Re-land "Concurrently unmap free pages."

src/heap/mark-compact.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/heap/mark-compact.h"
 
 #include "src/base/atomicops.h"
 #include "src/base/bits.h"
 #include "src/code-stubs.h"
 #include "src/compilation-cache.h"
@@ skipping 501 matching lines @@
 void MarkCompactCollector::EnsureSweepingCompleted() {
   DCHECK(sweeping_in_progress_ == true);
 
   // If sweeping is not completed or not running at all, we try to complete it
   // here.
   if (!heap()->concurrent_sweeping_enabled() || !IsSweepingCompleted()) {
     SweepInParallel(heap()->paged_space(OLD_SPACE), 0);
     SweepInParallel(heap()->paged_space(CODE_SPACE), 0);
     SweepInParallel(heap()->paged_space(MAP_SPACE), 0);
   }
-  // Wait twice for both jobs.
+
   if (heap()->concurrent_sweeping_enabled()) {
     pending_sweeper_jobs_semaphore_.Wait();
     pending_sweeper_jobs_semaphore_.Wait();
     pending_sweeper_jobs_semaphore_.Wait();
   }
+
+  heap()->WaitUntilUnmappingOfFreeChunksCompleted();
+
   ParallelSweepSpacesComplete();
   sweeping_in_progress_ = false;
   RefillFreeList(heap()->paged_space(OLD_SPACE));
   RefillFreeList(heap()->paged_space(CODE_SPACE));
   RefillFreeList(heap()->paged_space(MAP_SPACE));
   heap()->paged_space(OLD_SPACE)->ResetUnsweptFreeBytes();
   heap()->paged_space(CODE_SPACE)->ResetUnsweptFreeBytes();
   heap()->paged_space(MAP_SPACE)->ResetUnsweptFreeBytes();
 
 #ifdef VERIFY_HEAP
@@ skipping 4155 matching lines @@
   SlotsBuffer* buffer = *buffer_address;
   while (buffer != NULL) {
     SlotsBuffer* next_buffer = buffer->next();
     DeallocateBuffer(buffer);
     buffer = next_buffer;
   }
   *buffer_address = NULL;
 }
 }  // namespace internal
 }  // namespace v8