2 files changed, 9 insertions, 11 deletions

diff --git a/include/linux/swapops.h b/include/linux/swapops.h
index 792d16d9cbc7..47ead515c811 100644
--- a/include/linux/swapops.h
+++ b/include/linux/swapops.h
@@ -9,13 +9,15 @@
  * get good packing density in that tree, so the index should be dense in
  * the low-order bits.
  *
- * We arrange the `type' and `offset' fields so that `type' is at the five
+ * We arrange the `type' and `offset' fields so that `type' is at the seven
  * high-order bits of the swp_entry_t and `offset' is right-aligned in the
- * remaining bits.
+ * remaining bits.  Although `type' itself needs only five bits, we allow for
+ * shmem/tmpfs to shift it all up a further two bits: see swp_to_radix_entry().
  *
  * swp_entry_t's are *never* stored anywhere in their arch-dependent format.
  */
-#define SWP_TYPE_SHIFT(e)       (sizeof(e.val) * 8 - MAX_SWAPFILES_SHIFT)
+#define SWP_TYPE_SHIFT(e)       ((sizeof(e.val) * 8) - \
+                        (MAX_SWAPFILES_SHIFT + RADIX_TREE_EXCEPTIONAL_SHIFT))
 #define SWP_OFFSET_MASK(e)      ((1UL << SWP_TYPE_SHIFT(e)) - 1)
 
 /*
diff --git a/mm/swapfile.c b/mm/swapfile.c
index de5bc51c4a66..71373d03fcee 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1916,24 +1916,20 @@ static unsigned long read_swap_header(struct swap_info_struct *p,
 
         /*
          * Find out how many pages are allowed for a single swap
-         * device. There are three limiting factors: 1) the number
+         * 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 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
+         * different architectures. 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
          * 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.  Then the same is done for a radix_tree entry.
+         * swap pte.
          */
         maxpages = swp_offset(pte_to_swp_entry(
-                        swp_entry_to_pte(swp_entry(0, ~0UL))));
-        maxpages = swp_offset(radix_to_swp_entry(
-                        swp_to_radix_entry(swp_entry(0, maxpages)))) + 1;
-
+                        swp_entry_to_pte(swp_entry(0, ~0UL)))) + 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 */