--- old/src/share/vm/gc_implementation/g1/g1RegionToSpaceMapper.hpp	2015-02-03 11:18:17.617404662 +0100
+++ new/src/share/vm/gc_implementation/g1/g1RegionToSpaceMapper.hpp	2015-02-03 11:18:17.545402583 +0100
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2014, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2014, 2015 Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -51,7 +51,7 @@
   // Mapping management
   BitMap _commit_map;
 
-  G1RegionToSpaceMapper(ReservedSpace rs, size_t commit_granularity, size_t region_granularity, MemoryType type);
+  G1RegionToSpaceMapper(ReservedSpace rs, size_t actual_size, size_t commit_granularity, size_t region_granularity, MemoryType type);
 
   void fire_on_commit(uint start_idx, size_t num_regions, bool zero_filled);
  public:
@@ -71,11 +71,15 @@
   virtual void uncommit_regions(uintptr_t start_idx, size_t num_regions = 1) = 0;
 
   // Creates an appropriate G1RegionToSpaceMapper for the given parameters.
+  // The actual space to be used within the given reservation is given by actual_size.
+  // This is because some OSes need to round up the reservation size to guarantee
+  // alignment of os_commit_granularity.
   // The byte_translation_factor defines how many bytes in a region correspond to
   // a single byte in the data structure this mapper is for.
   // Eg. in the card table, this value corresponds to the size a single card
-  // table entry corresponds to.
+  // table entry corresponds to in the heap.
   static G1RegionToSpaceMapper* create_mapper(ReservedSpace rs,
+                                              size_t actual_size,
                                               size_t os_commit_granularity,
                                               size_t region_granularity,
                                               size_t byte_translation_factor,