Re: [PATCH v2 3/5] mm: userfaultfd: allow to register continue for guest_memfd

[Nikita Kalyazin]

On 02/04/2025 22:25, James Houghton wrote:
> On Wed, Apr 2, 2025 at 9:08 AM Nikita Kalyazin <kalyazin@xxxxxxxxxx> wrote:
> > Signed-off-by: Nikita Kalyazin <kalyazin@xxxxxxxxxx>
> > ---
> >   include/linux/userfaultfd_k.h | 13 ++++++++++++-
> >   1 file changed, 12 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
> > index 75342022d144..bc184edfbb85 100644
> > --- a/include/linux/userfaultfd_k.h
> > +++ b/include/linux/userfaultfd_k.h
> > @@ -212,6 +212,10 @@ static inline bool userfaultfd_armed(struct vm_area_struct *vma)
> >          return vma->vm_flags & __VM_UFFD_FLAGS;
> >   }
> >
> > +#ifdef CONFIG_KVM_PRIVATE_MEM
> > +bool kvm_gmem_vma_is_gmem(struct vm_area_struct *vma);
> > +#endif
> > +
> >   static inline bool vma_can_userfault(struct vm_area_struct *vma,
> >                                       unsigned long vm_flags,
> >                                       bool wp_async)
> > @@ -222,7 +226,11 @@ static inline bool vma_can_userfault(struct vm_area_struct *vma,
> >                  return false;
> >
> >          if ((vm_flags & VM_UFFD_MINOR) &&
> > -           (!is_vm_hugetlb_page(vma) && !vma_is_shmem(vma)))
> > +           (!is_vm_hugetlb_page(vma) && !vma_is_shmem(vma))
> > +#ifdef CONFIG_KVM_PRIVATE_MEM
> > +            && !kvm_gmem_vma_is_gmem(vma)
> > +#endif
>
> Maybe a better way to do this is to add a vm_ops->can_userfault() or
> something, so we could write something like this:
>
> if (vma->vm_ops && !vma->vm_ops->can_userfault)
>    return false;
> if (vma->vm_ops && !vma->vm_ops->can_userfault(vm_flags))
>    return false;

I like that, thanks!  What do you see passing vm_flags being useful for? 
 Shall we pass the entire vma struct like in most of other callbacks?

> And shmem/hugetlbfs can advertise support for everything they already
> support that way.
>
> > +           )
> >                  return false;
> >
> >          /*
> > @@ -244,6 +252,9 @@ static inline bool vma_can_userfault(struct vm_area_struct *vma,
> >
> >          /* By default, allow any of anon|shmem|hugetlb */
> >          return vma_is_anonymous(vma) || is_vm_hugetlb_page(vma) ||
> > +#ifdef CONFIG_KVM_PRIVATE_MEM
> > +           kvm_gmem_vma_is_gmem(vma) ||
> > +#endif
> >              vma_is_shmem(vma);
> >   }
> >
> > --
> > 2.47.1