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Date: Fri, 16 Jan 2026 17:36:59 +0000
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Subject: Re: [PATCH v9 07/13] KVM: guest_memfd: Add flag to remove from direct
 map

On Fri, 2026-01-16 at 17:28 +0000, Nikita Kalyazin wrote:
> > 
> > I imagine this feature is really targeted towards machines running
> > a bunch of untrusted VMs, so cloud hypervisors really. In that case
> > the direct map will probably be carved up pretty quick. Did you
> > consider just breaking the full direct map to 4k at the start when
> > it's in use?
> 
> That's an interesting point, I haven't thought about it from this 
> perspective.  We should run some tests internally to see if it'd
> help.  This will likely change with support for huge pages coming in
> though.

The thing is, those no_flush() helpers actually still flush if they
need to split a page. Plus if they need to clear out lazy vmalloc
aliases it could be another flush. There are probably a lot of
opportunities to reduce flushing even beyond pre-split.

Just curious... as far as performance, have you tested this on a big
multi-socket system, where that flushing will hurt more? It's something
that has always been a fear for these directmap unmapping solutions

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