Re: [PATCH v3 0/4] mm/userfaultfd: modulize memory types

Liam R. Howlett
2025-09-30T19:50:07Z

* Peter Xu <peterx@redhat.com> [250926 17:17]:
> [based on latest akpm/mm-new of Sep 26th, commit e612c80ae0aeb]
>
> v3 changelog:
> - Fixed checkpatch issues on spaces or typedef
> - Dropped uffd_copy() API
> - Refined commit messages here and there to reflect the removal of uffd_copy()
>
> v1: https://lore.kernel.org/r/20250620190342.1780170-1-peterx@redhat.com
> v2: https://lore.kernel.org/r/20250627154655.2085903-1-peterx@redhat.com
>
> This series is an alternative proposal of what Nikita proposed here on the
> initial three patches:
>
> https://lore.kernel.org/r/20250404154352.23078-1-kalyazin@amazon.com
>
> This is not yet relevant to any guest-memfd support, but paving way for it.

It would be much easier to review this with the guest-memfd support in
this patch set. Any chance of including the target user?

> Here, the major goal is to make kernel modules be able to opt-in with any
> form of userfaultfd supports, like guest-memfd. This alternative option
> should hopefully be cleaner, and avoid leaking userfault details into
> vm_ops.fault().
>
> It also means this series does not depend on anything. It's a pure
> refactoring of userfaultfd internals to provide a generic API, so that
> other types of files, especially RAM based, can support userfaultfd without
> touching mm/ at all.
>
> To achieve it, this series introduced a file operation called vm_uffd_ops.
> The ops needs to be provided when a file type supports any of userfaultfd.
>
> With that, I moved both hugetlbfs and shmem over, whenever possible. So
> far due to concerns on exposing an uffd_copy() API, the MISSING faults are
> still separately processed and can only be done within mm/. Hugetlbfs kept
> its special paths untouched.
>
> An example of shmem uffd_ops:
>
> static const vm_uffd_ops shmem_uffd_ops = {
> .uffd_features = __VM_UFFD_FLAGS,
> .uffd_ioctls = BIT(_UFFDIO_COPY) |
> BIT(_UFFDIO_ZEROPAGE) |
> BIT(_UFFDIO_WRITEPROTECT) |
> BIT(_UFFDIO_CONTINUE) |
> BIT(_UFFDIO_POISON),
> .uffd_get_folio = shmem_uffd_get_folio,
> };
>
> No functional change expected at all after the whole series applied. There
> might be some slightly stricter check on uffd ops here and there in the
> last patch, but that really shouldn't stand out anywhere to anyone.
>
> For testing: besides the cross-compilation tests, I did also try with
> uffd-stress in a VM to measure any perf difference before/after the change;
> The static call becomes a pointer now. I really cannot measure anything
> different, which is more or less expected.
>
> Comments welcomed, thanks.
>
> Peter Xu (4):
> mm: Introduce vm_uffd_ops API
> mm/shmem: Support vm_uffd_ops API
> mm/hugetlb: Support vm_uffd_ops API
> mm: Apply vm_uffd_ops API to core mm
>
> include/linux/mm.h | 9 +++
> include/linux/userfaultfd_k.h | 83 ++++++++++++++++-----------
> mm/hugetlb.c | 19 +++++++
> mm/shmem.c | 25 +++++++++
> mm/userfaultfd.c | 102 ++++++++++++++++++++++++++--------
> 5 files changed, 181 insertions(+), 57 deletions(-)
>
> --
> 2.50.1
>