Dave Hansen wrote: [Fri May 05 2006, 10:57:44AM EDT]
> On Fri, 2006-05-05 at 10:50 -0400, Bob Picco wrote:
> > Dave Hansen wrote: [Fri May 05 2006, 10:33:10AM EDT]
> > > The page_zonenum() checks look good, but I'm not sure I understand the
> > > page_in_zone_hole() part. If a page is in a hole in a zone, it will
> > > still have a valid mem_map entry, right? It should also never have been
> > > put into the allocator, so it also won't ever be coalesced.
> > This has always been subtle and not too revealing. It probably should
> > have a comment. The page_in_zone_hole check is for ia64
> > VIRTUAL_MEM_MAP. You might compute a page structure which is in a hole not
> > backed by memory; an unallocated page which covers pages structures.
> > VIRTUAL_MEM_MAP uses a contiguous virtual region with virtual space holes
> > not backed by memory. Take a look at ia64_pfn_valid.
>
> Ahhh. I hadn't made the ia64 connection. I wonder if it is worth
> making CONFIG_HOLES_IN_ZONE say ia64 or something about vmem_map in it
> somewhere. Might be worth at least a comment like this:
>
> + if (page_in_zone_hole(buddy)) /* noop on all but ia64 */
> + break;
> + else if (page_zonenum(buddy) != page_zonenum(page))
> + break;
> + else if (!page_is_buddy(buddy, order))
> break; /* Move the buddy up one level. */
>
> BTW, wasn't the whole idea of discontig to have holes in zones (before
> NUMA) without tricks like this? ;)
Sure you could boot ia64 with just DISCONTIGMEM and no VIRTUAL_MEM_MAP.
In fact that's exactly what I did to test code added in alloc_node_mem_map.
Unfortunately I was missing 1Gb from free memory after booting. The
missing 1Gb was consumed by reserved pages structures :)
>
> -- Dave
>
bob
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