linux-kernel - Re: [RFC 12/13] mm, compaction: more reliably increase direct compaction priority

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Date:	Tue, 10 May 2016 14:55:14 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Michal Hocko <mhocko@...nel.org>
Cc:	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Rik van Riel <riel@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Mel Gorman <mgorman@...hsingularity.net>,
	Johannes Weiner <hannes@...xchg.org>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC 12/13] mm, compaction: more reliably increase direct
 compaction priority

On 05/10/2016 09:36 AM, Vlastimil Babka wrote:
>   	/*
> -	 * 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 insufficient priority
> +	 * Compaction backed off due to watermark checks for order-0
> +	 * so the regular reclaim has to try harder and reclaim something
> +	 * Retry only if it looks like reclaim might have a chance.
>   	 */
> -	if (compaction_failed(compact_result)) {
> -		if (*compact_priority > 0) {
> -			(*compact_priority)--;
> -			return true;
> -		}
> -		return false;
> -	}

Oops, looks like my editing resulted in compaction_failed() check to be
removed completely, which wasn't intentional and can lead to infinite
loops. This should be added on top.

----8<----
>From 59a2b38689aa451f661c964dc9bfb990736ad92d Mon Sep 17 00:00:00 2001
From: Vlastimil Babka <vbabka@...e.cz>
Date: Tue, 10 May 2016 14:51:03 +0200
Subject: [PATCH 15/15] fixup! mm, compaction: more reliably increase direct
 compaction priority

---
 mm/page_alloc.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index fa49eb4a5919..e8a0d33cfb67 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3268,6 +3268,14 @@ should_compact_retry(struct alloc_context *ac, int order, int alloc_flags,
 	}
 
 	/*
+	 * Compaction considers all the zones as unfixably fragmented and we
+	 * are on the highest priority, which means it can't be due to
+	 * heuristics and it doesn't really make much sense to retry.
+	 */
+	if (compaction_failed(compact_result))
+		return false;
+
+	/*
 	 * The remaining possibility is that compaction made progress and
 	 * created a high-order page, but it was allocated by somebody else.
 	 * To prevent thrashing, limit the number of retries in such case.
-- 
2.8.2