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

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Date:   Tue, 18 May 2021 15:05:54 +0100
From:   Aaron Tomlin <atomlin@...hat.com>
To:     Michal Hocko <mhocko@...e.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

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.

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).

-- 
Aaron Tomlin

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