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

Christoph Lameter
2007-08-10T17:37:02Z

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?