Use a bitmap to record PageMemoryRegion usage.

third_party/WebKit/Source/platform/heap/PageMemory.cpp

 // Copyright 2016 The Chromium 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 "platform/heap/PageMemory.h"
 
 #include "platform/heap/Heap.h"
 #include "wtf/Assertions.h"
 #include "wtf/PageAllocator.h"
 
@@ skipping 13 matching lines @@
 void MemoryRegion::decommit()
 {
     WTF::decommitSystemPages(m_base, m_size);
     WTF::setSystemPagesInaccessible(m_base, m_size);
 }
 
 
 PageMemoryRegion::PageMemoryRegion(Address base, size_t size, unsigned numPages)
     : MemoryRegion(base, size)
     , m_isLargePage(numPages == 1)
+    , m_inUseBitmap(0)
     , m_numPages(numPages)
 {
+    static_assert(blinkPagesPerRegion < 8 * sizeof(unsigned), "PageMemoryRegion's in-use bitmap must fit within a word.");
     Heap::addPageMemoryRegion(this);
-    for (size_t i = 0; i < blinkPagesPerRegion; ++i)
-        m_inUse[i] = false;
 }
 
 PageMemoryRegion::~PageMemoryRegion()
 {
     Heap::removePageMemoryRegion(this);
     release();
 }
 
 // TODO(haraken): Like partitionOutOfMemoryWithLotsOfUncommitedPages(),
 // we should probably have a way to distinguish physical memory OOM from
@@ skipping 107 matching lines @@
     // Overallocate by 2 times OS page size to have space for a
     // guard page at the beginning and end of blink heap page.
     size_t allocationSize = payloadSize + 2 * blinkGuardPageSize;
     PageMemoryRegion* pageMemoryRegion = PageMemoryRegion::allocateLargePage(allocationSize);
     PageMemory* storage = setupPageMemoryInRegion(pageMemoryRegion, 0, payloadSize);
     RELEASE_ASSERT(storage->commit());
     return storage;
 }
 
 } // namespace blink