linux-kernel - Re: [PATCH 6/6] mm: page_alloc: Reduce cost of the fair zone allocation policy

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Date:	Fri, 08 Aug 2014 17:27:15 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>
CC:	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>,
	Linux-FSDevel <linux-fsdevel@...r.kernel.org>,
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH 6/6] mm: page_alloc: Reduce cost of the fair zone allocation
 policy

On 07/09/2014 10:13 AM, Mel Gorman wrote:
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1604,6 +1604,9 @@ again:
>  	}
>  
>  	__mod_zone_page_state(zone, NR_ALLOC_BATCH, -(1 << order));

This can underflow zero, right?

> +	if (zone_page_state(zone, NR_ALLOC_BATCH) == 0 &&

AFAICS, zone_page_state will correct negative values to zero only for
CONFIG_SMP. Won't this check be broken on !CONFIG_SMP?

I just stumbled upon this when trying to optimize the function. I didn't check
how rest of the design copes with negative NR_ALLOC_BATCH values.

> +	    !zone_is_fair_depleted(zone))
> +		zone_set_flag(zone, ZONE_FAIR_DEPLETED);
>  
>  	__count_zone_vm_events(PGALLOC, zone, 1 << order);
>  	zone_statistics(preferred_zone, zone, gfp_flags);
> @@ -1915,6 +1918,18 @@ static bool zone_allows_reclaim(struct zone *local_zone, struct zone *zone)
>  
>  #endif	/* CONFIG_NUMA */
>  

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