linux-kernel - Re: [PATCH 12/14] mm, oom: protect !costly allocations some more

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Message-ID: <20160504182049.GC21490@dhcp22.suse.cz>
Date:	Wed, 4 May 2016 20:20:49 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Joonsoo Kim <js1304@...il.com>
Cc:	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Mel Gorman <mgorman@...e.de>,
	David Rientjes <rientjes@...gle.com>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	Hillf Danton <hillf.zj@...baba-inc.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Linux Memory Management List <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 12/14] mm, oom: protect !costly allocations some more

On Wed 04-05-16 23:39:14, Joonsoo Kim wrote:
> 2016-05-04 17:53 GMT+09:00 Michal Hocko <mhocko@...nel.org>:
> > On Wed 04-05-16 15:01:24, Joonsoo Kim wrote:
> >> On Wed, Apr 20, 2016 at 03:47:25PM -0400, Michal Hocko wrote:
> > [...]
> >
> > Please try to trim your responses it makes it much easier to follow the
> > discussion
> 
> Okay.
> 
> >> > +static inline bool
> >> > +should_compact_retry(unsigned int order, enum compact_result compact_result,
> >> > +                enum migrate_mode *migrate_mode,
> >> > +                int compaction_retries)
> >> > +{
> >> > +   if (!order)
> >> > +           return false;
> >> > +
> >> > +   /*
> >> > +    * compaction considers all the zone as desperately out of memory
> >> > +    * so it doesn't really make much sense to retry except when the
> >> > +    * failure could be caused by weak migration mode.
> >> > +    */
> >> > +   if (compaction_failed(compact_result)) {
> >>
> >> IIUC, this compaction_failed() means that at least one zone is
> >> compacted and failed. This is not same with your assumption in the
> >> comment. If compaction is done and failed on ZONE_DMA, it would be
> >> premature decision.
> >
> > Not really, because if other zones are making some progress then their
> > result will override COMPACT_COMPLETE
> 
> Think about the situation that DMA zone fails to compact and
> the other zones are deferred or skipped. In this case, COMPACT_COMPLETE
> will be returned as a final result and should_compact_retry() return false.
> I don't think that it means all the zones are desperately out of memory.

But that would mean that the ZONE_DMA would be eligible for compaction,
no? And considering the watermark check this zone should COMPACT_SKIP
for most allocation request. Or am I missing something?
-- 
Michal Hocko
SUSE Labs