diff -r 63f4675ac87d -r 2d160770d2e5 src/share/vm/gc_implementation/g1/heapRegion.cpp
--- a/src/share/vm/gc_implementation/g1/heapRegion.cpp	Sat Jul 31 15:10:59 2010 +0100
+++ b/src/share/vm/gc_implementation/g1/heapRegion.cpp	Mon Aug 02 12:51:43 2010 -0700
@@ -683,6 +683,8 @@
     return NULL;
   }
 
+  assert(!is_young(), "check value of filter_young");
+
   // We used to use "block_start_careful" here.  But we're actually happy
   // to update the BOT while we do this...
   HeapWord* cur = block_start(mr.start());