Merge of patch for bug#11756764 from mysql-5.1.

storage/myisam/mi_close.c

@@ -84,7 +84,12 @@ int mi_close(register MI_INFO *info)
     }
 #ifdef HAVE_MMAP
     if (share->file_map)
-      _mi_unmap_file(info);
+    {
+      if (share->options & HA_OPTION_COMPRESS_RECORD)
+        _mi_unmap_file(info);
+      else
+        mi_munmap_file(info);
+    }
 #endif
     if (share->decode_trees)
     {

storage/myisam/mi_packrec.c

@@ -1549,13 +1549,14 @@ my_bool _mi_memmap_file(MI_INFO *info)
 
 void _mi_unmap_file(MI_INFO *info)
 {
-  (void) my_munmap((char*) info->s->file_map,
-                   (size_t) info->s->mmaped_length + MEMMAP_EXTRA_MARGIN);
+  DBUG_ASSERT(info->s->options & HA_OPTION_COMPRESS_RECORD);
+
+  (void) my_munmap((char*) info->s->file_map, (size_t) info->s->mmaped_length);
 
   if (myisam_mmap_size != SIZE_T_MAX)
   {
     mysql_mutex_lock(&THR_LOCK_myisam_mmap);
-    myisam_mmap_used-= info->s->mmaped_length + MEMMAP_EXTRA_MARGIN;
+    myisam_mmap_used-= info->s->mmaped_length;
     mysql_mutex_unlock(&THR_LOCK_myisam_mmap);
   }
 }