linux-kernel - Re: [PATCH 24/27] Convert gfp_zone() to use a table of precalculated values

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Date:	Thu, 19 Mar 2009 09:04:56 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Mel Gorman <mel@....ul.ie>
Cc:	Christoph Lameter <cl@...ux-foundation.org>,
	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 24/27] Convert gfp_zone() to use a table of
 precalculated values

On Wed, 18 Mar 2009 19:46:04 +0000
Mel Gorman <mel@....ul.ie> wrote:

> On Wed, Mar 18, 2009 at 03:07:48PM -0400, Christoph Lameter wrote:
> > On Wed, 18 Mar 2009, Mel Gorman wrote:
> > 
> > > Thanks.At a quick glance, it looks ok but I haven't tested it. As the intention
> > > was to get one pass of patches that are not controversial and are "obvious",
> > > I have dropped my version of the gfp_zone patch and the subsequent flag
> > > cleanup and will revisit it after the first lot of patches has been dealt
> > > with. I'm testing again with the remaining patches.
> > 
> > This fixes buggy behavior of gfp_zone so it would deserve a higher
> > priority.
> > 
> 
> It is buggy behaviour in response to a flag combination that makes no sense
> which arguably is a buggy caller. Now that I get to think about it a bit more,
> you can't define a const table in a header. If it's declared extern, then
> the compiler doesn't know what the constant value is so it can't generate
> better code.  At best, you end up with equivalent code to what my patch did
> in the first place except __GFP_DMA32|__GFP_HIGHMEM will return ZONE_NORMAL.
> 

I wonder why you have to make the bad caller work insane way ?
Is this bad ?
==
const int gfp_zone_table[GFP_ZONEMASK] = {
	ZONE_NORMAL,		/* 00 No flags set */
	ZONE_DMA,		/* 01 Only GFP_DMA set */
	ZONE_HIGHMEM,		/* 02 Only GFP_HIGHMEM set */
	BAD_ZONE,		/* 03 GFP_HIGHMEM and GFP_DMA set */
	ZONE_DMA32,		/* 04 Only GFP_DMA32 set */
	BAD_ZONE,		/* 05 GFP_DMA and GFP_DMA32 set */
	BAD_ZONE,		/* 06 GFP_DMA32 and GFP_HIGHMEM set */
	BAD_ZONE,		/* 07 GFP_DMA, GFP_DMA32 and GFP_DMA32 set */
	ZONE_MOVABLE,		/* 08 Only ZONE_MOVABLE set */
	ZONE_DMA,		/* 09 MOVABLE + DMA */
	ZONE_MOVABLE,		/* 0A MOVABLE + HIGHMEM */
	BAD_ZONE,		/* 0B MOVABLE + DMA + HIGHMEM */
	ZONE_DMA32,		/* 0C MOVABLE + DMA32 */
	BAD_ZONE,		/* 0D MOVABLE + DMA + DMA32 */
	BAD_ZONE,		/* 0E MOVABLE + DMA32 + HIGHMEM */
	BAD_ZONE		/* 0F MOVABLE + DMA32 + HIGHMEM + DMA
};
==

Thanks,
-Kame



> -- 
> Mel Gorman
> Part-time Phd Student                          Linux Technology Center
> University of Limerick                         IBM Dublin Software Lab
> 
> --
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