mm, kvm: allow uffd suppot in guest_memfd

From: "Mike Rapoport (Microsoft)" <rppt@kernel.org>

Hi,

These patches enable support for userfaultfd in guest_memfd.
They are quite different from the latest posting [1] so I'm restarting the
versioning. As there was a lot of tension around the topic, this is an RFC
to get some feedback and see how we can move forward.

As the ground work I refactored userfaultfd handling of PTE-based memory types
(anonymous and shmem) and converted them to use vm_uffd_ops for allocating a
folio or getting an existing folio from the page cache. shmem also implements
callbacks that add a folio to the page cache after the data passed in
UFFDIO_COPY was copied and remove the folio from the page cache if page table
update fails.

In order for guest_memfd to notify userspace about page faults, there are new
VM_FAULT_UFFD_MINOR and VM_FAULT_UFFD_MISSING that a ->fault() handler can
return to inform the page fault handler that it needs to call
handle_userfault() to complete the fault.

Nikita helped to plumb these new goodies into guest_memfd and provided basic
tests to verify that guest_memfd works with userfaultfd.

I deliberately left hugetlb out, at least for the most part.
hugetlb handles acquisition of VMA and more importantly establishing of parent
page table entry differently than PTE-based memory types. This is a different
abstraction level than what vm_uffd_ops provides and people objected to
exposing such low level APIs as a part of VMA operations.

Also, to enable uffd in guest_memfd refactoring of hugetlb is not needed and I
prefer to delay it until the dust settles after the changes in this set.

[1] https://lore.kernel.org/all/20251130111812.699259-1-rppt@...

Mike Rapoport (Microsoft) (12):
  userfaultfd: introduce mfill_copy_folio_locked() helper
  userfaultfd: introduce struct mfill_state
  userfaultfd: introduce mfill_get_pmd() helper.
  userfaultfd: introduce mfill_get_vma() and mfill_put_vma()
  userfaultfd: retry copying with locks dropped in mfill_atomic_pte_copy()
  userfaultfd: move vma_can_userfault out of line
  userfaultfd: introduce vm_uffd_ops
  userfaultfd, shmem: use a VMA callback to handle UFFDIO_CONTINUE
  userfaultfd: introduce vm_uffd_ops->alloc_folio()
  shmem, userfaultfd: implement shmem uffd operations using vm_uffd_ops
  userfaultfd: mfill_atomic() remove retry logic
  mm: introduce VM_FAULT_UFFD_MINOR fault reason

Nikita Kalyazin (5):
  mm: introduce VM_FAULT_UFFD_MISSING fault reason
  KVM: guest_memfd: implement userfaultfd minor mode
  KVM: guest_memfd: implement userfaultfd missing mode
  KVM: selftests: test userfaultfd minor for guest_memfd
  KVM: selftests: test userfaultfd missing for guest_memfd

 include/linux/mm.h                            |   5 +
 include/linux/mm_types.h                      |  15 +-
 include/linux/shmem_fs.h                      |  14 -
 include/linux/userfaultfd_k.h                 |  74 +-
 mm/hugetlb.c                                  |  21 +
 mm/memory.c                                   |   8 +-
 mm/shmem.c                                    | 188 +++--
 mm/userfaultfd.c                              | 671 ++++++++++--------
 .../testing/selftests/kvm/guest_memfd_test.c  | 191 +++++
 virt/kvm/guest_memfd.c                        | 134 +++-
 10 files changed, 871 insertions(+), 450 deletions(-)


base-commit: f8f9c1f4d0c7a64600e2ca312dec824a0bc2f1da
--
2.51.0