Insert a filler at the new space top even if the top is at the limit.

src/heap/heap.cc

Index: src/heap/heap.cc
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index 274d9ef0c22e83a5cc1f92ad8a03da6e7b9c6244..9aef842684ed248f5544121b02aa1566a998078e 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -794,10 +794,14 @@ void Heap::EnsureFillerObjectAtTop() {
   // pointer of the new space page. We store a filler object there to
   // identify the unused space.
   Address from_top = new_space_.top();
-  Address from_limit = new_space_.limit();
-  if (from_top < from_limit) {
-    int remaining_in_page = static_cast<int>(from_limit - from_top);
-    CreateFillerObjectAt(from_top, remaining_in_page);
+  // Check that from_top is inside its page (i.e., not at the end).
+  Address space_end = new_space_.ToSpaceEnd();
+  if (from_top < space_end) {
+    Page* page = Page::FromAddress(from_top);
+    if (page->Contains(from_top)) {
+      int remaining_in_page = static_cast<int>(page->area_end() - from_top);
+      CreateFillerObjectAt(from_top, remaining_in_page);
+    }
   }
 }