linux-kernel - Re: [PATCH] mm/compaction: correct to flush migrated pages if pageblock skip happens

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Date:	Mon, 24 Aug 2015 10:51:29 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Minchan Kim <minchan@...nel.org>
Subject: Re: [PATCH] mm/compaction: correct to flush migrated pages if
 pageblock skip happens

On 08/21/2015 05:56 AM, Joonsoo Kim wrote:
> We cache isolate_start_pfn before entering isolate_migratepages().
> If pageblock is skipped in isolate_migratepages() due to whatever reason,
> cc->migrate_pfn could be far from isolate_start_pfn hence flushing pages
> that were freed happens. For example, following scenario can be possible.
>
> - assume order-9 compaction, pageblock order is 9
> - start_isolate_pfn is 0x200
> - isolate_migratepages()
>    - skip a number of pageblocks
>    - start to isolate from pfn 0x600
>    - cc->migrate_pfn = 0x620
>    - return
> - last_migrated_pfn is set to 0x200
> - check flushing condition
>    - current_block_start is set to 0x600
>    - last_migrated_pfn < current_block_start then do useless flush
>
> This wrong flush would not help the performance and success rate so
> this patch try to fix it. One simple way to know exact position
> where we start to isolate migratable pages is that we cache it
> in isolate_migratepages() before entering actual isolation. This patch
> implements it and fix the problem.

Yeah, that should work.

> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>

Acked-by: Vlastimil Babka <vbabka@...e.cz>


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