Revert of base: Use MADV_REMOVE instead of ftruncate to purge discardable memory segments.

base/memory/discardable_shared_memory.cc

Index: base/memory/discardable_shared_memory.cc
diff --git a/base/memory/discardable_shared_memory.cc b/base/memory/discardable_shared_memory.cc
index 1b50da260e09b566807fb40e72a1ab0638db11d3..d0eaca1a66219eafee72db30d3e91170e4055e98 100644
--- a/base/memory/discardable_shared_memory.cc
+++ b/base/memory/discardable_shared_memory.cc
@@ -4,9 +4,8 @@
 
 #include "base/memory/discardable_shared_memory.h"
 
-#if defined(OS_POSIX) && !defined(OS_NACL)
-// For madvise() which is available on all POSIX compatible systems.
-#include <sys/mman.h>
+#if defined(OS_POSIX)
+#include <unistd.h>
 #endif
 
 #include <algorithm>
@@ -302,7 +301,11 @@
 bool DiscardableSharedMemory::Purge(Time current_time) {
   // Calls to this function must be synchronized properly.
   DFAKE_SCOPED_LOCK(thread_collision_warner_);
-  DCHECK(shared_memory_.memory());
+
+  // Early out if not mapped. This can happen if the segment was previously
+  // unmapped using a call to Close().
+  if (!shared_memory_.memory())
+    return true;
 
   SharedState old_state(SharedState::UNLOCKED, last_known_usage_);
   SharedState new_state(SharedState::UNLOCKED, Time());
@@ -323,25 +326,6 @@
     return false;
   }
 
-#if defined(OS_POSIX) && !defined(OS_NACL)
-// Linux and Android provide MADV_REMOVE which is preferred as it has a
-// behavior that can be verified in tests. Other POSIX flavors (MacOSX, BSDs),
-// provide MADV_FREE which has the same result but memory is purged lazily.
-#if defined(OS_LINUX) || defined(OS_ANDROID)
-#define MADV_PURGE_ARGUMENT MADV_REMOVE
-#else
-#define MADV_PURGE_ARGUMENT MADV_FREE
-#endif
-  // Advise the kernel to remove resources associated with purged pages.
-  // Subsequent accesses of memory pages will succeed, but might result in
-  // zero-fill-on-demand pages.
-  if (madvise(reinterpret_cast<char*>(shared_memory_.memory()) +
-                  AlignToPageSize(sizeof(SharedState)),
-              AlignToPageSize(mapped_size_), MADV_PURGE_ARGUMENT)) {
-    DPLOG(ERROR) << "madvise() failed";
-  }
-#endif
-
   last_known_usage_ = Time();
   return true;
 }
@@ -369,6 +353,26 @@
   shared_memory_.Close();
 }
 
+#if defined(DISCARDABLE_SHARED_MEMORY_SHRINKING)
+void DiscardableSharedMemory::Shrink() {
+#if defined(OS_POSIX)
+  SharedMemoryHandle handle = shared_memory_.handle();
+  if (!SharedMemory::IsHandleValid(handle))
+    return;
+
+  // Truncate shared memory to size of SharedState.
+  if (HANDLE_EINTR(ftruncate(SharedMemory::GetFdFromSharedMemoryHandle(handle),
+                             AlignToPageSize(sizeof(SharedState)))) != 0) {
+    DPLOG(ERROR) << "ftruncate() failed";
+    return;
+  }
+  mapped_size_ = 0;
+#else
+  NOTIMPLEMENTED();
+#endif
+}
+#endif
+
 Time DiscardableSharedMemory::Now() const {
   return Time::Now();
 }