Re: [rfc][patch] mm: vmap rewrite

Jeremy Fitzhardinge
2008-06-07T17:38:52Z

Nick Piggin wrote:
> Hi. RFC.
>
> Rewrite the vmap allocator to use rbtrees and lazy tlb flushing, and provide a
> fast, scalable percpu frontend for small vmaps.
>
> XEN and PAT and such do not like deferred TLB flushing. They just need to call
> vm_unmap_aliases() in order to flush any deferred mappings. That call is very
> expensive (well, actually not a lot more expensive than a single vunmap under
> the old scheme), however it should be OK if not called too often.
>

What are the performance characteristics? Can it be fast-pathed if
there are no outstanding aliases?

For Xen, I'd need to do the alias unmap each time it allocates a page
for use in a pagetable. For initial process construction that could be
deferred, but creating mappings on a live process could get fairly
expensive as a result. The ideal interface for me would be a way of
testing if a given page has vmap aliases, so that we need only do the
unmap if really necessary. I'm guessing that goes into "need a new page
flag" territory though...

J