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

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?

> 
> guest_memfd already notifies KVM of page invalidations, so users of guest_memfd
> within KVM only need to respect the MMU invalidation protocol to safely rely on
> guest_memfd to ensure page presence.

Yes, the invalidation protocol is the crucial part. If we get that wrong, we're
in holy CVE land.

For GUP-fast, there was a similar discussion with MMU notifiers, but to this
day, KVM actually grabs+drops references.

[...]

> Removing struct page from kvm_gmem_get_pfn() also moves KVM closer toward
> supporting memory backends that are not backed by struct page.

Agreed, they should not be messing with the struct page at all.

-- 
Cheers,

David