Re: [PATCH 09/10] mm/hugetlb: don't leave a uffd-wp marker when zapping an RWP entry

[Kiryl Shutsemau]

On Thu, Oct 15, 2026 at 06:07:15PM +0200, David Hildenbrand (Arm) wrote:
> From: Josef Bacik <josef@xxxxxxxxxxxxxx>
> 
> An entry that is protected with UFFDIO_RWPROTECT is PAGE_NONE with the
> uffd bit set, the bit that userfaultfd write protection uses too.  When
> __unmap_hugepage_range() zaps an entry with that bit it leaves a
> PTE_MARKER_UFFD_WP behind, whatever the VMA is registered for.
> 
> In a VMA registered with UFFDIO_REGISTER_MODE_RWP that marker means
> nothing.  The next fault has hugetlb_no_page() turn it into a present,
> read-only entry with the uffd bit.  That is not an RWP entry, since it is
> not PAGE_NONE, and the VMA has no VM_UFFD_WP (the two modes can not be
> registered together).  An earlier fix solved some side effects, now just
> stop the zap from wrongly installing a marker.
> 
> Leave a marker only if the VMA is registered for write protection, both
> for present and for non-present entries.  The zap of ordinary page
> tables has the same test, in cond_install_uffd_wp_ptes(), and
> hugetlb_change_protection() says that "uffd_rwp does not install markers".
> 
> After a zap the range of an RWP VMA is simply empty, as it is for
> anonymous memory and shmem.  Nothing changes for VMAs with VM_UFFD_WP.  In
> a VMA that is not registered at all a leftover uffd bit or marker is now
> dropped by the zap and no longer kept alive.
> 
> Original fix generated by an LLM and sent in private by Josef.
> 
> Fixes: 2d427bb01639 ("mm/userfaultfd: add RWP fault delivery and expose UFFDIO_REGISTER_MODE_RWP")
> Cc: stable@xxxxxxxxxxxxxxx
> Reported-by: Chris Mason <mason@xxxxxxxxxx>
> Signed-off-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
> [ david: a bit of description overhaul+simplification ]
> Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>

Acked-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>

-- 
  Kiryl Shutsemau / Kirill A. Shutemov