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

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Date:	Tue, 19 Jul 2016 08:54:55 +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 07/19/2016 08:44 AM, Joonsoo Kim wrote:
> On Mon, Jul 18, 2016 at 11:12:51AM +0200, Vlastimil Babka wrote:
>> 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...
>
> whole_zone would come along with ignore_skip_hint so I think that
> there is no problem on cached position updating.

Right, that's true.

>>
>>> 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?
>
> My point is that we should not initialize zone's cached pfn in case of
> the whole_zone because what compaction with COMPACT_PRIO_SYNC_FULL
> want is just to scan whole range. zone's cached pfn exists for
> efficiency and there is no reason to initialize it by compaction with
> COMPACT_PRIO_SYNC_FULL. If there are some parallel compaction users,
> they could be benefit from un-initialized zone's cached pfn so I'd
> like to leave them.

I doubt they will benefit much, but OK, I'll update the patch.

> Thanks.
>