Re: [PATCH v4 0/4] mm/userfaultfd: modulize memory types

[Peter Xu <peterx@redhat.com>]

On Mon, Oct 20, 2025 at 03:34:47PM +0200, David Hildenbrand wrote:
> On 15.10.25 01:14, Peter Xu wrote:
> > [based on latest akpm/mm-new of Oct 14th, commit 36c6c5ce1b275]
> > 
> > v4:
> > - Some cleanups within vma_can_userfault() [David]
> > - Rename uffd_get_folio() to minor_get_folio() [David]
> > - Remove uffd_features in vm_uffd_ops, deduce it from supported ioctls [David]
> > 
> > v1: https://lore.kernel.org/r/20250620190342.1780170-1-peterx@redhat.com
> > v2: https://lore.kernel.org/r/20250627154655.2085903-1-peterx@redhat.com
> > v3: https://lore.kernel.org/r/20250926211650.525109-1-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, whenever possible.  So
> > far due to concerns on exposing an uffd_copy() API, the MISSING faults are
> > still separately processed and can only be done within mm/.  Hugetlbfs kept
> > its special paths untouched.
> > 
> > An example of shmem uffd_ops:
> > 
> > static const struct vm_uffd_ops shmem_uffd_ops = {
> > 	.supported_ioctls	=	BIT(_UFFDIO_COPY) |
> > 					BIT(_UFFDIO_ZEROPAGE) |
> > 					BIT(_UFFDIO_WRITEPROTECT) |
> > 					BIT(_UFFDIO_CONTINUE) |
> > 					BIT(_UFFDIO_POISON),
> > 	.minor_get_folio	=	shmem_uffd_get_folio,
> > };
> 
> This looks better than the previous version to me.
> 
> Long term the goal should be to move all hugetlb/shmem specific stuff out of
> mm/hugetlb.c and of course, we won't be adding any new ones to
> mm/userfaultfd.c
> 
> I agree with Liam that a better interface could be providing default
> handlers for the separate ioctls [1], but there is always the option to
> evolve this interface into something like that later.

Thanks for accepting this current form.

> 
> 
> [1] https://lkml.kernel.org/r/frnos5jtmlqvzpcrredcoummuzvllweku5dgp5ii5in6epwnw5@anu4dqsz6shy

I have replied to that, here:

https://lore.kernel.org/all/aOVEDii4HPB6outm@x1.local/

If we ignore hugetlbfs, most of the hooks may not be needed, as explained.

If we introduce hooks only for hugetlbfs, IMHO it's going backwards.  When
we want to get rid of hugetlbfs paths, we will have something more to get
rid of..

We can wait until some use cases to appear, that may help us to better
understand what might be more suitable to be abstracted. Especially, if
there will be any consumer for missing faults.  However if guest-memfd has
it optional, my expectation so far is there'll be no real demand on that
for the foreseeable future.

Thanks,

-- 
Peter Xu