Re: [PATCH v3 1/4] mm: Introduce vm_uffd_ops API

David Hildenbrand
2025-09-30T09:37:02Z

On 26.09.25 23:16, Peter Xu wrote:
> Currently, most of the userfaultfd features are implemented directly in the
> core mm. It will invoke VMA specific functions whenever necessary. So far
> it is fine because it almost only interacts with shmem and hugetlbfs.
>
> Introduce a generic userfaultfd API extension for vm_operations_struct,
> so that any code that implements vm_operations_struct (including kernel
> modules that can be compiled separately from the kernel core) can support
> userfaults without modifying the core files.
>
> With this API applied, if a module wants to support userfaultfd, the
> module should only need to properly define vm_uffd_ops and hook it to
> vm_operations_struct, instead of changing anything in core mm.
>
> This API will not work for anonymous memory. Handling of userfault
> operations for anonymous memory remains unchanged in core mm.
>
> Due to a security concern while reviewing older versions of this series
> [1], uffd_copy() will be temprorarily removed. IOW, so far MISSING-capable
> memory types can only be hard-coded and implemented in mm/. It would also
> affect UFFDIO_COPY and UFFDIO_ZEROPAGE. Other functions should still be
> able to be provided from vm_uffd_ops.
>
> Introduces the API only so that existing userfaultfd users can be moved
> over without breaking them.
>
> [1] https://lore.kernel.org/all/20250627154655.2085903-1-peterx@redhat.com/
>

Looks much better with the uffdio_copy stuff removed for now.

> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
> include/linux/mm.h | 9 +++++++++
> include/linux/userfaultfd_k.h | 37 +++++++++++++++++++++++++++++++++++
> 2 files changed, 46 insertions(+)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 6b6c6980f46c2..8afb93387e2c6 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -620,6 +620,8 @@ struct vm_fault {
> */
> };
>
> +struct vm_uffd_ops;
> +
> /*
> * These are the virtual MM functions - opening of an area, closing and
> * unmapping it (needed to keep files on disk up-to-date etc), pointer
> @@ -705,6 +707,13 @@ struct vm_operations_struct {
> struct page *(*find_normal_page)(struct vm_area_struct *vma,
> unsigned long addr);
> #endif /* CONFIG_FIND_NORMAL_PAGE */
> +#ifdef CONFIG_USERFAULTFD
> + /*
> + * Userfaultfd related ops. Modules need to define this to support
> + * userfaultfd.
> + */
> + const struct vm_uffd_ops *userfaultfd_ops;
> +#endif
> };
>
> #ifdef CONFIG_NUMA_BALANCING
> diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
> index c0e716aec26aa..b1949d8611238 100644
> --- a/include/linux/userfaultfd_k.h
> +++ b/include/linux/userfaultfd_k.h
> @@ -92,6 +92,43 @@ enum mfill_atomic_mode {
> NR_MFILL_ATOMIC_MODES,
> };
>
> +/* VMA userfaultfd operations */
> +struct vm_uffd_ops {
> + /**
> + * @uffd_features: features supported in bitmask.
> + *
> + * When the ops is defined, the driver must set non-zero features
> + * to be a subset (or all) of: VM_UFFD_MISSING|WP|MINOR.
> + *
> + * NOTE: VM_UFFD_MISSING is still only supported under mm/ so far.
> + */
> + unsigned long uffd_features;

This variable name is a bit confusing , because it's all about vma
flags, not uffd features. Just reading the variable, I would rather
connect it to things like UFFD_FEATURE_WP_UNPOPULATED.

As currently used for VM flags, maybe you should call this

unsigned long uffd_vm_flags;

or sth like that.

I briefly wondered whether we could use actual UFFD_FEATURE_* here, but
they are rather unsuited for this case here (e.g., different feature
flags for hugetlb support/shmem support etc).

But reading "uffd_ioctls" below, can't we derive the suitable vma flags
from the supported ioctls?

_UFFDIO_COPY | _UFDIO_ZEROPAGE -> VM_UFFD_MISSING
_UFFDIO_WRITEPROTECT -> VM_UFFD_WP
_UFFDIO_CONTINUE -> VM_UFFD_MINOR

> + /**
> + * @uffd_ioctls: ioctls supported in bitmask.
> + *
> + * Userfaultfd ioctls supported by the module. Below will always
> + * be supported by default whenever a module provides vm_uffd_ops:
> + *
> + * _UFFDIO_API, _UFFDIO_REGISTER, _UFFDIO_UNREGISTER, _UFFDIO_WAKE
> + *
> + * The module needs to provide all the rest optionally supported
> + * ioctls. For example, when VM_UFFD_MINOR is supported,
> + * _UFFDIO_CONTINUE must be supported as an ioctl.
> + */
> + unsigned long uffd_ioctls;
> + /**
> + * uffd_get_folio: Handler to resolve UFFDIO_CONTINUE request.

Just wondering if we could incorporate the "continue" / "minor" aspect
into the callback name.

uffd_minor_get_folio / uffd_continue_get_folio

Or do you see use of that callback in the context of other uffd features?

--
Cheers

David / dhildenb