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

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Date:	Wed, 29 Jul 2015 13:25:02 +0100
From:	Mel Gorman <mgorman@...hsingularity.net>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	Linux-MM <linux-mm@...ck.org>,
	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>
Subject: Re: [PATCH 08/10] mm, page_alloc: Remove MIGRATE_RESERVE

On Wed, Jul 29, 2015 at 11:59:15AM +0200, Vlastimil Babka wrote:
> 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?

That is exactly what I meant.

> 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.
> 

Yes, the commit that introduced MIGRATE_RESERVE discusses it so I didn't
repeat it as the git digging is simply

1. Find the commit that introduced MIGRATE_HIGHATOMIC and see it
   replaced MIGRATE_RESERVE
2. Find the commit that introduced MIGRATE_RESERVE

That locates 56fd56b868f1 ("Bias the location of pages freed for
min_free_kbytes in the same MAX_ORDER_NR_PAGES blocks").

> > 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>
> 

Thanks.

-- 
Mel Gorman
SUSE Labs
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