Re: [PATCH v3 4/4] mm: Apply vm_uffd_ops API to core mm

Peter Xu
2025-09-30T18:52:48Z

On Tue, Sep 30, 2025 at 11:23:19AM +0200, David Hildenbrand wrote:
> On 26.09.25 23:16, Peter Xu wrote:
> > Move userfaultfd core to use new vm_uffd_ops API. After this change file
> > systems that implement vm_operations_struct can start using new API for
> > userfaultfd operations.
> >
> > When at it, moving vma_can_userfault() into mm/userfaultfd.c instead,
> > because it's getting too big. It's only used in slow paths so it shouldn't
> > be an issue. Move the pte marker check before wp_async, which might be
> > more intuitive because wp_async depends on pte markers. That shouldn't
> > cause any functional change though because only one check would take effect
> > depending on whether pte marker was selected in config.
> >
> > This will also remove quite some hard-coded checks for either shmem or
> > hugetlbfs. Now all the old checks should still work but with vm_uffd_ops.
> >
> > Note that anonymous memory will still need to be processed separately
> > because it doesn't have vm_ops at all.
> >
> > Reviewed-by: James Houghton <jthoughton@google.com>
> > Acked-by: Mike Rapoport <rppt@kernel.org>
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> > ---
>
> [...]
>
> > +++ b/mm/userfaultfd.c
> > @@ -20,6 +20,43 @@
> > #include "internal.h"
> > #include "swap.h"
> > +bool vma_can_userfault(struct vm_area_struct *vma, vm_flags_t vm_flags,
> > + bool wp_async)
> > +{
> > + unsigned long supported;
> > +
> > + if (vma->vm_flags & VM_DROPPABLE)
> > + return false;
> > +
> > + vm_flags &= __VM_UFFD_FLAGS;
> > +
> > +#ifndef CONFIG_PTE_MARKER_UFFD_WP
>
> While at it, you can turn that into an
> !IS_ENABLED(CONFIG_PTE_MARKER_UFFD_WP) to avoid the ifdef.
>
> > + /*
> > + * If user requested uffd-wp but not enabled pte markers for
> > + * uffd-wp, then any file system (like shmem or hugetlbfs) are not
> > + * supported but only anonymous.
> > + */
> > + if ((vm_flags & VM_UFFD_WP) && !vma_is_anonymous(vma))
> > + return false;
> > +#endif
> > + /*
> > + * If wp async enabled, and WP is the only mode enabled, allow any
> > + * memory type.
> > + */
> > + if (wp_async && (vm_flags == VM_UFFD_WP))
> > + return true;
>
>
> > +
> > + if (vma_is_anonymous(vma))
> > + /* Anonymous has no page cache, MINOR not supported */
> > + supported = VM_UFFD_MISSING | VM_UFFD_WP;
> > + else if (vma_get_uffd_ops(vma))
> > + supported = vma_get_uffd_ops(vma)->uffd_features;
> > + else
> > + return false;
>
> To avoid the hidde return here, I think you can just do
>
> supported = 0;
>
>
> Or even cleaner, just do
>
> unsigned long supported = 0
> ...
> if (vma_is_anonymous(vma))
> supported = ...
> else if (vma_get_uffd_ops(vma))
> supported = ...
> return ...
>
> > +
> > + return !(vm_flags & (~supported));
>
> I think this can just be:
>
> return !(vm_flags & ~supported);

Sure thing, I'll apply everything you mentioned above.

Thanks,

--
Peter Xu