diff --git a/mm/swapfile.c b/mm/swapfile.c
index 1b8c339..17bc224 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1924,20 +1924,24 @@
 
 	/*
 	 * Find out how many pages are allowed for a single swap
-	 * device. There are two limiting factors: 1) the number of
-	 * bits for the swap offset in the swp_entry_t type and
-	 * 2) the number of bits in the a swap pte as defined by
-	 * the different architectures. In order to find the
-	 * largest possible bit mask a swap entry with swap type 0
+	 * device. There are three limiting factors: 1) the number
+	 * of bits for the swap offset in the swp_entry_t type, and
+	 * 2) the number of bits in the swap pte as defined by the
+	 * the different architectures, and 3) the number of free bits
+	 * in an exceptional radix_tree entry. In order to find the
+	 * largest possible bit mask, a swap entry with swap type 0
 	 * and swap offset ~0UL is created, encoded to a swap pte,
-	 * decoded to a swp_entry_t again and finally the swap
+	 * decoded to a swp_entry_t again, and finally the swap
 	 * offset is extracted. This will mask all the bits from
 	 * the initial ~0UL mask that can't be encoded in either
 	 * the swp_entry_t or the architecture definition of a
-	 * swap pte.
+	 * swap pte.  Then the same is done for a radix_tree entry.
 	 */
 	maxpages = swp_offset(pte_to_swp_entry(
-			swp_entry_to_pte(swp_entry(0, ~0UL)))) + 1;
+			swp_entry_to_pte(swp_entry(0, ~0UL))));
+	maxpages = swp_offset(radix_to_swp_entry(
+			swp_to_radix_entry(swp_entry(0, maxpages)))) + 1;
+
 	if (maxpages > swap_header->info.last_page) {
 		maxpages = swap_header->info.last_page + 1;
 		/* p->max is an unsigned int: don't overflow it */