[PATCH v3 0/5] mm, kvm: add guest_memfd support for uffd minor faults

[Mike Rapoport <rppt@kernel.org>]

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

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_folio_noalloc() 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, there is a
new VM_FAULT_UFFD_MINOR that a ->fault() handler can return to inform the
page fault handler that it needs to call handle_userfault() to complete the
fault (patch 3).
 
Patch 4 plumbs these new goodies into guest_memfd.

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.

v3 changes:
* rename ->get_folio() to ->get_folio_noalloc()
* fix build errors reported by kbuild
* pull handling of UFFD_MINOR out of hotpath in __do_fault()
* update guest_memfs changes so its ->fault() and ->get_folio_noalloc()
  follow the same semantics as shmem and hugetlb.
* s/MISSING/MINOR/g in changelogs
* added review tags

v2: https://lore.kernel.org/all/20251125183840.2368510-1-rppt@kernel.org
* rename ->get_shared_folio() to ->get_folio()
* hardwire VM_FAULF_UFFD_MINOR to 0 when CONFIG_USERFAULTFD=n

v1: https://patch.msgid.link/20251123102707.559422-1-rppt@kernel.org
* Introduce VM_FAULF_UFFD_MINOR to avoid exporting handle_userfault()
* Simplify vma_can_mfill_atomic()
* Rename get_pagecache_folio() to get_shared_folio() and use inode
  instead of vma as its argument

rfc: https://patch.msgid.link/20251117114631.2029447-1-rppt@kernel.org

Mike Rapoport (Microsoft) (4):
  userfaultfd: move vma_can_userfault out of line
  userfaultfd, shmem: use a VMA callback to handle UFFDIO_CONTINUE
  mm: introduce VM_FAULT_UFFD_MINOR fault reason
  guest_memfd: add support for userfaultfd minor mode

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

 include/linux/mm.h                            |  9 ++
 include/linux/mm_types.h                      | 10 +-
 include/linux/userfaultfd_k.h                 | 36 +------
 mm/memory.c                                   |  5 +-
 mm/shmem.c                                    | 20 +++-
 mm/userfaultfd.c                              | 80 ++++++++++++---
 .../testing/selftests/kvm/guest_memfd_test.c  | 97 +++++++++++++++++++
 virt/kvm/guest_memfd.c                        | 33 ++++++-
 8 files changed, 236 insertions(+), 54 deletions(-)


base-commit: ac3fd01e4c1efce8f2c054cdeb2ddd2fc0fb150d
-- 
2.51.0