Date: Fri, 24 Aug 2007 18:44:38 +0100
To: Andi Kleen <ak@suse.de>
Cc: Andy Whitcroft <apw@shadowen.org>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] x86 Boot NUMA kernels on non-NUMA hardware with DISCONTIG memory model
Message-ID: <20070824174438.GG26374@skynet.ie>
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From: mel@skynet.ie (Mel Gorman)
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On (24/08/07 19:38), Andi Kleen didst pronounce:
> > Other than the fact that the memmap must be PMD aligned to use hugepage
> > entries for the memmap. 
> 
> Why is that so?  mem_map should be just part of lowmem anyways.
> 

Not in this case. memmap is allocated node local and mapped in the virtual
memory area normally occupied by the end of low memory. The objective was
to have memmap for the struct pages node-local. Hence, portions of
memmap are really in highmem.

> > It could be mapped with small pages in corner cases
> > but the complexity worth it?
> 
> You don't need to map it with small pages in the normal case,
> the only requirement is that c_p_a() is aware of it so it can
> split it if needed.
> 
> > I can't see this type of lifting being done any time soon. As SPARSEMEM works
> > and there is hope with the vmemmap work that DISCONTIG will finally go away,
> > it may not be the best investment of time.
> 
> It's a trivial change, probably less code than your original patch.
> 

I'll have to take your word for it because I haven't looked closely
enough. I'll try and find time to look at it but the earliest I'll get around
to it is post kernel-summit. In the meantime, SPARSEMEM works.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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