Messages in this thread | | | Date | Sat, 07 Jun 2008 18:38:01 +0100 | From | Jeremy Fitzhardinge <> | Subject | Re: [rfc][patch] mm: vmap rewrite |
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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
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