linux-kernel - Re: [PATCH 2/2] mm, oom: do not enfore OOM killer for __GFP

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Message-ID: <20161206192544.GB10273@dhcp22.suse.cz>
Date:   Tue, 6 Dec 2016 20:25:45 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        akpm@...ux-foundation.org, hannes@...xchg.org, mgorman@...e.de,
        rientjes@...gle.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm, oom: do not enfore OOM killer for __GFP_NOFAIL
 automatically

On Tue 06-12-16 12:03:02, Vlastimil Babka wrote:
> On 12/06/2016 11:38 AM, Tetsuo Handa wrote:
> >>
> >> So we are somewhere in the middle between pre-mature and pointless
> >> system disruption (GFP_NOFS with a lots of metadata or lowmem request)
> >> where the OOM killer even might not help and potential lockup which is
> >> inevitable with the current design. Dunno about you but I would rather
> >> go with the first option. To be honest I really fail to understand your
> >> line of argumentation. We have this
> >> 	do {
> >> 		cond_resched();
> >> 	} while (!(page = alloc_page(GFP_NOFS)));
> >> vs.
> >> 	page = alloc_page(GFP_NOFS | __GFP_NOFAIL);
> >>
> >> the first one doesn't invoke OOM killer while the later does. This
> >> discrepancy just cannot make any sense... The same is true for
> >>
> >> 	alloc_page(GFP_DMA) vs alloc_page(GFP_DMA|__GFP_NOFAIL)
> >>
> >> Now we can discuss whether it is a _good_ idea to not invoke OOM killer
> >> for those exceptions but whatever we do __GFP_NOFAIL is not a way to
> >> give such a subtle side effect. Or do you disagree even with that?
> > 
> > "[PATCH 1/2] mm: consolidate GFP_NOFAIL checks in the allocator slowpath"
> > silently changes __GFP_NOFAIL vs. __GFP_NORETRY priority.
> 
> I guess that wasn't intended?

I even didn't think about that possibility because it just doesn't make
any sense.

> > Currently, __GFP_NORETRY is stronger than __GFP_NOFAIL; __GFP_NOFAIL
> > allocation requests fail without invoking the OOM killer when both
> > __GFP_NORETRY and __GFP_NOFAIL are given.
> > 
> > With [PATCH 1/2], __GFP_NOFAIL becomes stronger than __GFP_NORETRY;
> > __GFP_NOFAIL allocation requests will loop forever without invoking
> > the OOM killer when both __GFP_NORETRY and __GFP_NOFAIL are given.
> 
> Does such combination of flag make sense? Should we warn about it, or
> even silently remove __GFP_NORETRY in such case?

No this combination doesn't make any sense. I seriously doubt we should
even care about it and simply following the stronger requirement makes
more sense from a semantic point of view.

-- 
Michal Hocko
SUSE Labs