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

[David Hildenbrand]

On 30.09.25 20:48, Peter Xu wrote:
> 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?

The former would make things clearer when we are at least not talking 
about uffd features.

> > 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.

I rather dislike the current split approach between vm-flags and ioctls.

I briefly thought about abstracting it for internal purposes further and 
just have some internal backend ("memory type") flags.

UFFD_BACKEND_FEAT_MISSING -> _UFFDIO_COPY and VM_UFFD_MISSING
UFFD_BACKEND_FEAT_ZEROPAGE -> _UFDIO_ZEROPAGE
UFFD_BACKEND_FEAT_WP -> _UFFDIO_WRITEPROTECT and VM_UFFD_WP
UFFD_BACKEND_FEAT_MINOR -> _UFFDIO_CONTINUE and VM_UFFD_MINOR
UFFD_BACKEND_FEAT_POISON -> _UFFDIO_POISON
> 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.

POISON is not a VM_ flag, so that wouldn't work completely, right?

As a side note, hugetlbfs support for ZEROPAGE should be fairly easy: 
similar to shmem support, simply allocate a zeroed hugetlb folio.

> Do you mind I still keep it as-is?

I would prefer if we find a way to not have this dependency between both 
feature/ioctl thingies. It just looks rather odd.

But let's hear if there are other opinions.

--
Cheers

David / dhildenb