Re: [PATCH] userfaultfd: prevent registration of special VMAs

[Mike Rapoport]

On Thu, Jun 18, 2026 at 10:47:19AM +0200, David Hildenbrand (Arm) wrote:
> On 6/18/26 10:43, Mike Rapoport wrote:
> > On Thu, Jun 18, 2026 at 11:34:12AM +0300, Mike Rapoport wrote:
> >> On Thu, Jun 18, 2026 at 10:19:17AM +0200, David Hildenbrand (Arm) wrote:
> >>>
> >>> I'm sure you considered VM_SPECIAL, which additionally includes VM_DONTEXPAND.
> >>>
> >>> Would that be better, or what was the reason to allow VM_DONTEXPAND?
> >>
> >> By itself VM_DONTEXPAND won't matter, as uffd can't resize a VMA.
> >> But thinking more about it, it's better to make vma_can_userfault() more
> >> restrictive and just use VM_SPECIAL.
> > 
> > Ah, hugetlb sets VM_DONTEXPAND, so it must me excluded to allow uffd with
> > hugetlb.
> 
> It would probably be cleaner to just allow hugetlb, and then check for
> VM_SPECIAL if not hugetlb.

Cleaner in what sense?
Will be uglier for sure, just take a look at vma_can_userfault().
 
> -- 
> Cheers,
> David

-- 
Sincerely yours,
Mike.