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
+// Define DISCARDABLE_SHARED_MEMORY_REMOVE if platform supports MADV_REMOVE
+// and the behavior of using madvise() to synchronsouly purge memory reliably
+// results in zero-filled-on-demand pages. madvise() will still be used on
+// other POSIX compatible platforms but doesn't provide the zero-filled pages
+// guarantee.
+#if defined(OS_POSIX) && !defined(OS_MACOSX) && !defined(OS_NACL)

[danakj, 2015/10/21 18:34:25]

Thanks for the extra comments, I'm finally wrapping my head around this a bit.
From what I understand MADV_REMOVE only exists on linux (and android cuz it's
linux)?

This define as is would include FREEBSD and NETBSD etc.. which it won't exist
on? I tried to google search and I didn't find anything saying it would exist.

So I guess LINUX||ANDROID is actually what you want, but how come you don't just
use the existance of MADV_REMOVE to inform this decision?

I like that you explain madvice will always be used tho, this comment is good.
Maybe you could say this is a linux-specific extension to madvise() and android
has it also (if this is true)?

[reveman, 2015/10/21 19:28:53]

Changed things a bit in latest patch, including a less cryptic name for this
define. I think it's in line with what you're asking for. Let me know what you
think.

+#define DISCARDABLE_SHARED_MEMORY_REMOVE
 #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_