Re-land: base: Use MADV_REMOVE instead of ftruncate to purge discardable memory segments.

base/memory/discardable_shared_memory.h

Index: base/memory/discardable_shared_memory.h
diff --git a/base/memory/discardable_shared_memory.h b/base/memory/discardable_shared_memory.h
index 5dca884ca82e2d6665930905c92e990b8a5515c2..5720fad3e4755d25c8263c0785084fc4314053ac 100644
--- a/base/memory/discardable_shared_memory.h
+++ b/base/memory/discardable_shared_memory.h
@@ -15,10 +15,13 @@
 #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 {
@@ -124,12 +127,6 @@ class BASE_EXPORT DiscardableSharedMemory {
     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;