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

[Nikita Kalyazin <kalyazin@amazon.com>]

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/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
>