Re: RFC: Transparent Hugepage support

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Wed, 2009-10-28 at 09:54 -0500, Adam Litke wrote:
> 
> 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 . 

Note: this is server powerpc's. Embedded ones are more flexible but on
server we have this limitation and not much we can do about it.

Note also that the "slice" sizes are a SW thing. HW segments are either
256M or 1T (the later being supported only on some processors), and
linux maintains that concept of "slices" in order to simplify the
tracking of said segments and to use 1T when available.

Ben.


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