--- a/src/hotspot/share/services/virtualMemoryTracker.cpp	Mon Jan 29 11:55:40 2018 -0500
+++ b/src/hotspot/share/services/virtualMemoryTracker.cpp	Mon Jan 29 15:11:33 2018 +0100
@@ -65,16 +65,26 @@
     }
 
     if (rgn->adjacent_to(addr, size)) {
-      // special case to expand prior region if there is no next region
-      LinkedListNode<CommittedMemoryRegion>* next = node->next();
-      if (next == NULL && rgn->call_stack()->equals(stack)) {
-        VirtualMemorySummary::record_uncommitted_memory(rgn->size(), flag());
+      if (rgn->call_stack()->equals(stack)) {
         // the two adjacent regions have the same call stack, merge them
         rgn->expand_region(addr, size);
-        VirtualMemorySummary::record_committed_memory(rgn->size(), flag());
+        VirtualMemorySummary::record_committed_memory(size, flag());
+
+        // maybe merge with the next region
+        LinkedListNode<CommittedMemoryRegion>* next_node = node->next();
+        if (next_node != NULL) {
+          CommittedMemoryRegion* next = next_node->data();
+          if (next->call_stack()->equals(stack) && rgn->adjacent_to(next->base(), next->size())) {
+            // the two adjacent regions have the same call stack, merge them
+            rgn->expand_region(next->base(), next->size());
+
+            // the merge next_node needs to be removed from the list
+            _committed_regions.remove(next_node);
+          }
+        }
         return true;
       }
-      }
+    }
 
     if (rgn->overlap_region(addr, size)) {
       // Clear a space for this region in the case it overlaps with any regions.
@@ -87,10 +97,10 @@
     node = node->next();
   }
 
-    // New committed region
-    VirtualMemorySummary::record_committed_memory(size, flag());
-    return add_committed_region(committed_rgn);
-  }
+  // New committed region
+  VirtualMemorySummary::record_committed_memory(size, flag());
+  return add_committed_region(committed_rgn);
+}
 
 void ReservedMemoryRegion::set_all_committed(bool b) {
   if (all_committed() != b) {

