Re: RFC: Transparent Hugepage support

[Adam Litke <agl@us.ibm.com>]

On Wed, 2009-10-28 at 15:18 +0100, Andi Kleen wrote:
> > My worry are the archs like powerpc where a hugepage doesn't fit in a
> > pmd_trans_huge. I think x86 will fit the pmd/pud_trans_huge approach
> > in my patch even of 1G pages in the long run, so there is no actual
> > long term limitation with regard to x86. The fact is that the generic
> > pagetable code is tuned for x86 so no problem there.
> > 
> > What I am unsure about and worries me more are those archs that don't
> > use a pmd to map hugepages and to create hugetlb. I am unsure if those
> > archs will be able to take advantage of my patch with minor changes to
> > it given it is wired to pmd_trans_huge availability.
> 
> I see. Some archs (like IA64 or POWER?) require special VA address
>  ranges for huge pages, for those doing it fully transparent without 
> a mmap time flag is likely hard.

PowerPC does not require specific virtual addresses for huge pages, but
does require that a consistent page size be used for each slice of the
virtual address space.  Slices are 256M in size from 0 to 4G and 1TB in
size above 1TB while huge pages are 64k, 16M, or 16G.  Unless the PPC
guys can work some more magic with their mmu, split_huge_page() in its
current form just plain won't work on PowerPC.  That doesn't even take
into account the (already discussed) page table layout differences
between x86 and ppc: http://linux-mm.org/PageTableStructure .

-- 
Thanks,
Adam

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