linux-kernel - Re: [rfc][patch] mm: vmap rewrite

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Date:	Sat, 07 Jun 2008 18:38:01 +0100
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Nick Piggin <npiggin@...e.de>
CC:	Linux Memory Management List <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [rfc][patch] mm: vmap rewrite

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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