linux-kernel - Re: [PATCH 3/4] Embed zone_id information within the zonelist->zones pointer

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Date:	Fri, 10 Aug 2007 11:47:49 +0100
From:	mel@...net.ie (Mel Gorman)
To:	Christoph Lameter <clameter@....com>
Cc:	Lee.Schermerhorn@...com, ak@...e.de, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH 3/4] Embed zone_id information within the zonelist->zones pointer

On (09/08/07 18:44), Christoph Lameter didst pronounce:
> 
> On Fri, 10 Aug 2007, Mel Gorman wrote:
> 
> > > > +#if defined(CONFIG_SMP) && INTERNODE_CACHE_SHIFT > ZONES_SHIFT
> > > 
> > > Is this necessary? ZONES_SHIFT is always <= 2 so it will work with 
> > > any pointer. Why disable this for UP?
> > > 
> > 
> > Caution in case the number of zones increases. There was no guarantee of
> > zone alignment. It's the same reason I have a BUG_ON in the encode
> > function so that if we don't catch problems at compile-time, it'll go
> > BANG in a nice predictable fashion.
> 
> Caution would lead to a BUG_ON but why the #if? Why exclude UP?

On x86_64 would have ZONE_DMA, ZONE_DMA32, ZONE_NORMAL, ZONE_HIGHMEM and
ZONE_MOVABLE. On SMP, that's more than two bits worth and would fail t
runtime. Well, it should at least I didn't actually try it out.

However, I accept that the SMP check is less than than ideal. I considered
comparing it against MAX_NR_ZONES but as it's an enum, it can't be checked
at compile time. What else would make a better check?

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