1// Copyright (c) 2013 The Chromium Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style license that can be
3// found in the LICENSE file.
4
5#ifndef BASE_MEMORY_DISCARDABLE_MEMORY_H_
6#define BASE_MEMORY_DISCARDABLE_MEMORY_H_
7
8#include "base/base_export.h"
9#include "base/basictypes.h"
10#include "base/compiler_specific.h"
11
12namespace base {
13
14enum LockDiscardableMemoryStatus {
15  DISCARDABLE_MEMORY_FAILED = -1,
16  DISCARDABLE_MEMORY_PURGED = 0,
17  DISCARDABLE_MEMORY_SUCCESS = 1
18};
19
20// Platform abstraction for discardable memory. DiscardableMemory is used to
21// cache large objects without worrying about blowing out memory, both on mobile
22// devices where there is no swap, and desktop devices where unused free memory
23// should be used to help the user experience. This is preferable to releasing
24// memory in response to an OOM signal because it is simpler, though it has less
25// flexibility as to which objects get discarded.
26//
27// Discardable memory has two states: locked and unlocked. While the memory is
28// locked, it will not be discarded. Unlocking the memory allows the OS to
29// reclaim it if needed. Locks do not nest.
30//
31// Notes:
32//   - The paging behavior of memory while it is locked is not specified. While
33//     mobile platforms will not swap it out, it may qualify for swapping
34//     on desktop platforms. It is not expected that this will matter, as the
35//     preferred pattern of usage for DiscardableMemory is to lock down the
36//     memory, use it as quickly as possible, and then unlock it.
37//   - Because of memory alignment, the amount of memory allocated can be
38//     larger than the requested memory size. It is not very efficient for
39//     small allocations.
40//
41// References:
42//   - Linux: http://lwn.net/Articles/452035/
43//   - Mac: http://trac.webkit.org/browser/trunk/Source/WebCore/platform/mac/PurgeableBufferMac.cpp
44//          the comment starting with "vm_object_purgable_control" at
45//            http://www.opensource.apple.com/source/xnu/xnu-792.13.8/osfmk/vm/vm_object.c
46class BASE_EXPORT DiscardableMemory {
47 public:
48  DiscardableMemory();
49
50  // If the discardable memory is locked, the destructor will unlock it.
51  // The opened file will also be closed after this.
52  ~DiscardableMemory();
53
54  // Check whether the system supports discardable memory.
55  static bool Supported();
56
57  // Initialize the DiscardableMemory object. On success, this function returns
58  // true and the memory is locked. This should only be called once.
59  // This call could fail because of platform-specific limitations and the user
60  // should stop using the DiscardableMemory afterwards.
61  bool InitializeAndLock(size_t size);
62
63  // Lock the memory so that it will not be purged by the system. Returns
64  // DISCARDABLE_MEMORY_SUCCESS on success. If the return value is
65  // DISCARDABLE_MEMORY_FAILED then this object should be discarded and
66  // a new one should be created. If the return value is
67  // DISCARDABLE_MEMORY_PURGED then the memory is present but any data that
68  // was in it is gone.
69  LockDiscardableMemoryStatus Lock() WARN_UNUSED_RESULT;
70
71  // Unlock the memory so that it can be purged by the system. Must be called
72  // after every successful lock call.
73  void Unlock();
74
75  // Return the memory address held by this object. The object must be locked
76  // before calling this. Otherwise, this will cause a DCHECK error.
77  void* Memory() const;
78
79  // Testing utility calls.
80
81  // Check whether a purge of all discardable memory in the system is supported.
82  // Use only for testing!
83  static bool PurgeForTestingSupported();
84
85  // Purge all discardable memory in the system. This call has global effects
86  // across all running processes, so it should only be used for testing!
87  static void PurgeForTesting();
88
89 private:
90#if defined(OS_ANDROID)
91  // Maps the discardable memory into the caller's address space.
92  // Returns true on success, false otherwise.
93  bool Map();
94
95  // Unmaps the discardable memory from the caller's address space.
96  void Unmap();
97
98  // Reserve a file descriptor. When reaching the fd limit, this call returns
99  // false and initialization should fail.
100  bool ReserveFileDescriptor();
101
102  // Release a file descriptor so that others can reserve it.
103  void ReleaseFileDescriptor();
104#endif  // OS_ANDROID
105
106  void* memory_;
107  size_t size_;
108  bool is_locked_;
109#if defined(OS_ANDROID)
110  int fd_;
111#endif  // OS_ANDROID
112
113  DISALLOW_COPY_AND_ASSIGN(DiscardableMemory);
114};
115
116}  // namespace base
117
118#endif  // BASE_MEMORY_DISCARDABLE_MEMORY_H_
119