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Date: Fri, 16 Jan 2026 17:28:06 +0000
From: Nikita Kalyazin <kalyazin@...zon.com>
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Subject: Re: [PATCH v9 07/13] KVM: guest_memfd: Add flag to remove from direct
 map



On 16/01/2026 15:34, Edgecombe, Rick P wrote:
> On Fri, 2026-01-16 at 15:00 +0000, Nikita Kalyazin wrote:
>>> Does this assume the folio would not have been split after it was
>>> zapped? As in, if it was zapped at 2MB granularity (no 4KB direct
>>> map split required) but then restored at 4KB (split required)? Or
>>> it gets merged somehow before this?
>>
>> AFAIK it can't be zapped at 2MB granularity as the zapping code will
>> inevitably cause splitting because guest_memfd faults occur at the
>> base page granularity as of now.
> 
> Ah, right since there are no huge pages currently. Then the huge page
> series will need to keep this in mind and figure out some solution.
> Probably worth a comment on that assumption to help anyone that changes
> it.

Makes sense.  I'll leave a comment.

> 
> 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.

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