linux-kernel - Re: [PATCH] mm: add the block to the tail of the list in expand()

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Date:	Fri, 31 Jul 2015 16:24:02 -0700
From:	Dave Hansen <dave.hansen@...el.com>
To:	Xishi Qiu <qiuxishi@...wei.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Vlastimil Babka <vbabka@...e.cz>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>, iamjoonsoo.kim@....com,
	alexander.h.duyck@...hat.com, sasha.levin@...cle.com
CC:	Linux MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: add the block to the tail of the list in expand()

On 07/31/2015 02:30 AM, Xishi Qiu wrote:
> __free_one_page() will judge whether the the next-highest order is free,
> then add the block to the tail or not. So when we split large order block, 
> add the small block to the tail, it will reduce fragment.

It's an interesting idea, but what does it do in practice?  Can you
measure a decrease in fragmentation?

Further, the comment above the function says:
 * The order of subdivision here is critical for the IO subsystem.
 * Please do not alter this order without good reasons and regression
 * testing.

Has there been regression testing?

Also, this might not do very much good in practice.  If you are
splitting a high-order page, you are doing the split because the
lower-order lists are empty.  So won't that list_add() be to an empty
list most of the time?  Or does the __rmqueue_fallback()
largest->smallest logic dominate?
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