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/test/hotspot/jtreg/runtime/NMT/VirtualAllocCommitMerge.java	Mon Jan 29 15:11:33 2018 +0100
@@ -0,0 +1,283 @@
+/*
+ * Copyright (c) 2014, 2016, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/*
+ * @test
+ * @summary Test merging of committed virtual memory and that we track it correctly
+ * @key nmt jcmd
+ * @library /test/lib
+ * @modules java.base/jdk.internal.misc
+ *          java.management
+ * @build sun.hotspot.WhiteBox
+ * @run main ClassFileInstaller sun.hotspot.WhiteBox
+ * @run main/othervm -Xbootclasspath/a:. -XX:+UnlockDiagnosticVMOptions -XX:+WhiteBoxAPI -XX:NativeMemoryTracking=detail VirtualAllocCommitMerge
+ *
+ */
+
+import jdk.test.lib.process.ProcessTools;
+import jdk.test.lib.process.OutputAnalyzer;
+import jdk.test.lib.JDKToolFinder;
+
+import sun.hotspot.WhiteBox;
+
+public class VirtualAllocCommitMerge {
+
+    public static WhiteBox wb = WhiteBox.getWhiteBox();
+
+    public static void main(String args[]) throws Exception {
+        OutputAnalyzer output;
+        long commitSize = 128 * 1024; // 128KB
+        long reserveSize = 4 * 1024 * 1024; // 4096KB
+        long addr;
+
+        String pid = Long.toString(ProcessTools.getProcessId());
+        ProcessBuilder pb = new ProcessBuilder();
+
+        // reserve
+        addr = wb.NMTReserveMemory(reserveSize);
+        pb.command(new String[] { JDKToolFinder.getJDKTool("jcmd"), pid,
+                "VM.native_memory", "detail" });
+
+        output = new OutputAnalyzer(pb.start());
+        checkReservedCommittedSummary(output, "4096KB", "0KB");
+        checkReserved(output, addr, reserveSize, "4096KB");
+
+        long addrA = addr + (0 * commitSize);
+        long addrB = addr + (1 * commitSize);
+        long addrC = addr + (2 * commitSize);
+        long addrD = addr + (3 * commitSize);
+        long addrE = addr + (4 * commitSize);
+
+        // Test discontigous areas
+        {
+            // commit ACE
+            wb.NMTCommitMemory(addrA, commitSize);
+            wb.NMTCommitMemory(addrC, commitSize);
+            wb.NMTCommitMemory(addrE, commitSize);
+
+            output = new OutputAnalyzer(pb.start());
+            checkReservedCommittedSummary(output, "4096KB", "384KB");
+            checkReserved(output, addr, reserveSize, "4096KB");
+
+            checkCommitted(output, addrA, commitSize, "128KB");
+            checkCommitted(output, addrC, commitSize, "128KB");
+            checkCommitted(output, addrE, commitSize, "128KB");
+
+            // uncommit ACE
+            wb.NMTUncommitMemory(addrA, commitSize);
+            wb.NMTUncommitMemory(addrC, commitSize);
+            wb.NMTUncommitMemory(addrE, commitSize);
+
+            output = new OutputAnalyzer(pb.start());
+            checkReservedCommittedSummary(output, "4096KB", "0KB");
+        }
+
+        // Test contiguous areas
+        {
+            // commit AB
+            wb.NMTCommitMemory(addrA, commitSize);
+            wb.NMTCommitMemory(addrB, commitSize);
+
+            output = new OutputAnalyzer(pb.start());
+            checkReservedCommittedSummary(output, "4096KB", "256KB");
+            checkReserved(output, addr, reserveSize, "4096KB");
+
+            checkCommitted(output, addrA, 2 * commitSize, "256KB");
+
+            // uncommit AB
+            wb.NMTUncommitMemory(addrA, commitSize);
+            wb.NMTUncommitMemory(addrB, commitSize);
+
+            output = new OutputAnalyzer(pb.start());
+            checkReservedCommittedSummary(output, "4096KB", "0KB");
+        }
+
+        {
+            // commit BA
+            wb.NMTCommitMemory(addrB, commitSize);
+            wb.NMTCommitMemory(addrA, commitSize);
+
+            output = new OutputAnalyzer(pb.start());
+            checkReservedCommittedSummary(output, "4096KB", "256KB");
+            checkReserved(output, addr, reserveSize, "4096KB");
+
+            checkCommitted(output, addrA, 2 * commitSize, "256KB");
+
+            // uncommit AB
+            wb.NMTUncommitMemory(addrB, commitSize);
+            wb.NMTUncommitMemory(addrA, commitSize);
+
+            output = new OutputAnalyzer(pb.start());
+            checkReservedCommittedSummary(output, "4096KB", "0KB");
+        }
+
+        {
+            // commit ABC
+            wb.NMTCommitMemory(addrA, commitSize);
+            wb.NMTCommitMemory(addrB, commitSize);
+            wb.NMTCommitMemory(addrC, commitSize);
+
+            output = new OutputAnalyzer(pb.start());
+            checkReservedCommittedSummary(output, "4096KB", "384KB");
+            checkReserved(output, addr, reserveSize, "4096KB");
+
+            checkCommitted(output, addrA, 3 * commitSize, "384KB");
+
+            // uncommit
+            wb.NMTUncommitMemory(addrA, commitSize);
+            wb.NMTUncommitMemory(addrB, commitSize);
+            wb.NMTUncommitMemory(addrC, commitSize);
+
+            output = new OutputAnalyzer(pb.start());
+            checkReservedCommittedSummary(output, "4096KB", "0KB");
+        }
+
+        {
+            // commit ACB
