linux-kernel - Re: [PATCH] mm: __rmqueue_fallback() should respect pageblock type

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Message-ID: <5271E3F8.80108@gmail.com>
Date:	Thu, 31 Oct 2013 01:00:40 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	kosaki.motohiro@...il.com, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH] mm: __rmqueue_fallback() should respect pageblock type

(10/31/13 12:35 AM), Andrew Morton wrote:
> On Thu, 31 Oct 2013 00:24:49 -0400 kosaki.motohiro@...il.com wrote:
>
>> When __rmqueue_fallback() don't find out a free block with the same size
>> of required, it splits a larger page and puts back rest peiece of the page
>> to free list.
>>
>> But it has one serious mistake. When putting back, __rmqueue_fallback()
>> always use start_migratetype if type is not CMA. However, __rmqueue_fallback()
>> is only called when all of start_migratetype queue are empty. That said,
>> __rmqueue_fallback always put back memory to wrong queue except
>> try_to_steal_freepages() changed pageblock type (i.e. requested size is
>> smaller than half of page block). Finally, antifragmentation framework
>> increase fragmenation instead of decrease.
>>
>> Mel's original anti fragmentation do the right thing. But commit 47118af076
>> (mm: mmzone: MIGRATE_CMA migration type added) broke it.
>>
>> This patch restores sane and old behavior.
>
> What are the user-visible runtime effects of this change?

Memory fragmentation may increase compaction rate. (And then system get unnecessary
slow down)


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