[PATCH v3 0/4] Stop returning struct page from guest_memfd PFN lookup

[Ackerley Tng <ackerleytng@google.com>]

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.

CoCo shared-to-private conversion handling must inspect folio refcounts to
ensure exclusive ownership by guest_memfd. A concurrent guest page fault
taking a temporary reference on the folio causes conversions to fail due to
an elevated refcount.

While this refcount is also taken on host userspace page faults, that
refcount is taken on behalf of the host userspace page tables. This
refcount will be dropped when conversions unmaps the page. Either way, once
there's an mmap() or userspace mapping, the pages are open to way more
refcounts, transient or not. This series focuses on just dropping refcounts
before handing KVM a page.

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

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.

This series first prepares the SEV-SNP handlers by treating unassigned RMP
entries as benign races on PSMASH failure (which can occur on concurrent
truncation) and dropping page references early in the RMP fault and VMSA reload
paths. It then updates kvm_gmem_get_pfn() to drop the folio reference internally
and stop returning a struct page pointer across x86 and arm64.

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

This is built off Sean's sample code [1].

[1] https://lore.kernel.org/all/an5RJYTwlYeym--O@google.com/

Thank you everybody for your quick reviews and testing, I really appreciate
it!

Changes from v2:

+ Picked up Reviewed-bys and Tested-bys
+ Addressed comments

v2: https://patch.msgid.link/20260818-gmem-no-return-page-v2-0-5298f42d49bb@google.com
v1: https://patch.msgid.link/20260818-gmem-no-return-page-v1-0-4f8d939efdbc@google.com

Signed-off-by: Ackerley Tng <ackerleytng@google.com>
---
Ackerley Tng (2):
      KVM: SEV: Treat unassigned RMP entry as benign race on PSMASH failure
      KVM: SEV: Drop page refcount early in VMSA reload

Sean Christopherson (2):
      KVM: SEV: Drop page refcount early during RMP fault handling
      KVM: guest_memfd: Stop returning struct page from PFN lookup

 arch/arm64/kvm/mmu.c     |  4 +---
 arch/arm64/kvm/nested.c  |  4 ++--
 arch/x86/kvm/mmu/mmu.c   |  2 +-
 arch/x86/kvm/svm/sev.c   | 53 ++++++++++++++++++++++++++++--------------------
 include/linux/kvm_host.h |  6 ++----
 virt/kvm/guest_memfd.c   |  9 ++------
 6 files changed, 39 insertions(+), 39 deletions(-)
---
base-commit: 1b731e5ded480bd1e5546aed35584238661ce72e
change-id: 20260818-gmem-no-return-page-614927a29f97

Best regards,
--  
Ackerley Tng <ackerleytng@google.com>