[PATCH v9 07/13] KVM: guest_memfd: Add flag to remove from direct map

[Nikita Kalyazin]

On 16/01/2026 17:36, Edgecombe, Rick P wrote:
> 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

Yes, this is a problem that we'd like to address.  We have been 
discussing it in [1].  The effect of flushing on memory population that 
we see on x86 is 5-7x elongation.  We are thinking of making use of the 
no-direct-map memory allocator that Brendan is working on [2].

[1] 
https://lore.kernel.org/lkml/d1b58114-9b88-4535-b28c-09d9cc1ff3be@amazon.com
[2] https://lore.kernel.org/kvm/DDVS9ITBCE2Z.RSTLCU79EX8G@google.com
>