Re: [PATCH v4 07/15] userfaultfd: introduce vm_uffd_ops

[Mike Rapoport <rppt@kernel.org>]

On Thu, Apr 02, 2026 at 03:32:58PM -0400, Tal Zussman wrote:
> Hi Mike,
> 
> On 4/2/26 12:11 AM, Mike Rapoport wrote:
> > @@ -2021,34 +2040,33 @@ ssize_t move_pages(struct userfaultfd_ctx *ctx, unsigned long dst_start,
> >  bool vma_can_userfault(struct vm_area_struct *vma, vm_flags_t vm_flags,
> >  		       bool wp_async)
> >  {
> > -	vm_flags &= __VM_UFFD_FLAGS;
> > +	const struct vm_uffd_ops *ops = vma_uffd_ops(vma);
> >  
> > -	if (vma->vm_flags & VM_DROPPABLE)
> > -		return false;
> > -
> > -	if ((vm_flags & VM_UFFD_MINOR) &&
> > -	    (!is_vm_hugetlb_page(vma) && !vma_is_shmem(vma)))
> > -		return false;
> > +	vm_flags &= __VM_UFFD_FLAGS;
> >  
> >  	/*
> > -	 * If wp async enabled, and WP is the only mode enabled, allow any
> > +	 * If WP is the only mode enabled and context is wp async, allow any
> >  	 * memory type.
> >  	 */
> >  	if (wp_async && (vm_flags == VM_UFFD_WP))
> >  		return true;
> >  
> > +	/* For any other mode reject VMAs that don't implement vm_uffd_ops */
> > +	if (!ops)
> > +		return false;
> > +
> > +	if (vma->vm_flags & VM_DROPPABLE)
> > +		return false;
> > +
> 
> Doesn't moving the VM_DROPPABLE check after the wp_async early return make
> the issue David raised in [1] possible again (i.e. moving swapbacked pages
> into a droppable region using UFFDIO_MOVE)? With this change, VM_DROPPABLE
> pages can now be registered with uffd under wp_async, which allows
> UFFDIO_MOVE.
> 
> I think the VM_DROPPABLE check should go right before the wp_async check.
> And it would probably benefit from a comment...

You are right.

Andrew, if it's not too late can you please add this fixup:

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index f1828106484b..75290483ffc7 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -2019,6 +2019,9 @@ 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)
+		return false;
+
 	vm_flags &= __VM_UFFD_FLAGS;
 
 	/*
@@ -2032,9 +2035,6 @@ bool vma_can_userfault(struct vm_area_struct *vma, vm_flags_t vm_flags,
 	if (!ops)
 		return false;
 
-	if (vma->vm_flags & VM_DROPPABLE)
-		return false;
-
 	/*
 	 * If user requested uffd-wp but not enabled pte markers for
 	 * uffd-wp, then only anonymous memory is supported

 
> Thanks,
> Tal

-- 
Sincerely yours,
Mike.