Re: [PATCH RFC 0/3] KVM: guest_memfd: folio migration for non-confidential VMs

["David Hildenbrand (Arm)" <david@kernel.org>]

On 6/15/26 12:43, Alexandru Elisei wrote:
> Hi,
> 
> On Thu, Jun 11, 2026 at 01:05:07PM +0000, Shivank Garg wrote:
>> guest_memfd folios are currently marked unmovable, so the kernel cannot
>> perform NUMA-balancing, memory compaction, etc. This is unavoidable for
>> confidential VMs (SEV-SNP, TDX), since memory is encrypted and copying it
>> needs firmware assistance. However, for non-confidential VMs (like
>> Firecracker), we can migrate the folios.
>>
>> This series enables folio migration for non-confidential guest_memfd and
>> also lays the groundwork for migrating confidential guest_memfd later.
>> Once firmware-assisted copying support is available, those VMs can be
>> made movable, the confidential folio content can be copied separately,
>> and the destination folio marked with FOLIO_CONTENT_COPIED so
>> __migrate_folio() skips the host-side folio_mc_copy().
> 
> I always thought that one of the nice things about using guest_memfd as a
> memory backend, as opposed to host userspace mappings, is that the host
> cannot unmap VM memory because of KSM, automatic NUMA balancing, hugepage
> collapse, compaction, etc, acting on the host userspace mapping of the
> VM memory, and outside of the VMM's or KVM's control.

Yeah, but it doesn't play nice with THPs / large folios. So if you want to run
something else on a hypervisor than just confidential VMs, you definitely want
guest_memfd to be as nice to the system.

That is, support page migration if nothing speaks against it.

Now, if something speaks against it, for sure we can just leave the pages be
unmovable.

Fortunately, the patch is rather trivial.

-- 
Cheers,

David