diff --git a/mm/swapfile.c b/mm/swapfile.c
index 4a98612..84374d8 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -2161,7 +2161,11 @@
 }
 
 /*
- * increase reference count of swap entry by 1.
+ * Increase reference count of swap entry by 1.
+ * Returns 0 for success, or -ENOMEM if a swap_count_continuation is required
+ * but could not be atomically allocated.  Returns 0, just as if it succeeded,
+ * if __swap_duplicate() fails for another reason (-EINVAL or -ENOENT), which
+ * might occur if a page table entry has got corrupted.
  */
 int swap_duplicate(swp_entry_t entry)
 {