Re: [PATCH v2 07/14] mm: handle VM_UFFD_RWP in khugepaged, rmap, and GUP

[Mike Rapoport <rppt@kernel.org>]

On Fri, May 08, 2026 at 04:55:19PM +0100, Kiryl Shutsemau (Meta) wrote:
> Three mm paths outside the fault handler gate on the uffd PTE bit
> today: khugepaged (skip collapse on ranges carrying markers), rmap
> (cap unmap batching), and GUP (force a fault through
> gup_can_follow_protnone). Extend each to treat VM_UFFD_RWP the same
> as VM_UFFD_WP; otherwise per-PTE RWP state is silently destroyed or
> bypassed.
> 
> khugepaged: try_collapse_pte_mapped_thp() and
> file_backed_vma_is_retractable() already refuse to collapse or
> retract page tables on ranges carrying the uffd PTE bit. Broaden the
> VMA predicate from userfaultfd_wp() to userfaultfd_protected() so
> VM_UFFD_RWP ranges get the same protection. hpage_collapse_scan_pmd()
> needs no change — its existing pte_uffd() check already catches an
> RWP PTE because it carries the uffd bit.
> 
> rmap: folio_unmap_pte_batch() caps batching at 1 for VM_UFFD_RWP so
> the restore path handles each PTE with its own marker.
> 
> GUP: gup_can_follow_protnone() forces a fault on VM_UFFD_RWP VMAs
> regardless of FOLL_HONOR_NUMA_FAULT. RWP uses protnone as an
> access-tracking marker, not for NUMA hinting, so any GUP — read or
> write — must go through the userfaultfd fault path.
> 
> Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
> Assisted-by: Claude:claude-opus-4-6

Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

> ---
>  include/linux/mm.h | 10 +++++++++-
>  mm/khugepaged.c    | 18 +++++++++++-------
>  mm/rmap.c          |  2 +-
>  3 files changed, 21 insertions(+), 9 deletions(-)

-- 
Sincerely yours,
Mike.