mm/swap: fix pte_same_as_swp() not removing uffd-wp bit when compare

I found it by pure code review, that pte_same_as_swp() of unuse_vma()
didn't take uffd-wp bit into account when comparing ptes.
pte_same_as_swp() returning false negative could cause failure to
swapoff swap ptes that was wr-protected by userfaultfd.

Link: https://lkml.kernel.org/r/20210603180546.9083-1-peterx@redhat.com
Fixes: f45ec5ff ("userfaultfd: wp: support swap and page migration")
Signed-off-by: Peter Xu <peterx@redhat.com>
Acked-by: Hugh Dickins <hughd@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: <stable@vger.kernel.org>	[5.7+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/swapops.h

@@ -23,6 +23,16 @@
 #define SWP_TYPE_SHIFT	(BITS_PER_XA_VALUE - MAX_SWAPFILES_SHIFT)
 #define SWP_OFFSET_MASK	((1UL << SWP_TYPE_SHIFT) - 1)
 
+/* Clear all flags but only keep swp_entry_t related information */
+static inline pte_t pte_swp_clear_flags(pte_t pte)
+{
+	if (pte_swp_soft_dirty(pte))
+		pte = pte_swp_clear_soft_dirty(pte);
+	if (pte_swp_uffd_wp(pte))
+		pte = pte_swp_clear_uffd_wp(pte);
+	return pte;
+}
+
 /*
  * Store a type+offset into a swp_entry_t in an arch-independent format
  */
@@ -66,10 +76,7 @@ static inline swp_entry_t pte_to_swp_entry(pte_t pte)
 {
 	swp_entry_t arch_entry;
 
-	if (pte_swp_soft_dirty(pte))
-		pte = pte_swp_clear_soft_dirty(pte);
-	if (pte_swp_uffd_wp(pte))
-		pte = pte_swp_clear_uffd_wp(pte);
+	pte = pte_swp_clear_flags(pte);
 	arch_entry = __pte_to_swp_entry(pte);
 	return swp_entry(__swp_type(arch_entry), __swp_offset(arch_entry));
 }

mm/swapfile.c

@@ -1900,7 +1900,7 @@ unsigned int count_swap_pages(int type, int free)
 
 static inline int pte_same_as_swp(pte_t pte, pte_t swp_pte)
 {
-	return pte_same(pte_swp_clear_soft_dirty(pte), swp_pte);
+	return pte_same(pte_swp_clear_flags(pte), swp_pte);
 }
 
 /*