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Subject: Re: [PATCH v4] mm/page_alloc: bail out on fatal signal during
 reclaim/compaction retry attempt
To:     Aaron Tomlin <atomlin@redhat.com>, linux-mm@kvack.org
Cc:     akpm@linux-foundation.org, mhocko@suse.com, willy@infradead.org,
        linux-kernel@vger.kernel.org
References: <YKZObDpduqwWi/Zm@casper.infradead.org>
 <20210520142901.3371299-1-atomlin@redhat.com>
From:   Vlastimil Babka <vbabka@suse.cz>
Message-ID: <c94f4323-1f63-e9f0-6dfd-402cbbc39a87@suse.cz>
Date:   Fri, 28 May 2021 14:53:38 +0200
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On 5/20/21 4:29 PM, Aaron Tomlin wrote:
> A customer experienced a low-memory situation and decided to issue a
> SIGKILL (i.e. a fatal signal). Instead of promptly terminating as one
> would expect, the aforementioned task remained unresponsive.
> 
> Further investigation indicated that the task was "stuck" in the
> reclaim/compaction retry loop. Now, it does not make sense to retry
> compaction when a fatal signal is pending.
> 
> In the context of 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()). For example, if a zone was found
> to succeed, then reclaim/compaction would be tried again
> (notwithstanding the above).
> 
> This patch ensures that compaction is not needlessly retried
> irrespective of the last known compaction result e.g. if it was skipped,
> in the unlikely case a fatal signal is found pending.
> So, OOM is at least attempted.
> 
> Signed-off-by: Aaron Tomlin <atomlin@redhat.com>

Reviewed-by: Vlastimil Babka <vbabka@suse.cz>

> ---
>  mm/page_alloc.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index aaa1655cf682..b317057ac186 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4252,6 +4252,9 @@ should_compact_retry(struct alloc_context *ac, int order, int alloc_flags,
>  	if (!order)
>  		return false;
>  
> +	if (fatal_signal_pending(current))
> +		return false;
> +
>  	if (compaction_made_progress(compact_result))
>  		(*compaction_retries)++;
>  
>