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

[Jeremy Fitzhardinge <jeremy@goop.org>]

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