Re: [PATCH v2 1/4] mm: Introduce vm_uffd_ops API

Peter Xu
2025-07-04T15:00:02Z

On Fri, Jul 04, 2025 at 11:34:15AM +0200, David Hildenbrand wrote:
> On 03.07.25 19:48, Mike Rapoport wrote:
> > On Wed, Jul 02, 2025 at 03:46:57PM -0400, Peter Xu wrote:
> > > On Wed, Jul 02, 2025 at 08:39:32PM +0300, Mike Rapoport wrote:
> > >
> > > [...]
> > >
> > > > > The main target of this change is the implementation of UFFD for
> > > > > KVM/guest_memfd (examples: [1], [2]) to avoid bringing KVM-specific code
> > > > > into the mm codebase. We usually mean KVM by the "drivers" in this context,
> > > > > and it is already somewhat "knowledgeable" of the mm. I don't think there
> > > > > are existing use cases for other drivers to implement this at the moment.
> > > > >
> > > > > Although I can't see new exports in this series, there is now a way to limit
> > > > > exports to particular modules [3]. Would it help if we only do it for KVM
> > > > > initially (if/when actually needed)?
> > > >
> > > > There were talks about pulling out guest_memfd core into mm, but I don't
> > > > remember patches about it. If parts of guest_memfd were already in mm/ that
> > > > would make easier to export uffd ops to it.
> > >
> > > Do we have a link to such discussion? I'm also curious whether that idea
> > > was acknowledged by KVM maintainers.
> >
> > AFAIR it was discussed at one of David's guest_memfd calls
>
> While it was discussed in the call a couple of times in different context
> (guest_memfd as a library / guest_memfd shim), I think we already discussed
> it back at LPC last year.
>
> One of the main reasons for doing that is supporting guest_memfd in other
> hypervisors -- the gunyah hypervisor in the kernel wants to make use of it
> as well.

I see, thanks for the info. I found the series, it's here:

https://lore.kernel.org/all/20241113-guestmem-library-v3-0-71fdee85676b@quicinc.com/

Here, the question is whether do we still want to keep explicit calls to
shmem, hugetlbfs and in the future, guest-memfd. The library-ize of
guest-memfd doesn't change a huge lot on answering this question, IIUC.

It definitely reduces the use of mfill_atomic_install_pte() so that we
don't need to export it.

However if we want to generalize userfaultfd capability for a type of
memory, we will still need something like the vm_uffd_ops hook to report
such information. It means drivers can still overwrite these, with/without
an exported mfill_atomic_install_pte() functions. I'm not sure whether
that eases the concern.

So to me, generalizing the mem type looks helpful with/without moving
guest-memfd under mm/.

We do have the option to keep hard-code guest-memfd like shmem or
hugetlbfs. This is still "doable", but this likely means guest-memfd
support for userfaultfd needs to be done after that work. I did quickly
check the status of gunyah hypervisor [1,2,3], I found that all of the
efforts are not yet continued in 2025. The hypervisor last update was Jan
2024 with a batch push [1].

I still prefer generalizing uffd capabilities using the ops. That makes
guest-memfd support on MINOR not be blocked and it should be able to be
done concurrently v.s. guest-memfd library. If guest-memfd library idea
didn't move on, it's non-issue either.

I've considered dropping uffd_copy() and MISSING support for vm_uffd_ops if
I'm going to repost - that looks like the only thing that people are
against with, even though that is not my preference, as that'll make the
API half-broken on its own. Said that, I still prefer this against
hard-code and/or use CONFIG_GUESTMEM in userfaultfd code.

I'll wait for a few more days to see whether there's comment on above plan
to drop uffd_copy().

Thanks,

[1] https://github.com/quic/gunyah-hypervisor/tree/develop/hyp
[2] https://lore.kernel.org/all/20240516143356.1739402-1-quic_svaddagi@quicinc.com/
[3] https://lore.kernel.org/lkml/20240222-gunyah-v17-0-1e9da6763d38@quicinc.com

--
Peter Xu