Re-land: base: Use MADV_REMOVE instead of ftruncate to purge discardable memory segments.

base/memory/discardable_shared_memory.h

 // 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.
 
 #ifndef BASE_MEMORY_DISCARDABLE_SHARED_MEMORY_H_
 #define BASE_MEMORY_DISCARDABLE_SHARED_MEMORY_H_
 
 #include "base/base_export.h"
 #include "base/logging.h"
 #include "base/memory/shared_memory.h"
 #include "base/threading/thread_collision_warner.h"
 #include "base/time/time.h"
 
 #if DCHECK_IS_ON()
 #include <set>
 #endif
 
-// Define DISCARDABLE_SHARED_MEMORY_SHRINKING if platform supports shrinking
-// of discardable shared memory segments.
-#if defined(OS_POSIX) && !defined(OS_ANDROID)
-#define DISCARDABLE_SHARED_MEMORY_SHRINKING
+// Linux (including Android) support the MADV_REMOVE argument with madvise()
+// which has the behavior of reliably causing zero-fill-on-demand pages to
+// be returned after a call. Here we define
+// DISCARDABLE_SHARED_MEMORY_ZERO_FILL_ON_DEMAND_PAGES_AFTER_PURGE on Linux
+// and Android to indicate that this type of behavior can be expected on
+// those platforms. Note that madvise() will still be used on other POSIX
+// platforms but doesn't provide the zero-fill-on-demand pages guarantee.
+#if defined(OS_LINUX) || defined(OS_ANDROID)
+#define DISCARDABLE_SHARED_MEMORY_ZERO_FILL_ON_DEMAND_PAGES_AFTER_PURGE
 #endif
 
 namespace base {
 
 // Platform abstraction for discardable shared memory.
 //
 // This class is not thread-safe. Clients are responsible for synchronizing
 // access to an instance of this class.
 class BASE_EXPORT DiscardableSharedMemory {
  public:
@@ skipping 85 matching lines @@
   // Shares the discardable memory segment to another process. Attempts to
   // create a platform-specific |new_handle| which can be used in a remote
   // process to access the discardable memory segment. |new_handle| is an
   // output parameter to receive the handle for use in the remote process.
   // Returns true on success, false otherwise.
   bool ShareToProcess(ProcessHandle process_handle,
                       SharedMemoryHandle* new_handle) {
     return shared_memory_.ShareToProcess(process_handle, new_handle);
   }
 
-#if defined(DISCARDABLE_SHARED_MEMORY_SHRINKING)
-  // Release as much memory as possible to the OS. The change in size will
-  // be reflected by the return value of mapped_size().
-  void Shrink();
-#endif
-
  private:
   // Virtual for tests.
   virtual Time Now() const;
 
   SharedMemory shared_memory_;
   size_t mapped_size_;
   size_t locked_page_count_;
 #if DCHECK_IS_ON()
   std::set<size_t> locked_pages_;
 #endif
   // Implementation is not thread-safe but still usable if clients are
   // synchronized somehow. Use a collision warner to detect incorrect usage.
   DFAKE_MUTEX(thread_collision_warner_);
   Time last_known_usage_;
 
   DISALLOW_COPY_AND_ASSIGN(DiscardableSharedMemory);
 };
 
 }  // namespace base
 
 #endif  // BASE_MEMORY_DISCARDABLE_SHARED_MEMORY_H_