Re: [RFC PATCH v5 06/15] KVM: guest_memfd: Handle final folio_put() of guestmem pages

[Vishal Annapurve <vannapurve@google.com>]

On Fri, Jan 17, 2025 at 8:30 AM Fuad Tabba <tabba@google.com> wrote:
>
> Before transitioning a guest_memfd folio to unshared, thereby
> disallowing access by the host and allowing the hypervisor to
> transition its view of the guest page as private, we need to be
> sure that the host doesn't have any references to the folio.
>
> This patch introduces a new type for guest_memfd folios, and uses
> that to register a callback that informs the guest_memfd
> subsystem when the last reference is dropped, therefore knowing
> that the host doesn't have any remaining references.
>
> Signed-off-by: Fuad Tabba <tabba@google.com>
> ---
> The function kvm_slot_gmem_register_callback() isn't used in this
> series. It will be used later in code that performs unsharing of
> memory. I have tested it with pKVM, based on downstream code [*].
> It's included in this RFC since it demonstrates the plan to
> handle unsharing of private folios.
>
> [*] https://android-kvm.googlesource.com/linux/+/refs/heads/tabba/guestmem-6.13-v5-pkvm

Should the invocation of kvm_slot_gmem_register_callback() happen in
the same critical block as setting the guest memfd range mappability
to NONE, otherwise conversion/truncation could race with registration
of callback?