linux-kernel - Re: [PATCH 05/12] mm, page_alloc: Remove unecessary recheck of nodemask

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Date:	Tue, 25 Aug 2015 16:32:05 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Mel Gorman <mgorman@...hsingularity.net>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Johannes Weiner <hannes@...xchg.org>,
	Rik van Riel <riel@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Michal Hocko <mhocko@...nel.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 05/12] mm, page_alloc: Remove unecessary recheck of
 nodemask

On 08/24/2015 02:09 PM, Mel Gorman wrote:
> An allocation request will either use the given nodemask or the cpuset
> current tasks mems_allowed. A cpuset retry will recheck the callers nodemask
> and while it's trivial overhead during an extremely rare operation, also
> unnecessary. This patch fixes it.
>
> Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
> ---
>   mm/page_alloc.c | 5 ++---
>   1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 2c1c3bf54d15..32d1cec124bc 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3171,7 +3171,7 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order,
>   	gfp_t alloc_mask; /* The gfp_t that was actually used for allocation */
>   	struct alloc_context ac = {
>   		.high_zoneidx = gfp_zone(gfp_mask),
> -		.nodemask = nodemask,
> +		.nodemask = nodemask ? : &cpuset_current_mems_allowed,

Hm this is a functional change for atomic allocations with NULL 
nodemask. ac.nodemask is passed down to __alloc_pages_slowpath() which 
might determine that ALLOC_CPUSET is not to be used (because it's 
atomic). Yet it would use the restricted ac.nodemask in 
get_page_from_freelist() and elsewhere.

>   		.migratetype = gfpflags_to_migratetype(gfp_mask),
>   	};
>
> @@ -3206,8 +3206,7 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order,
>
>   	/* The preferred zone is used for statistics later */
>   	preferred_zoneref = first_zones_zonelist(ac.zonelist, ac.high_zoneidx,
> -				ac.nodemask ? : &cpuset_current_mems_allowed,
> -				&ac.preferred_zone);
> +				ac.nodemask, &ac.preferred_zone);
>   	if (!ac.preferred_zone)
>   		goto out;
>   	ac.classzone_idx = zonelist_zone_idx(preferred_zoneref);
>

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