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

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Message-ID: <Pine.LNX.4.64.0708101035020.12758@schroedinger.engr.sgi.com>
Date:	Fri, 10 Aug 2007 10:37:02 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Mel Gorman <mel@...net.ie>
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 Fri, 10 Aug 2007, Mel Gorman wrote:

> 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.

x86_64 does not support ZONE_HIGHMEM. The number of zones is 
depending on SMP?
 
> 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?

You could do a BUILD_BUG_ON() instead?
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