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

Mel Gorman
2007-08-10T10:47:49Z

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