mm: handle VM_UFFD_RWP in khugepaged, rmap, and GUP

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.

Link: https://lore.kernel.org/20260708111417.173443-9-kirill@shutemov.name
Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Assisted-by: Claude:claude-opus-4-6
Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: James Houghton <jthoughton@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam Howlett <liam@infradead.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: SeongJae Park <sj@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 15 insertions, 1 deletions

diff --git a/include/linux/mm.h b/include/linux/mm.h
index be4457269a38..25e669632d2c 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -4654,12 +4654,26 @@ static inline int vm_fault_to_errno(vm_fault_t vm_fault, int foll_flags)
 
 /*
  * Indicates whether GUP can follow a PROT_NONE mapped page, or whether
- * a (NUMA hinting) fault is required.
+ * a (NUMA hinting or userfaultfd RWP) fault is required.
  */
 static inline bool gup_can_follow_protnone(const struct vm_area_struct *vma,
 					   unsigned int flags)
 {
 	/*
+	 * VM_UFFD_RWP uses protnone as an access-tracking marker, not for
+	 * NUMA hinting. GUP must always take a fault so the access is
+	 * delivered to userfaultfd, regardless of FOLL_HONOR_NUMA_FAULT.
+	 *
+	 * Only do so while the VMA is accessible. If it has been made
+	 * inaccessible (e.g. mprotect(PROT_NONE)), fall through to the guard
+	 * below: forcing a fault there would loop, as handle_mm_fault() makes
+	 * no progress on protnone in an inaccessible VMA, and the access is
+	 * denied regardless of RWP anyway.
+	 */
+	if (vma_test_single_mask(vma, VMA_UFFD_RWP) && vma_is_accessible(vma))
+		return false;
+
+	/*
 	 * If callers don't want to honor NUMA hinting faults, no need to
 	 * determine if we would actually have to trigger a NUMA hinting fault.
 	 */