linux-kernel - Re: [PATCH] mm: page_alloc: Fix setting of ZONE_FAIR

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Date:	Tue, 9 Sep 2014 12:53:18 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Leon Romanovsky <leon@...n.nu>, Vlastimil Babka <vbabka@...e.cz>,
	Johannes Weiner <hannes@...xchg.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>,
	Linux-FSDevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] mm: page_alloc: Fix setting of ZONE_FAIR_DEPLETED on UP
 v2

On Mon, 8 Sep 2014 12:57:18 +0100 Mel Gorman <mgorman@...e.de> wrote:

> zone_page_state is an API hazard because of the difference in behaviour
> between SMP and UP is very surprising. There is a good reason to allow
> NR_ALLOC_BATCH to go negative -- when the counter is reset the negative
> value takes recent activity into account. This patch makes zone_page_state
> behave the same on SMP and UP as saving one branch on UP is not likely to
> make a measurable performance difference.
> 
> ...
>
> --- a/include/linux/vmstat.h
> +++ b/include/linux/vmstat.h
> @@ -131,10 +131,8 @@ static inline unsigned long zone_page_state(struct zone *zone,
>  					enum zone_stat_item item)
>  {
>  	long x = atomic_long_read(&zone->vm_stat[item]);
> -#ifdef CONFIG_SMP
>  	if (x < 0)
>  		x = 0;
> -#endif
>  	return x;
>  }

We now have three fixes for the same thing.  I'm presently holding on
to hannes's mm-page_alloc-fix-zone-allocation-fairness-on-up.patch.

Regularizing zone_page_state() in this fashion seems a good idea and is
presumably safe because callers have been tested with SMP.  So unless
shouted at I think I'll queue this one for 3.18?

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