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

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"
@@ skipping 16 matching lines @@
 
   DiscardableSharedMemory();
 
   // Create a new DiscardableSharedMemory object from an existing, open shared
   // memory file. Memory must be locked.
   explicit DiscardableSharedMemory(SharedMemoryHandle handle);
 
   // Closes any open files.
   virtual ~DiscardableSharedMemory();
 
+  // Return true iff shrinking of discardable shared memory is supported.
+  static bool IsShrinkingSupported();
+
   // Creates and maps a locked DiscardableSharedMemory object with |size|.
   // Returns true on success and false on failure.
   bool CreateAndMap(size_t size);
 
   // Maps the locked discardable memory into the caller's address space.
   // Returns true on success, false otherwise.
   bool Map(size_t size);
 
   // The actual size of the mapped memory (may be larger than requested).
   size_t mapped_size() const { return mapped_size_; }
@@ skipping 39 matching lines @@
   // if locked or the actual last usage timestamp if unlocked. It is often
   // necessary to call this function twice for the object to successfully be
   // purged. First call, updates |last_known_usage_|. Second call, successfully
   // purges the object using the updated |last_known_usage_|.
   // Note: there is no guarantee that multiple calls to this function will
   // successfully purge object. DiscardableSharedMemory object might be locked
   // or another thread/process might be able to lock and unlock it in between
   // each call.
   bool Purge(Time current_time);
 
-  // Purge and release as much memory as possible to the OS.
-  // Note: The amount of memory that can be released to the OS is platform
-  // specific. Best case, all but one page is released. Worst case, nothing
-  // is released.
-  bool PurgeAndTruncate(Time current_time);
+  // Release as much memory as possible to the OS. The change in size will
+  // be reflected by the return value of mapped_size().
+  // Note: It is an error to call this if IsShrinkingSupported() returns false.
+  void Shrink();
 
   // Returns true if memory is still resident.
   bool IsMemoryResident() const;
 
   // Closes the open discardable memory segment.
   // It is safe to call Close repeatedly.
   void Close();
 
   // Shares the discardable memory segment to another process. Attempts to
   // create a platform-specific |new_handle| which can be used in a remote
@@ skipping 19 matching lines @@
   // 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_