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

[James Houghton <jthoughton@google.com>]

On Wed, Apr 2, 2025 at 9:08 AM Nikita Kalyazin <kalyazin@amazon.com> wrote:
>
> Signed-off-by: Nikita Kalyazin <kalyazin@amazon.com>
> ---
>  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;

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
>