On 17/11/2025 11:46, Mike Rapoport wrote:
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_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/snapshotti...
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