Re-land: base: Replace PurgeAndTruncate with Shrink function.

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"
 
 #if defined(OS_POSIX)
 #include <unistd.h>
 #endif
 
@@ skipping 307 matching lines @@
     last_known_usage_ = result.GetLockState() == SharedState::LOCKED
                             ? current_time
                             : result.GetTimestamp();
     return false;
   }
 
   last_known_usage_ = Time();
   return true;
 }
 
-bool DiscardableSharedMemory::PurgeAndTruncate(Time current_time) {
-  if (!Purge(current_time))
-    return false;
-
-#if defined(OS_POSIX)
-  // Truncate shared memory to size of SharedState.
-  SharedMemoryHandle handle = shared_memory_.handle();
-  if (SharedMemory::IsHandleValid(handle)) {
-    if (HANDLE_EINTR(ftruncate(handle.fd, sizeof(SharedState))) != 0)
-      DPLOG(ERROR) << "ftruncate() failed";
-  }
-#endif
-
-  return true;
-}
-
 bool DiscardableSharedMemory::IsMemoryResident() const {
   DCHECK(shared_memory_.memory());
 
   SharedState result(subtle::NoBarrier_Load(
       &SharedStateFromSharedMemory(shared_memory_)->value.i));
 
   return result.GetLockState() == SharedState::LOCKED ||
          !result.GetTimestamp().is_null();
 }
 
 void DiscardableSharedMemory::Close() {
   shared_memory_.Unmap();
   shared_memory_.Close();
   mapped_size_ = 0;
 }
 
+#if defined(DISCARDABLE_SHARED_MEMORY_SHRINKING)
+void DiscardableSharedMemory::Shrink() {
+#if defined(OS_POSIX)
+  SharedMemoryHandle handle = shared_memory_.handle();
+  if (!SharedMemory::IsHandleValid(handle))
+    return;
+
+  // Truncate shared memory to size of SharedState.
+  if (HANDLE_EINTR(
+          ftruncate(handle.fd, AlignToPageSize(sizeof(SharedState)))) != 0) {
+    DPLOG(ERROR) << "ftruncate() failed";
+    return;
+  }
+  mapped_size_ = 0;
+#else
+  NOTIMPLEMENTED();
+#endif
+}
+#endif
+
 Time DiscardableSharedMemory::Now() const {
   return Time::Now();
 }
 
 }  // namespace base