On Thu, 29 Jul 2010, Dave Hansen wrote:

> SPARSEMEM_EXTREME would be a bit different.  It's a 2-level lookup.
> You'd have 16 "section roots", each representing 256MB of address space.
> Each time we put memory under one of those roots, we'd fill in a
> 512-section second-level table, which is designed to always fit into one
> page.  If you start at 256MB, you won't waste all those entries.

That is certain a solution to the !MMU case and it would work very much
like a page table. If you have an MMU then the vmemmap sparsemem
configuration can take advantage of of that to avoid the 2 level lookup.