From: "Bob Picco" <bob.picco@hp.com>
Date: Fri, 5 May 2006 10:50:18 -0400
To: Dave Hansen <haveblue@us.ibm.com>
Cc: Bob Picco <bob.picco@hp.com>, Andy Whitcroft <apw@shadowen.org>,
       Ingo Molnar <mingo@elte.hu>, Nick Piggin <nickpiggin@yahoo.com.au>,
       "Martin J. Bligh" <mbligh@mbligh.org>, Andi Kleen <ak@suse.de>,
       linux-kernel@vger.kernel.org, Andrew Morton <akpm@osdl.org>,
       Linux Memory Management <linux-mm@kvack.org>
Subject: Re: assert/crash in __rmqueue() when enabling CONFIG_NUMA
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Dave Hansen wrote:	[Fri May 05 2006, 10:33:10AM EDT]
> On Fri, 2006-05-05 at 09:55 -0400, Bob Picco wrote:
> > -               if (!page_is_buddy(buddy, order))
> > +               if (page_in_zone_hole(buddy))
> > +                       break;
> > +               else if (page_zonenum(buddy) != page_zonenum(page))
> > +                       break;
> > +               else if (!page_is_buddy(buddy, order))
> >                         break;          /* Move the buddy up one level. */ 
> 
> 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.
> 
> I'm a bit confused. :(
> 
> BTW, I like the idea of just aligning HIGHMEM's start because it has no
> runtime cost.  Buuuuut, it is still just a shift and compare of the two
> page->flags, which should already be (or will soon anyway be) in the
> cache.
Yes. I'll defer to Andy whether he wants the zonenum check or to align
HIGHMEM corrrectly.
> 
> -- Dave
> 
bob
-
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