Re: [PATCH v2 0/4] Stop returning struct page from guest_memfd PFN lookup

[Sean Christopherson <seanjc@google.com>]

On Tue, Aug 18, 2026, David Hildenbrand (Arm) wrote:
> On 8/18/26 11:15, Ackerley Tng wrote:
> > KVM currently expects kvm_gmem_get_pfn() to return a refcounted struct
> > page. Callers (such as x86 TDP MMU, arm64 Stage-2 fault handler, and SEV-SNP
> > VMSA / RMP handlers) hold this refcount across page fault handling.
> > 
> > Holding a page refcount across fault handling is problematic for guest_memfd.
> > In-place memory conversions between confidential computing shared and private
> > states inspect folio refcounts to ensure exclusive ownership by guest_memfd.  A
> > concurrent guest page fault taking a reference on the folio causes conversions
> > to fail due to an elevated refcount.
> 
> Right. Won't we still, at least temporarily, grab a reference while looking up
> the folio in the page cache, or will we be preventing that concurrent race with
> locking?

The latter.  What I want to aim for is that if the relevant guest_memfd range
has never been mmap()'d and there are no memory failures, then conversion is
guaranteed to not fail due to elevated refcounts.

Or to put it a different way, I want KVM's ABI to be that pausing vCPU is *NOT*
required to perform an in-place conversion.