base: Improve DiscardableSharedMemory support for ashmem.

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
 
 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:
+  enum LockResult { SUCCESS, PURGED, FAILED };
+
   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();
 
   // 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_; }
 
   // Returns a shared memory handle for this DiscardableSharedMemory object.
   SharedMemoryHandle handle() const { return shared_memory_.handle(); }
 
   // Locks a range of memory so that it will not be purged by the system.
-  // Returns true if successful and the memory is still resident. Locking can
-  // fail for three reasons; object might have been purged, our last known usage
-  // timestamp might be out of date or memory might already be locked. Last
-  // know usage time is updated to the actual last usage timestamp if memory
-  // is still resident or 0 if not. The range of memory must be unlocked. The
-  // result of trying to lock an already locked range is undefined.
-  // |offset| and |length| must both be a multiple of the page size as returned
-  // by GetPageSize().
+  // The range of memory must be unlocked. The result of trying to lock an
+  // already locked range is undefined. |offset| and |length| must both be
+  // a multiple of the page size as returned by GetPageSize().
   // Passing 0 for |length| means "everything onward".
-  bool Lock(size_t offset, size_t length);
+  // Returns SUCCESS if range was successfully locked and the memory is still
+  // resident, PURGED if range was successfully locked but has been purged
+  // since last time it was locked and FAILED if range could not be locked.
+  // Locking can fail for two reasons; object might have been purged, our
+  // last known usage timestamp might be out of date. Last know usage time

[danakj, 2015/01/24 00:16:06]

Last known

[reveman, 2015/01/24 00:27:48]

Done.

+  // is updated to the actual last usage timestamp if memory

[danakj, 2015/01/24 00:16:06]

wrapping

[reveman, 2015/01/24 00:27:48]

Done.

+  // is still resident or 0 if not.
+  LockResult Lock(size_t offset, size_t length);
 
   // Unlock a previously successfully locked range of memory. The range of
   // memory must be locked. The result of trying to unlock a not
   // previously locked range is undefined.
   // |offset| and |length| must both be a multiple of the page size as returned
   // by GetPageSize().
   // Passing 0 for |length| means "everything onward".
   void Unlock(size_t offset, size_t length);
 
   // Gets a pointer to the opened discardable memory space. Discardable memory
@@ skipping 56 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_