Re: assert/crash in __rmqueue() when enabling CONFIG_NUMA

["Bob Picco" <bob.picco@hp.com>]

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