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Date:	Mon, 16 May 2016 11:27:56 +0200
From:	Vlastimil Babka <>
To:	Michal Hocko <>
Cc:, Andrew Morton <>,
	Joonsoo Kim <>,
	Rik van Riel <>,
	David Rientjes <>,
	Mel Gorman <>,
	Johannes Weiner <>,
	Tetsuo Handa <>,,
	Linus Torvalds <>
Subject: Re: [RFC 12/13] mm, compaction: more reliably increase direct
 compaction priority

On 05/16/2016 10:14 AM, Michal Hocko wrote:
> On Mon 16-05-16 09:31:44, Vlastimil Babka wrote:
>> Yeah it should work, my only worry was that this may get subtly wrong (as
>> experience shows us) and due to e.g. slightly different watermark checks
>> and/or a corner-case zone such as ZONE_DMA, should_reclaim_retry() would
>> keep returning true, even if reclaim couldn't/wouldn't help anything. Then
>> compaction would be needlessly kept at ineffective priority.
> watermark check for ZONE_DMA should always fail because it fails even
> when is completely free to the lowmem reserves. I had a subtle bug in
> the original code to check highzone_idx rather than classzone_idx but
> that should the fix has been posted recently:

Sure, but that just adds to the experience of being subtly wrong in this 
area :) But sure we can leave this part alone until proven wrong, I 
don't insist strongly.

>> Also my understanding of the initial compaction priorities is to lower the
>> latency if fragmentation is just light and there's enough memory. Once we
>> start struggling, I don't see much point in not switching to the full
>> compaction priority quickly.
> That is true but why to compact when there are high order pages and they
> are just hidden by the watermark check.

Compaction should skip such zone regardless of priority.

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