On 17/11/2025 11:46, Mike Rapoport wrote:
From: "Mike Rapoport (Microsoft)" <[email protected]>
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.
Hi Mike,
Thanks for posting it!
In our use case, Firecracker snapshot-restore using UFFD [1], we will
use UFFD minor/continue to respond to guest_memfd faults in user
mappings primarily due to VMM accesses that are required for PV (virtio)
device emulation and also KVM accesses when decoding MMIO operations on x86.
Nikita
[1]
https://github.com/firecracker-microvm/firecracker/blob/main/docs/snapshotting/handling-page-faults-on-snapshot-resume.md
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
