[PATCH v2 0/5] mm, kvm: add guest_memfd support for uffd minor faults

Mike Rapoport
2025-11-25T18:38:49Z

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_shmem_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, 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.

v2 changes:
* 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 | 2 +
mm/shmem.c | 20 +++-
mm/userfaultfd.c | 80 +++++++++++---
.../testing/selftests/kvm/guest_memfd_test.c | 103 ++++++++++++++++++
virt/kvm/guest_memfd.c | 28 +++++
8 files changed, 236 insertions(+), 52 deletions(-)

base-commit: 6a23ae0a96a600d1d12557add110e0bb6e32730c
--
2.50.1