[PATCH v2 0/4] mm/userfaultfd: modulize memory types

Peter Xu
2025-06-27T15:47:03Z

[based on latest akpm/mm-new of June 27th, commit 9be7387ae43f]

v2 changelog:
- Patch 1
- update English in commit log [David]
- move vm_uffd_ops definition to userfaultfd_k.h [Mike]
- Patch 4
- fix sparse warning on bitwise type conversions [syzbot]
- Commit message updates on explanation of vma_can_userfault check [James]

v1: https://lore.kernel.org/r/20250620190342.1780170-1-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.
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.

Hugetlbfs is still very special that it will only use partial of the
vm_uffd_ops API, due to similar reason why hugetlb_vm_op_fault() has a
BUG() and so far hard-coded into core mm. But this should still be better,
because at least hugetlbfs is still always involved in feature probing
(e.g. where it used to not support ZEROPAGE and we have a hard-coded line
to fail that, and some more). Meanwhile after this series, shmem will be
completely converted to the new vm_uffd_ops API; the final vm_uffd_ops for
shmem looks like this:

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,
.uffd_copy = shmem_mfill_atomic_pte,
};

As I mentioned in one of my reply to Nikita, I don't like the current
interface of uffd_copy(), but this will be the minimum change version of
such API to support complete extrenal-module-ready userfaultfd. Here, very
minimal change will be needed from shmem side to support that.

Meanwhile, the vm_uffd_ops is also not the only place one will need to
provide to support userfaultfd. Normally vm_ops.fault() will also need to
be updated, but that's a generic function and it'll play together with the
new vm_uffd_ops to make everything fly.

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/shmem_fs.h | 14 -----
include/linux/userfaultfd_k.h | 98 +++++++++++++++++++----------
mm/hugetlb.c | 19 ++++++
mm/shmem.c | 28 ++++++++-
mm/userfaultfd.c | 115 +++++++++++++++++++++++++---------
6 files changed, 207 insertions(+), 76 deletions(-)

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2.49.0