macOS: Use MADV_FREE_REUSABLE for discardable shared memory.

base/memory/discardable_shared_memory.cc

 // Copyright 2014 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 "base/memory/discardable_shared_memory.h"
 
 #include <stdint.h>
 
 #include <algorithm>
 
@@ skipping 326 matching lines @@
 // purge and releases its own references.
 // Note: this memory will not be accessed again.  The segment will be
 // freed asynchronously at a later time, so just do the best
 // immediately.
 #if defined(OS_POSIX) && !defined(OS_NACL)
 // Linux and Android provide MADV_REMOVE which is preferred as it has a
 // behavior that can be verified in tests. Other POSIX flavors (MacOSX, BSDs),
 // provide MADV_FREE which has the same result but memory is purged lazily.
 #if defined(OS_LINUX) || defined(OS_ANDROID)
 #define MADV_PURGE_ARGUMENT MADV_REMOVE
+#elif defined(OS_MACOSX)
+// MADV_FREE_REUSABLE is similar to MADV_FREE, but also marks the pages with the
+// reusable bit, which allows both Activity Monitor and memory-infra to
+// correctly track the pages.
+#define MADV_PURGE_ARGUMENT MADV_FREE_REUSABLE
 #else
 #define MADV_PURGE_ARGUMENT MADV_FREE
 #endif
   // Advise the kernel to remove resources associated with purged pages.
   // Subsequent accesses of memory pages will succeed, but might result in
   // zero-fill-on-demand pages.
   if (madvise(reinterpret_cast<char*>(shared_memory_.memory()) +
                   AlignToPageSize(sizeof(SharedState)),
               AlignToPageSize(mapped_size_), MADV_PURGE_ARGUMENT)) {
     DPLOG(ERROR) << "madvise() failed";
@@ skipping 33 matching lines @@
 
 void DiscardableSharedMemory::Close() {
   shared_memory_.Close();
 }
 
 Time DiscardableSharedMemory::Now() const {
   return Time::Now();
 }
 
 }  // namespace base