[PATCH 07/10] mm/hugetlb: tolerate uffd bits set without uffd for PROT_NONE PTEs

[David Hildenbrand (Arm)]

The whole uffd handling when a VMA is unregistered from UFFD is shaky:
there are still some scenarios where we might have stale uffd PTE bits
in VMAs no longer registered for UFFD, which confuses other code.

Let's just tolerate that for now and simply clear the PTE bit. Add a
TODO to really clean up any uffd leftovers when unregistering a VMA from
userfaultfd.

The handling is now more similar to what we have in handle_pte_fault().

Fixes: 2d427bb01639 ("mm/userfaultfd: add RWP fault delivery and expose UFFDIO_REGISTER_MODE_RWP")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
---
 mm/hugetlb.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index c3823f1958568..ba6860b9b7f4c 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -6127,16 +6127,20 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 	 * NUMA hinting), so let normal hugetlb fault handling proceed.
 	 */
 	if (pte_protnone(vmf.orig_pte) && vma_is_accessible(vma) &&
-	    userfaultfd_rwp(vma) && huge_pte_uffd(vmf.orig_pte)) {
+	    huge_pte_uffd(vmf.orig_pte)) {
 		spinlock_t *ptl;
 		pte_t pte;
 
 		/* Sync: drop hugetlb locks before blocking in handle_userfault() */
-		if (!userfaultfd_rwp_async(vma))
+		if (userfaultfd_rwp(vma) && !userfaultfd_rwp_async(vma))
 			return hugetlb_handle_userfault(&vmf, mapping, VM_UFFD_RWP);
 
 		ptl = huge_pte_lock(h, mm, vmf.pte);
 		pte = huge_ptep_get(mm, vmf.address, vmf.pte);
+		/*
+		 * TODO: fix all cases where uffd PTE bits remain in VMAs that
+		 * are no longer registered with uffd.
+		 */
 		if (pte_protnone(pte) && huge_pte_uffd(pte)) {
 			unsigned int shift = huge_page_shift(h);
 

-- 
2.43.0