Re: [PATCH v6 3/6] mm/cma: populate ZONE_CMA

[Vlastimil Babka]

On 10/26/2016 06:31 AM, Joonsoo Kim wrote:
> Hello,
>
> Here comes fixed one.
>
> ----------->8------------
> From 93fb05a83d74f9e2c8caebc2fa6d1a8807c9ffb6 Mon Sep 17 00:00:00 2001
> From: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> Date: Thu, 24 Mar 2016 22:29:10 +0900
> Subject: [PATCH] mm/cma: populate ZONE_CMA
>
> Until now, reserved pages for CMA are managed in the ordinary zones
> where page's pfn are belong to. This approach has numorous problems
> and fixing them isn't easy. (It is mentioned on previous patch.)
> To fix this situation, ZONE_CMA is introduced in previous patch, but,
> not yet populated. This patch implement population of ZONE_CMA
> by stealing reserved pages from the ordinary zones.
>
> Unlike previous implementation that kernel allocation request with
> __GFP_MOVABLE could be serviced from CMA region, allocation request only
> with GFP_HIGHUSER_MOVABLE can be serviced from CMA region in the new
> approach. This is an inevitable design decision to use the zone
> implementation because ZONE_CMA could contain highmem. Due to this
> decision, ZONE_CMA will work like as ZONE_HIGHMEM or ZONE_MOVABLE.
>
> I don't think it would be a problem because most of file cache pages
> and anonymous pages are requested with GFP_HIGHUSER_MOVABLE. It could
> be proved by the fact that there are many systems with ZONE_HIGHMEM and
> they work fine. Notable disadvantage is that we cannot use these pages
> for blockdev file cache page, because it usually has __GFP_MOVABLE but
> not __GFP_HIGHMEM and __GFP_USER. But, in this case, there is pros and
> cons. In my experience, blockdev file cache pages are one of the top
> reason that causes cma_alloc() to fail temporarily. So, we can get more
> guarantee of cma_alloc() success by discarding that case.
>
> Implementation itself is very easy to understand. Steal when cma area is
> initialized and recalculate various per zone stat/threshold.
>
> Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>