[PATCH v5 31/38] mm/uffd: use predicates for userfaultfd checks

[David Hildenbrand (Arm)]

On 10/9/26 20:04, Lorenzo Stoakes (ARM) wrote:
> Rather than directly checking VMA flags, use the newly introduced
> vma_is_mm_managed() and vma_is_mm_backed() 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 mm-managed mappings may not set
> VMA_IO_BIT so the check is now redundant.
> 
> Acked-by: Zi Yan <ziy at nvidia.com>
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs at kernel.org>
> ---
>  mm/userfaultfd.c | 19 +++++++++++++------
>  1 file changed, 13 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index 949017e60608..f33b04700838 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -1754,10 +1754,16 @@ 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 mappings not managed by core mm. */
> +	if (!vma_is_mm_managed(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. */
> +	return !vma_is_hugetlb(vma);
>  }
>  
>  static int validate_move_areas(struct userfaultfd_ctx *ctx,
> @@ -2146,10 +2152,11 @@ 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))
> +	/* Only mm-backed memory can have its faults handled by userspace. */
> +	if (!vma_is_mm_backed(vma))
>  		return false;

Again, for both I would prefer keeping the droppable check separate.

Acked-by: David Hildenbrand (Arm) <david at kernel.org>

-- 
Cheers,

David