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

Peter Xu
2025-09-30T18:48:57Z

On Tue, Sep 30, 2025 at 11:36:53AM +0200, David Hildenbrand wrote:
> > +/* 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.

Indeed it's slightly confusing. However uffd_vm_flags is confusing in
another way, where it seems to imply some flags similar to vm_flags that is
prone to change.

How about uffd_vm_flags_supported / uffd_modes_supported?

>
> 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

Yes we can deduce that, but it'll be unclear then when one stares at a
bunch of ioctls and cannot easily digest the modes the memory type
supports. Here, the modes should be the most straightforward way to
describe the capability of a memory type.

If hugetlbfs supported ZEROPAGE, then we can deduce the ioctls the other
way round, and we can drop the uffd_ioctls. However we need the ioctls now
for hugetlbfs to make everything generic.

Do you mind I still keep it as-is? So far that's still the clearest I can
think of. It's only set when some support is added to a memory type, so
it's a one-time shot.

Thanks,

--
Peter Xu