linux-kernel - Re: [PATCH v3 09/17] mm, compaction: make whole_zone flag ignore cached scanner positions

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Date:	Mon, 18 Jul 2016 11:12:51 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Michal Hocko <mhocko@...nel.org>,
	Mel Gorman <mgorman@...hsingularity.net>,
	David Rientjes <rientjes@...gle.com>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH v3 09/17] mm, compaction: make whole_zone flag ignore
 cached scanner positions

On 07/06/2016 07:09 AM, Joonsoo Kim wrote:
> On Fri, Jun 24, 2016 at 11:54:29AM +0200, Vlastimil Babka wrote:
>> A recent patch has added whole_zone flag that compaction sets when scanning
>> starts from the zone boundary, in order to report that zone has been fully
>> scanned in one attempt. For allocations that want to try really hard or cannot
>> fail, we will want to introduce a mode where scanning whole zone is guaranteed
>> regardless of the cached positions.
>>
>> This patch reuses the whole_zone flag in a way that if it's already passed true
>> to compaction, the cached scanner positions are ignored. Employing this flag
>
> Okay. But, please don't reset cached scanner position even if whole_zone
> flag is set. Just set cc->migrate_pfn and free_pfn, appropriately. With

Won't that result in confusion on cached position updates during 
compaction where it checks the previous cached position? I wonder what 
kinds of corner cases it can bring...

> your following patches, whole_zone could be set without any compaction
> try

I don't understand what you mean here? Even after whole series, 
whole_zone is only checked, and positions thus reset, after passing the 
compaction_suitable() call from compact_zone(). So at that point we can 
say that compaction is being actually tried and it's not a drive-by reset?

Thanks