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>

3 files changed, 27 insertions, 9 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.
 	 */
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 0b5fb0d0a0c1..55157567dc4c 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1893,8 +1893,11 @@ static enum scan_result try_collapse_pte_mapped_thp(struct mm_struct *mm, unsign
 	if (!thp_vma_allowable_order(vma, vma->vm_flags, TVA_FORCED_COLLAPSE, PMD_ORDER))
 		return SCAN_VMA_CHECK;
 
-	/* Keep pmd pgtable for uffd-wp; see comment in retract_page_tables() */
-	if (userfaultfd_wp(vma))
+	/*
+	 * Keep pmd pgtable while the uffd bit is in use; see comment in
+	 * retract_page_tables().
+	 */
+	if (userfaultfd_protected(vma))
 		return SCAN_PTE_UFFD;
 
 	folio = filemap_lock_folio(vma->vm_file->f_mapping,
@@ -2107,13 +2110,14 @@ static bool file_backed_vma_is_retractable(struct vm_area_struct *vma)
 		return false;
 
 	/*
-	 * When a vma is registered with uffd-wp, we cannot recycle
+	 * When a vma is registered with uffd-wp or RWP, we cannot recycle
 	 * the page table because there may be pte markers installed.
-	 * Other vmas can still have the same file mapped hugely, but
-	 * skip this one: it will always be mapped in small page size
-	 * for uffd-wp registered ranges.
+	 * VM_UFFD_RWP ranges similarly rely on per-PTE uffd state
+	 * and cannot be recycled to a shared PMD. Other vmas can still
+	 * have the same file mapped hugely, but skip this one: it will
+	 * always be mapped in small page size for these registrations.
 	 */
-	if (userfaultfd_wp(vma))
+	if (userfaultfd_protected(vma))
 		return false;
 
 	/*
diff --git a/mm/rmap.c b/mm/rmap.c
index 50f973b63f1c..26166a6b8cb9 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1965,7 +1965,7 @@ static inline unsigned int folio_unmap_pte_batch(struct folio *folio,
 	if (pte_unused(pte))
 		return 1;
 
-	if (userfaultfd_wp(vma))
+	if (userfaultfd_protected(vma))
 		return 1;
 
 	/*