linux-kernel - Re: [RFC PATCH v3 1/3] mm/cma: change fallback behaviour for CMA freepage

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Date:	Mon, 2 Feb 2015 16:29:07 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Vlastimil Babka <vbabka@...e.cz>
Cc:	Mel Gorman <mgorman@...e.de>, David Rientjes <rientjes@...gle.com>,
	Rik van Riel <riel@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Zhang Yanfei <zhangyanfei@...fujitsu.com>
Subject: Re: [RFC PATCH v3 1/3] mm/cma: change fallback behaviour for CMA
 freepage

On Mon, Feb 02, 2015 at 04:15:46PM +0900, Joonsoo Kim wrote:
> freepage with MIGRATE_CMA can be used only for MIGRATE_MOVABLE and
> they should not be expanded to other migratetype buddy list
> to protect them from unmovable/reclaimable allocation. Implementing
> these requirements in __rmqueue_fallback(), that is, finding largest
> possible block of freepage has bad effect that high order freepage
> with MIGRATE_CMA are broken continually although there are suitable
> order CMA freepage. Reason is that they are not be expanded to other
> migratetype buddy list and next __rmqueue_fallback() invocation try to
> finds another largest block of freepage and break it again. So,
> MIGRATE_CMA fallback should be handled separately. This patch
> introduces __rmqueue_cma_fallback(), that just wrapper of
> __rmqueue_smallest() and call it before __rmqueue_fallback()
> if migratetype == MIGRATE_MOVABLE.
> 
> This results in unintended behaviour change that MIGRATE_CMA freepage
> is always used first rather than other migratetype as movable
> allocation's fallback. But, as already mentioned above,
> MIGRATE_CMA can be used only for MIGRATE_MOVABLE, so it is better
> to use MIGRATE_CMA freepage first as much as possible. Otherwise,
> we needlessly take up precious freepages with other migratetype and
> increase chance of fragmentation.
> 
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>
> ---

Hello, Vlastimil.

This RFC is targeted to you, but, I mistakenly omit your e-mail
on CC list. Sorry about that. :/

How about this v3 which try to clean-up __rmqueue_fallback() much more?

Thanks.
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