Add discardable memory emulation for non-android/mac platforms

base/memory/discardable_memory.h

 // Copyright (c) 2013 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_MEMORY_H_
 #define BASE_MEMORY_DISCARDABLE_MEMORY_H_
 
 #include "base/base_export.h"
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
+#include "base/threading/non_thread_safe.h"
 
 namespace base {
 
 enum LockDiscardableMemoryStatus {
   DISCARDABLE_MEMORY_FAILED = -1,
   DISCARDABLE_MEMORY_PURGED = 0,
   DISCARDABLE_MEMORY_SUCCESS = 1
 };
 
+#if defined(OS_MACOSX) || defined(OS_ANDROID)
+#define DISCARDABLE_MEMORY_ALWAYS_SUPPORTED_NATIVELY
+#else
+#define DISCARDABLE_MEMORY_NEVER_SUPPORTED_NATIVELY
+#endif
+
 // Platform abstraction for discardable memory. DiscardableMemory is used to
 // cache large objects without worrying about blowing out memory, both on mobile
 // devices where there is no swap, and desktop devices where unused free memory
 // should be used to help the user experience. This is preferable to releasing
 // memory in response to an OOM signal because it is simpler, though it has less
 // flexibility as to which objects get discarded.
 //
 // Discardable memory has two states: locked and unlocked. While the memory is
 // locked, it will not be discarded. Unlocking the memory allows the OS to
 // reclaim it if needed. Locks do not nest.
 //
 // Notes:
 //   - The paging behavior of memory while it is locked is not specified. While
 //     mobile platforms will not swap it out, it may qualify for swapping
 //     on desktop platforms. It is not expected that this will matter, as the
 //     preferred pattern of usage for DiscardableMemory is to lock down the
 //     memory, use it as quickly as possible, and then unlock it.
 //   - Because of memory alignment, the amount of memory allocated can be
 //     larger than the requested memory size. It is not very efficient for
 //     small allocations.
+//   - A discardable memory instance is not thread safe. If the need to pass
+//     ownership of a discardable memory instance from one thread to another,
+//     NonThreadSafe::DetachFromThread could be used, but this is unnecessary
+//     at the moment.
 //
 // References:
 //   - Linux: http://lwn.net/Articles/452035/
 //   - Mac: http://trac.webkit.org/browser/trunk/Source/WebCore/platform/mac/PurgeableBufferMac.cpp
 //          the comment starting with "vm_object_purgable_control" at
 //            http://www.opensource.apple.com/source/xnu/xnu-792.13.8/osfmk/vm/vm_object.c
-class BASE_EXPORT DiscardableMemory {
+class BASE_EXPORT DiscardableMemory : public NonThreadSafe {
  public:
   DiscardableMemory();
 
   // If the discardable memory is locked, the destructor will unlock it.
   // The opened file will also be closed after this.
   ~DiscardableMemory();
 
-  // Check whether the system supports discardable memory.
-  static bool Supported();
+  // Check whether the system supports discardable memory natively. Returns
+  // false if the support is emulated.
+  static bool SupportedNatively();
 
   // Initialize the DiscardableMemory object. On success, this function returns
   // true and the memory is locked. This should only be called once.
   // This call could fail because of platform-specific limitations and the user
   // should stop using the DiscardableMemory afterwards.
   bool InitializeAndLock(size_t size);
 
   // Lock the memory so that it will not be purged by the system. Returns
   // DISCARDABLE_MEMORY_SUCCESS on success. If the return value is
   // DISCARDABLE_MEMORY_FAILED then this object should be discarded and
   // a new one should be created. If the return value is
   // DISCARDABLE_MEMORY_PURGED then the memory is present but any data that
   // was in it is gone.
   LockDiscardableMemoryStatus Lock() WARN_UNUSED_RESULT;
 
   // Unlock the memory so that it can be purged by the system. Must be called
   // after every successful lock call.
   void Unlock();
 
+  bool is_locked() const { return is_locked_; }
+
   // Return the memory address held by this object. The object must be locked
   // before calling this. Otherwise, this will cause a DCHECK error.
   void* Memory() const;
 
   // Testing utility calls.
 
   // Check whether a purge of all discardable memory in the system is supported.
   // Use only for testing!
   static bool PurgeForTestingSupported();
 
   // Purge all discardable memory in the system. This call has global effects
   // across all running processes, so it should only be used for testing!
   static void PurgeForTesting();
 
  private:
+#if !defined(DISCARDABLE_MEMORY_ALWAYS_SUPPORTED_NATIVELY)
+  friend class DiscardableMemoryProvider;
+  friend class DiscardableMemoryProviderTest;
+
+  // Used by the DiscardableMemoryProvider to allocate and free memory.
+  bool Allocate();
+  void Deallocate();
+#endif
+
 #if defined(OS_ANDROID)
   // Maps the discardable memory into the caller's address space.
   // Returns true on success, false otherwise.
   bool Map();
 
   // Unmaps the discardable memory from the caller's address space.
   void Unmap();
 
   // Reserve a file descriptor. When reaching the fd limit, this call returns
   // false and initialization should fail.
   bool ReserveFileDescriptor();
 
   // Release a file descriptor so that others can reserve it.
   void ReleaseFileDescriptor();
 #endif  // OS_ANDROID
 
   void* memory_;
   size_t size_;
   bool is_locked_;
 #if defined(OS_ANDROID)
   int fd_;
 #endif  // OS_ANDROID
 
   DISALLOW_COPY_AND_ASSIGN(DiscardableMemory);
 };
 
 }  // namespace base
 
 #endif  // BASE_MEMORY_DISCARDABLE_MEMORY_H_