Re: [PATCH v3 32/40] mm/uffd: use predicates for userfaultfd checks

On Fri, Oct 02, 2026 at 09:04:17AM +0200, David Hildenbrand (Arm) wrote:

On 9/17/26 18:22, Lorenzo Stoakes (ARM) wrote:

quoted

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) <ljs@kernel.org>
---
 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.

quoted

 }

 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.

quoted

-
-	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