--- old/src/hotspot/share/gc/g1/g1RemSet.cpp	2019-11-18 17:32:49.497578021 +0100
+++ new/src/hotspot/share/gc/g1/g1RemSet.cpp	2019-11-18 17:32:49.065575879 +0100
@@ -304,11 +304,6 @@
   }
 
   void prepare() {
-    for (size_t i = 0; i < _max_regions; i++) {
-      _collection_set_iter_state[i] = false;
-      clear_scan_top((uint)i);
-    }
-
     _all_dirty_regions = new G1DirtyRegions(_max_regions);
     _next_dirty_regions = new G1DirtyRegions(_max_regions);
   }
@@ -403,6 +398,10 @@
     } while (cur != start_pos);
   }
 
+  void reset_region_claim(uint region_idx) {
+    _collection_set_iter_state[region_idx] = false;
+  }
+
   // Attempt to claim the given region in the collection set for iteration. Returns true
   // if this call caused the transition from Unclaimed to Claimed.
   inline bool claim_collection_set_region(uint region) {
@@ -882,10 +881,10 @@
   }
 }
 
-// Creates a snapshot of the current _top values at the start of collection to
-// filter out card marks that we do not want to scan.
-void G1RemSet::prepare_region_for_scanning(HeapRegion* region) {
+void G1RemSet::prepare_region_for_scan(HeapRegion* region) {
   uint hrm_index = region->hrm_index();
+
+  _scan_state->reset_region_claim(hrm_index);
   if (region->in_collection_set()) {
     // Young regions had their card table marked as young at their allocation;
     // we need to make sure that these marks are cleared at the end of GC, *but*
@@ -893,8 +892,10 @@
     // So directly add them to the "all_dirty_regions".
     // Same for regions in the (initial) collection set: they may contain cards from
     // the log buffers, make sure they are cleaned.
+    _scan_state->clear_scan_top(hrm_index);
     _scan_state->add_all_dirty_region(hrm_index);
-  } else if (region->is_old_or_humongous_or_archive()) {
+  } else {
+    assert(region->is_old_or_humongous_or_archive(), "All other regions should be in the collection set");
     _scan_state->set_scan_top(hrm_index, region->top());
   }
 }