linux-kernel - Re: [PATCH 04/19] Convert gfp_zone() to use a table of precalculated values

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Date:	Tue, 24 Feb 2009 11:43:29 -0500 (EST)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Mel Gorman <mel@....ul.ie>
cc:	Linux Memory Management List <linux-mm@...ck.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Lin Ming <ming.m.lin@...el.com>,
	Zhang Yanmin <yanmin_zhang@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 04/19] Convert gfp_zone() to use a table of precalculated
 values

On Tue, 24 Feb 2009, Mel Gorman wrote:

>  static inline enum zone_type gfp_zone(gfp_t flags)
>  {
> -#ifdef CONFIG_ZONE_DMA
> -	if (flags & __GFP_DMA)
> -		return ZONE_DMA;
> -#endif
> -#ifdef CONFIG_ZONE_DMA32
> -	if (flags & __GFP_DMA32)
> -		return ZONE_DMA32;
> -#endif
> -	if ((flags & (__GFP_HIGHMEM | __GFP_MOVABLE)) ==
> -			(__GFP_HIGHMEM | __GFP_MOVABLE))
> -		return ZONE_MOVABLE;
> -#ifdef CONFIG_HIGHMEM
> -	if (flags & __GFP_HIGHMEM)
> -		return ZONE_HIGHMEM;
> -#endif
> -	return ZONE_NORMAL;
> +	return gfp_zone_table[flags & GFP_ZONEMASK];
>  }

Aassume

GFP_DMA		= 0x01
GFP_DMA32	= 0x02
GFP_MOVABLE	= 0x04
GFP_HIGHMEM	= 0x08

ZONE_NORMAL	= 0
ZONE_DMA	= 1
ZONE_DMA32	= 2
ZONE_MOVABLE	= 3
ZONE_HIGHMEM	= 4

then we could implement gfp_zone simply as:

static inline enum zone_type gfp_zone(gfp_t flags)
{
	return ffs(flags & 0xf);
}

However, this would return ZONE_MOVABLE if only GFP_MOVABLE would be
set but not GFP_HIGHMEM.

If we could make sure that GFP_MOVABLE always includes GFP_HIGHMEM then
this would not be a problem.
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