linux-kernel - Re: [PATCH] mm/page_alloc: try oom if reclaim is unable to make forward progress

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Date:   Wed, 19 May 2021 13:10:33 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Aaron Tomlin <atomlin@...hat.com>
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org, Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH] mm/page_alloc: try oom if reclaim is unable to make
 forward progress

On Tue 18-05-21 15:05:54, Aaron Tomlin wrote:
> Michal,
> 
> On Fri 2021-03-26 16:36 +0100, Michal Hocko wrote:
> > OK, I kinda expected this would be not easily reproducible.
> 
> Unfortunately, I'm still waiting for feedback on this.
> 
> > We should be focusing on the compaction retry logic and see whether we
> > can have some "run away" scenarios there. Seeing so many retries without
> > compaction bailing out sounds like a bug in that retry logic.
> 
> I suspect so.
> 
> This is indeed a case of excessive reclaim/compaction retries (i.e. the
> last known value stored in the no_progress_loops variable was 31,611,688).
> 
> What might be particularly unique about this situation is that a fatal
> signal was found pending. In this context, if I understand correctly, it
> does not make sense to retry compaction when the last known compact result
> was skipped and a fatal signal is pending.

OK, this might be an interesting lead.

> Looking at try_to_compact_pages(), indeed COMPACT_SKIPPED can be returned;
> albeit, not every zone, on the zone list, would be considered in the case
> a fatal signal is found to be pending. Yet, in should_compact_retry(),
> given the last known compaction result, each zone, on the zone list, can be
> considered/or checked (see compaction_zonelist_suitable()). If a zone e.g.
> was found to succeed then reclaim/compaction would be tried again
> (notwithstanding the above).

I believe Vlastimil would be much better fit into looking into those
details but it smells like pending fatal signals can lead to a unbound
retry indeed.

Thanks!

-- 
Michal Hocko
SUSE Labs