--- old/src/hotspot/share/gc/g1/g1ConcurrentMark.hpp	2019-06-04 13:17:00.188651314 -0700
+++ new/src/hotspot/share/gc/g1/g1ConcurrentMark.hpp	2019-06-04 13:16:59.940651323 -0700
@@ -222,7 +222,7 @@
   template<typename Fn> void iterate(Fn fn) const PRODUCT_RETURN;
 };
 
-// Root Regions are regions that contain objects from nTAMS to top. These are roots
+// Root MemRegions are memory regions that contain objects from nTAMS to top. These are roots
 // for marking, i.e. their referenced objects must be kept alive to maintain the
 // SATB invariant.
 // We could scan and mark them through during the initial-mark pause, but for
@@ -230,10 +230,13 @@
 // We need to complete this procedure before the next GC because it might determine
 // that some of these "root objects" are dead, potentially dropping some required
 // references.
-// Root regions comprise of the complete contents of survivor regions, and any
+// Root MemRegions comprise of the complete contents of survivor regions, and any
 // objects copied into old gen during GC.
-class G1CMRootRegions {
-  HeapRegion** _root_regions;
+class G1CMRootMemRegions {
+  // The set of root MemRegions. We store the corresponding nTAMS and top() pointer
+  // for a given heap region into MemRegion to know the exact area to scan because
+  // the top of a reused survivor region changes over time.
+  MemRegion* _root_regions;
   size_t const _max_regions;
 
   volatile size_t _num_root_regions; // Actual number of root regions.
@@ -246,13 +249,13 @@
   void notify_scan_done();
 
 public:
-  G1CMRootRegions(uint const max_regions);
-  ~G1CMRootRegions();
+  G1CMRootMemRegions(uint const max_regions);
+  ~G1CMRootMemRegions();
 
   // Reset the data structure to allow addition of new root regions.
   void reset();
 
-  void add(HeapRegion* hr);
+  void add(HeapWord* start, HeapWord* end);
 
   // Reset the claiming / scanning of the root regions.
   void prepare_for_scan();
@@ -264,9 +267,9 @@
   // false otherwise.
   bool scan_in_progress() { return _scan_in_progress; }
 
-  // Claim the next root region to scan atomically, or return NULL if
+  // Claim the next root MemRegion to scan atomically, or return NULL if
   // all have been claimed.
-  HeapRegion* claim_next();
+  MemRegion* claim_next();
 
   // The number of root regions to scan.
   uint num_root_regions() const;
@@ -310,7 +313,7 @@
   MemRegion const         _heap;
 
   // Root region tracking and claiming
-  G1CMRootRegions         _root_regions;
+  G1CMRootMemRegions         _root_regions;
 
   // For grey objects
   G1CMMarkStack           _global_mark_stack; // Grey objects behind global finger
@@ -501,7 +504,7 @@
   size_t partial_mark_stack_size_target() const { return _global_mark_stack.capacity() / 3; }
   bool mark_stack_empty() const                 { return _global_mark_stack.is_empty(); }
 
-  G1CMRootRegions* root_regions() { return &_root_regions; }
+  G1CMRootMemRegions* root_regions() { return &_root_regions; }
 
   void concurrent_cycle_start();
   // Abandon current marking iteration due to a Full GC.
@@ -554,8 +557,8 @@
   // them.
   void scan_root_regions();
 
-  // Scan a single root region from nTAMS to top and mark everything reachable from it.
-  void scan_root_region(HeapRegion* hr, uint worker_id);
+  // Scan a single root MemRegion which holds from nTAMS to top and mark everything reachable from it.
+  void scan_root_region(MemRegion* region, uint worker_id);
 
   // Do concurrent phase of marking, to a tentative transitive closure.
   void mark_from_roots();