+            wb.NMTCommitMemory(addrA, commitSize);
+            wb.NMTCommitMemory(addrC, commitSize);
+            wb.NMTCommitMemory(addrB, commitSize);
+
+            output = new OutputAnalyzer(pb.start());
+            checkReservedCommittedSummary(output, "4096KB", "384KB");
+            checkReserved(output, addr, reserveSize, "4096KB");
+
+            checkCommitted(output, addrA, 3 * commitSize, "384KB");
+
+            // uncommit
+            wb.NMTUncommitMemory(addrA, commitSize);
+            wb.NMTUncommitMemory(addrC, commitSize);
+            wb.NMTUncommitMemory(addrB, commitSize);
+
+            output = new OutputAnalyzer(pb.start());
+            checkReservedCommittedSummary(output, "4096KB", "0KB");
+        }
+
+        {
+            // commit BAC
+            wb.NMTCommitMemory(addrB, commitSize);
+            wb.NMTCommitMemory(addrA, commitSize);
+            wb.NMTCommitMemory(addrC, commitSize);
+
+            output = new OutputAnalyzer(pb.start());
+            checkReservedCommittedSummary(output, "4096KB", "384KB");
+            checkReserved(output, addr, reserveSize, "4096KB");
+
+            checkCommitted(output, addrA, 3 * commitSize, "384KB");
+
+            // uncommit
+            wb.NMTUncommitMemory(addrB, commitSize);
+            wb.NMTUncommitMemory(addrA, commitSize);
+            wb.NMTUncommitMemory(addrC, commitSize);
+
+            output = new OutputAnalyzer(pb.start());
+            checkReservedCommittedSummary(output, "4096KB", "0KB");
+        }
+
+        {
+            // commit BCA
+            wb.NMTCommitMemory(addrB, commitSize);
+            wb.NMTCommitMemory(addrC, commitSize);
+            wb.NMTCommitMemory(addrA, commitSize);
+
+            output = new OutputAnalyzer(pb.start());
+            checkReservedCommittedSummary(output, "4096KB", "384KB");
+            checkReserved(output, addr, reserveSize, "4096KB");
+
+            checkCommitted(output, addrA, 3 * commitSize, "384KB");
+
+            // uncommit
+            wb.NMTUncommitMemory(addrB, commitSize);
+            wb.NMTUncommitMemory(addrC, commitSize);
+            wb.NMTUncommitMemory(addrA, commitSize);
+
+            output = new OutputAnalyzer(pb.start());
+            checkReservedCommittedSummary(output, "4096KB", "0KB");
+        }
+
+        {
+            // commit CAB
+            wb.NMTCommitMemory(addrC, commitSize);
+            wb.NMTCommitMemory(addrA, commitSize);
+            wb.NMTCommitMemory(addrB, commitSize);
+
+            output = new OutputAnalyzer(pb.start());
+            checkReservedCommittedSummary(output, "4096KB", "384KB");
+            checkReserved(output, addr, reserveSize, "4096KB");
+
+            checkCommitted(output, addrA, 3 * commitSize, "384KB");
+
+            // uncommit
+            wb.NMTUncommitMemory(addrC, commitSize);
+            wb.NMTUncommitMemory(addrA, commitSize);
+            wb.NMTUncommitMemory(addrB, commitSize);
+
+            output = new OutputAnalyzer(pb.start());
+            checkReservedCommittedSummary(output, "4096KB", "0KB");
+        }
+
+        {
+            // commit CBA
+            wb.NMTCommitMemory(addrC, commitSize);
+            wb.NMTCommitMemory(addrB, commitSize);
+            wb.NMTCommitMemory(addrA, commitSize);
+
+            output = new OutputAnalyzer(pb.start());
+            checkReservedCommittedSummary(output, "4096KB", "384KB");
+            checkReserved(output, addr, reserveSize, "4096KB");
+
+            checkCommitted(output, addrA, 3 * commitSize, "384KB");
+
+            // uncommit
+            wb.NMTUncommitMemory(addrC, commitSize);
+            wb.NMTUncommitMemory(addrB, commitSize);
+            wb.NMTUncommitMemory(addrA, commitSize);
+
+            output = new OutputAnalyzer(pb.start());
+            checkReservedCommittedSummary(output, "4096KB", "0KB");
+        }
+
+        // release
+        wb.NMTReleaseMemory(addr, reserveSize);
+        output = new OutputAnalyzer(pb.start());
+        output.shouldNotContain("Test (reserved=");
+        output.shouldNotMatch("\\[0x[0]*" + Long.toHexString(addr) + " - 0x[0]*"
+                + Long.toHexString(addr + reserveSize) + "\\] reserved 4096KB for Test");
+    }
+
+    public static void checkReservedCommittedSummary(OutputAnalyzer output, String reservedString, String committedString) {
+        output.shouldContain("Test (reserved=" + reservedString + ", committed=" + committedString + ")");
+    }
+
+    public static void checkReserved(OutputAnalyzer output, long addr, long size, String sizeString) {
+        output.shouldMatch("\\[0x[0]*" + Long.toHexString(addr) + " - 0x[0]*"
+                           + Long.toHexString(addr + size)
+                           + "\\] reserved 4096KB for Test");
+    }
+
+    public static void checkCommitted(OutputAnalyzer output, long addr, long size, String sizeString) {
+        output.shouldMatch("\\[0x[0]*" + Long.toHexString(addr) + " - 0x[0]*"
+                           + Long.toHexString(addr + size)
+                           + "\\] committed " + sizeString + " from.*");
+    }
+}