  	 * make sure exclude 0 mask - all other users should have at least
> >  	 * ___GFP_DIRECT_RECLAIM to get here.
> >  	 */
> > -	if (oc->gfp_mask && !(oc->gfp_mask & (__GFP_FS|__GFP_NOFAIL)))
> > +	if (oc->gfp_mask && !(oc->gfp_mask & __GFP_FS))
> >  		return true;
> 
> This makes sense, we should go back to what we had here. Because it's
> not that the reported OOMs are premature - there is genuinely no more
> memory reclaimable from the allocating context - but that this class
> of allocations should never invoke the OOM killer in the first place.

agreed, at least not with the current implementtion. If we had a proper
accounting where we know that the memory pinned by the fs is not really
there then we could invoke the oom killer and be safe

> > @@ -3737,6 +3752,16 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> >  		 */
> >  		WARN_ON_ONCE(order > PAGE_ALLOC_COSTLY_ORDER);
> >  
> > +		/*
> > +		 * Help non-failing allocations by giving them access to memory
> > +		 * reserves but do not use ALLOC_NO_WATERMARKS because this
> > +		 * could deplete whole memory reserves which would just make
> > +		 * the situation worse
> > +		 */
> > +		page = __alloc_pages_cpuset_fallback(gfp_mask, order, ALLOC_HARDER, ac);
> > +		if (page)
> > +			goto got_pg;
> > +
> 
> But this should be a separate patch, IMO.
> 
> Do we observe GFP_NOFS lockups when we don't do this? 

this is hard to tell but considering users like grow_dev_page we can get
stuck with a very slow progress I believe. Those allocations could see
some help.

> Don't we risk
> premature exhaustion of the memory reserves, and it's better to wait
> for other reclaimers to make some progress instead?

waiting for other reclaimers would be preferable but we should at least
give these some priority, which is what ALLOC_HARDER should help with.

> Should we give
> reserve access to all GFP_NOFS allocations, or just the ones from a
> reclaim/cleaning context?

I would focus only for those which are important enough. Which are those
is a harder question. But certainly those with GFP_NOFAIL are important
enough.

> All that should go into the changelog of a separate allocation booster
> patch, I think.

The reason I did both in the same patch is to address the concern about
potential lockups when NOFS|NOFAIL cannot make any progress. I've chosen
ALLOC_HARDER to give the minimum portion of the reserves so that we do
not risk other high priority users to be blocked out but still help a
bit at least and prevent from starvation when other reclaimers are
faster to consume the reclaimed memory.

I can extend the changelog of course but I believe that having both
changes together makes some sense. NOFS|NOFAIL allocations are not all
that rare and sometimes we really depend on them making a further
progress.

-- 
Michal Hocko
SUSE Labs