linux-kernel - Re: [PATCH 08/10] mm, page_alloc: Remove MIGRATE

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Date:	Wed, 29 Jul 2015 11:59:15 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Mel Gorman <mgorman@...e.com>, Linux-MM <linux-mm@...ck.org>
Cc:	Johannes Weiner <hannes@...xchg.org>,
	Rik van Riel <riel@...hat.com>,
	Pintu Kumar <pintu.k@...sung.com>,
	Xishi Qiu <qiuxishi@...wei.com>, Gioh Kim <gioh.kim@....com>,
	LKML <linux-kernel@...r.kernel.org>,
	Mel Gorman <mgorman@...hsingularity.net>
Subject: Re: [PATCH 08/10] mm, page_alloc: Remove MIGRATE_RESERVE

On 07/20/2015 10:00 AM, Mel Gorman wrote:
> From: Mel Gorman <mgorman@...e.de>
> 
> MIGRATE_RESERVE preserves an old property of the buddy allocator that existed
> prior to fragmentation avoidance -- min_free_kbytes worth of pages tended to
> remain free until the only alternative was to fail the allocation. At the

          ^ I think you meant contiguous instead of free? Is it because
splitting chooses lowest possible order, and grouping by mobility means you
might be splitting e.g. order-5 movable page instead of using order-0 unmovable
page? And that the fallback heuristics specifically select highest available
order? I think it's not that obvious, so worth mentioning.

> time it was discovered that high-order atomic allocations relied on this
> property so MIGRATE_RESERVE was introduced. A later patch will introduce
> an alternative MIGRATE_HIGHATOMIC so this patch deletes MIGRATE_RESERVE
> and supporting code so it'll be easier to review. Note that this patch
> in isolation may look like a false regression if someone was bisecting
> high-order atomic allocation failures.
> 
> Signed-off-by: Mel Gorman <mgorman@...e.de>

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

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