diff --git a/storage/myisam/mi_extra.c b/storage/myisam/mi_extra.c
index ae584b06173..729174b6f88 100644
--- a/storage/myisam/mi_extra.c
+++ b/storage/myisam/mi_extra.c
@@ -350,11 +350,13 @@ int mi_extra(MI_INFO *info, enum ha_extra_function function, void *extra_arg)
 #ifdef HAVE_MMAP
     pthread_mutex_lock(&share->intern_lock);
     /*
-      Memory map the data file if it is not already mapped and if there
-      are no other threads using this table. intern_lock prevents other
-      threads from starting to use the table while we are mapping it.
+      Memory map the data file if it is not already mapped. It is safe
+      to memory map a file while other threads are using file I/O on it.
+      Assigning a new address to a function pointer is an atomic
+      operation. intern_lock prevents that two or more mappings are done
+      at the same time.
     */
-    if (!share->file_map && (share->tot_locks == 1))
+    if (!share->file_map)
     {
       if (mi_dynmap_file(info, share->state.state.data_file_length))
       {