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

[David Hildenbrand (Arm)]

On 8/18/26 21:55, Sean Christopherson wrote:
> 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.

Having the VM access a page that is currently under conversion (triggered by the
VM) should not be the common case, no? Except, prefaulting, of course.

-- 
Cheers,

David