[RFC PATCH 0/4] mm, kvm: add guest_memfd support for uffd minor faults

[Mike Rapoport]

From: "Mike Rapoport (Microsoft)" <rppt@xxxxxxxxxx>

Hi,

These patches allow guest_memfd to notify userspace about minor page
faults using userfaultfd and let userspace to resolve these page faults
using UFFDIO_CONTINUE.

To allow UFFDIO_CONTINUE outside of the core mm I added a
get_pagecache_folio() callback to vm_ops that allows an address space
backing a VMA to return a folio that exists in it's page cache (patch 2)

In order for guest_memfd to notify userspace about page faults, it has to
call handle_userfault() and since guest_memfd may be a part of kvm module,
handle_userfault() is exported for kvm module (patch 3).

Note that patch 3 changelog does not provide motivation for enabling uffd
in guest_memfd, mainly because I can't say I understand why is that
required :)
Would be great to hear from KVM folks about it.

This series is the minimal change I've been able to come up with to allow
integration of guest_memfd with uffd and while refactoring uffd and making
mfill_atomic() flow more linear would have been a nice improvement, it's
way out of the scope of enabling uffd with guest_memfd.

Mike Rapoport (Microsoft) (3):
  userfaultfd: move vma_can_userfault out of line
  userfaultfd, shmem: use a VMA callback to handle UFFDIO_CONTINUE
  userfaultfd, guest_memfd: support userfault minor mode in guest_memfd

Nikita Kalyazin (1):
  KVM: selftests: test userfaultfd minor for guest_memfd

 fs/userfaultfd.c                              |   4 +-
 include/linux/mm.h                            |   9 ++
 include/linux/userfaultfd_k.h                 |  36 +-----
 include/uapi/linux/userfaultfd.h              |   8 +-
 mm/shmem.c                                    |  20 ++++
 mm/userfaultfd.c                              |  88 ++++++++++++---
 .../testing/selftests/kvm/guest_memfd_test.c  | 103 ++++++++++++++++++
 virt/kvm/guest_memfd.c                        |  30 +++++
 8 files changed, 245 insertions(+), 53 deletions(-)


base-commit: 6146a0f1dfae5d37442a9ddcba012add260bceb0
-- 
2.50.1