--- /dev/null	2019-02-06 07:17:25.258988811 +0100
+++ new/src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/gc/z/ZExternalBitMap.java	2019-02-11 14:47:36.698999579 +0100
@@ -0,0 +1,87 @@
+package sun.jvm.hotspot.gc.z;
+
+import java.util.HashMap;
+
+import sun.jvm.hotspot.runtime.VM;
+import sun.jvm.hotspot.utilities.BitMap;
+import sun.jvm.hotspot.utilities.BitMapInterface;
+
+/** Discontiguous bitmap for ZGC. */
+public class ZExternalBitMap implements BitMapInterface {
+    private ZPageTable pageTable;
+    private final long oopSize;
+
+    private HashMap<ZPage, BitMap> pageToBitMap = new HashMap<ZPage, BitMap>();
+
+    public ZExternalBitMap(ZCollectedHeap collectedHeap) {
+        pageTable = collectedHeap.heap().pageTable();
+        oopSize = VM.getVM().getOopSize();
+    }
+
+    private ZPage getPage(long zOffset) {
+        if (zOffset > ZGlobals.ZAddressOffsetMask) {
+            throw new RuntimeException("Not a Z offset: " + zOffset);
+        }
+
+        ZPage page = pageTable.get(ZOop.to_address(zOffset));
+        if (page == null) {
+            throw new RuntimeException("Address not in pageTable: " + zOffset);
+        }
+        return page;
+    }
+
+    private BitMap getOrAddBitMap(ZPage page) {
+        BitMap bitMap = pageToBitMap.get(page);
+        if (bitMap == null) {
+            long size = page.size();
+
+            long maxNumObjects = size >>> page.object_alignment_shift();
+            if (maxNumObjects > Integer.MAX_VALUE) {
+                throw new RuntimeException("int overflow");
+            }
+            int intMaxNumObjects = (int)maxNumObjects;
+
+            bitMap = new BitMap(intMaxNumObjects);
+            pageToBitMap.put(page,  bitMap);
+        }
+
+        return bitMap;
+    }
+
+    private int pageLocalBitMapIndex(ZPage page, long zOffset) {
+        long pageLocalZOffset = zOffset - page.start();
+        return (int)(pageLocalZOffset >>> page.object_alignment_shift());
+    }
+
+    private long convertToZOffset(long offset) {
+        long addr = ZGlobals.ZAddressSpaceStart + oopSize * offset;
+        return addr & ZGlobals.ZAddressOffsetMask;
+    }
+
+    @Override
+    public boolean at(long offset) {
+        long zOffset = convertToZOffset(offset);
+        ZPage page = getPage(zOffset);
+        BitMap bitMap = getOrAddBitMap(page);
+        int index = pageLocalBitMapIndex(page, zOffset);
+
+        return bitMap.at(index);
+    }
+
+    @Override
+    public void atPut(long offset, boolean value) {
+        long zOffset = convertToZOffset(offset);
+        ZPage page = getPage(zOffset);
+        BitMap bitMap = getOrAddBitMap(page);
+        int index = pageLocalBitMapIndex(page, zOffset);
+
+        bitMap.atPut(index, value);
+    }
+
+    @Override
+    public void clear() {
+        for (BitMap bitMap : pageToBitMap.values()) {
+            bitMap.clear();
+        }
+    }
+}