On Fri, Oct 02, 2026 at 09:04:17AM +0200, David Hildenbrand (Arm) wrote:
> On 9/17/26 18:22, Lorenzo Stoakes (ARM) wrote:
> > Rather than directly checking VMA flags, use the newly introduced
> > vma_is_kernel_owned() and vma_is_persistent() helpers in userfaultfd when
> > assessing VMA suitability for userfaultfd and UFFDIO_MOVE.
> >
> > Update vma_move_compatible() so it's expressed in terms of VMA
> > characteristics rather than arbitrary flags.
> >
> > Additionally, update the use of the deprecated VMA flag API when checking
> > VMA_SHADOW_STACK_BIT.
> >
> > A VMA_IO_BIT check is no longer required but that is fine as a hard
> > invariant has been established that only kernel-owned mappings may set
> > VMA_IO_BIT so the check is now redundant.
> >
> > Signed-off-by: Lorenzo Stoakes (ARM) <[email protected]>
> > ---
> > mm/userfaultfd.c | 21 +++++++++++++++------
> > 1 file changed, 15 insertions(+), 6 deletions(-)
> >
> > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> > index 949017e60608..ddf0a4a3d399 100644
> > --- a/mm/userfaultfd.c
> > +++ b/mm/userfaultfd.c
> > @@ -1754,10 +1754,18 @@ static inline bool move_splits_huge_pmd(unsigned
> > long dst_addr,
> > }
> > #endif
> >
> > -static inline bool vma_move_compatible(struct vm_area_struct *vma)
> > +static inline bool vma_move_compatible(const struct vm_area_struct *vma)
> > {
> > - return !(vma->vm_flags & (VM_PFNMAP | VM_IO | VM_HUGETLB |
> > - VM_MIXEDMAP | VM_SHADOW_STACK));
> > + /* uffd is generally incompatible with kernel-owned mappings. */
> > + if (vma_is_kernel_owned(vma))
> > + return false;
> > + /* The shadow stack should not be written to by userspace. */
> > + if (vma_test_single_mask(vma, VMA_SHADOW_STACK))
> > + return false;
> > + /* hugetlb mappings cannot be safely moved. */
> > + if (vma_is_hugetlb(vma))
> > + return false;
> > + return true;
>
> return !vma_is_hugetlb(vma);
Ack, done for v4.
>
> > }
> >
> > static int validate_move_areas(struct userfaultfd_ctx *ctx,
> > @@ -2146,10 +2154,11 @@ static bool vma_can_userfault(struct vm_area_struct
> > *vma, vm_flags_t vm_flags,
> > {
> > const struct vm_uffd_ops *ops = vma_uffd_ops(vma);
> >
> > - if (vma->vm_flags & (VM_DROPPABLE | VM_SHADOW_STACK))
> > + /* Non-persistent memory is inherently not controllable by userspace. */
> > + if (!vma_is_persistent(vma))
> > return false;
>
> Completely confusing. Just avoid the helper and open-code this here.
See the other thread on this. I think with this as vma_is_volatile() this is a
lot clearer.
>
> > -
> > - if (!vma_is_hugetlb(vma) && (vma->vm_flags & VM_SPECIAL))
> > + /* The shadow stack should not be written to by userspace. */
> > + if (vma_test_single_mask(vma, VMA_SHADOW_STACK))
> > return false;
> >
> > vm_flags &= __VM_UFFD_FLAGS;
> >
>
> Other stuff looks much better to me.
Thanks!
>
>
> --
> Cheers,
>
> David
--
Cheers, Lorenzo
