[PATCH v3 0/6] KVM: guest_memfd: support for uffd minor

[Nikita Kalyazin <kalyazin@amazon.com>]

This series is built on top of the Fuad's v7 "mapping guest_memfd backed
memory at the host" [1].

With James's KVM userfault [2], it is possible to handle stage-2 faults
in guest_memfd in userspace.  However, KVM itself also triggers faults
in guest_memfd in some cases, for example: PV interfaces like kvmclock,
PV EOI and page table walking code when fetching the MMIO instruction on
x86.  It was agreed in the guest_memfd upstream call on 23 Jan 2025 [3]
that KVM would be accessing those pages via userspace page tables.  In
order for such faults to be handled in userspace, guest_memfd needs to
support userfaultfd.

Changes since v2 [4]:
 - James: Fix sgp type when calling shmem_get_folio_gfp
 - James: Improved vm_ops->fault() error handling
 - James: Add and make use of the can_userfault() VMA operation
 - James: Add UFFD_FEATURE_MINOR_GUEST_MEMFD feature flag
 - James: Fix typos and add more checks in the test

Nikita

[1] https://lore.kernel.org/kvm/20250318161823.4005529-1-tabba@google.com/T/
[2] https://lore.kernel.org/kvm/20250109204929.1106563-1-jthoughton@google.com/T/
[3] https://docs.google.com/document/d/1M6766BzdY1Lhk7LiR5IqVR8B8mG3cr-cxTxOrAosPOk/edit?tab=t.0#heading=h.w1126rgli5e3
[4] https://lore.kernel.org/kvm/20250402160721.97596-1-kalyazin@amazon.com/T/

Nikita Kalyazin (6):
  mm: userfaultfd: generic continue for non hugetlbfs
  mm: provide can_userfault vma operation
  mm: userfaultfd: use can_userfault vma operation
  KVM: guest_memfd: add support for userfaultfd minor
  mm: userfaultfd: add UFFD_FEATURE_MINOR_GUEST_MEMFD
  KVM: selftests: test userfaultfd minor for guest_memfd

 fs/userfaultfd.c                              |  3 +-
 include/linux/mm.h                            |  5 +
 include/linux/mm_types.h                      |  4 +
 include/linux/userfaultfd_k.h                 | 10 +-
 include/uapi/linux/userfaultfd.h              |  8 +-
 mm/hugetlb.c                                  |  9 +-
 mm/shmem.c                                    | 17 +++-
 mm/userfaultfd.c                              | 47 ++++++---
 .../testing/selftests/kvm/guest_memfd_test.c  | 99 +++++++++++++++++++
 virt/kvm/guest_memfd.c                        | 10 ++
 10 files changed, 188 insertions(+), 24 deletions(-)


base-commit: 3cc51efc17a2c41a480eed36b31c1773936717e0
-- 
2.47.1