[Discardable Shared Memory] Remove Windows (un)pinning support.

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 acae29eabadec0a16de0ed5e0e43f672d3faddca..c04e5c41c58d0b7945afcafb25e7d5cd3b5aee05 100644
--- a/base/memory/discardable_shared_memory.cc
+++ b/base/memory/discardable_shared_memory.cc
@@ -24,10 +24,6 @@
 #include "third_party/ashmem/ashmem.h"
 #endif
 
-#if defined(OS_WIN)
-#include "base/win/windows_version.h"
-#endif
-
 namespace base {
 namespace {
 
@@ -234,14 +230,6 @@ DiscardableSharedMemory::LockResult DiscardableSharedMemory::Lock(
       return PURGED;
     }
   }
-#elif defined(OS_WIN)
-  if (base::win::GetVersion() >= base::win::VERSION_WIN8) {
-    if (!VirtualAlloc(reinterpret_cast<char*>(shared_memory_.memory()) +
-                          AlignToPageSize(sizeof(SharedState)) + offset,
-                      length, MEM_RESET_UNDO, PAGE_READWRITE)) {

[danakj, 2016/03/25 23:49:39]

I'm confused by this code heh.

https://msdn.microsoft.com/en-us/library/windows/desktop/aa366887(v=vs.85).aspx
says MEM_RESET_UNDO "should only be called on an address range to which
MEM_RESET was successfully applied earlier", but Lock() is usually called before
Unlock(), or not in this case?

Were we using this incorrectly? I don't suppose that could be related to our
woes.

[reveman, 2016/03/26 07:01:10]

Discardable memory is initially locked. The only time we'd call Lock is as a
result of first calling Unlock.

-      return PURGED;
-    }
-  }
 #endif
 
   return SUCCESS;
@@ -269,18 +257,6 @@ void DiscardableSharedMemory::Unlock(size_t offset, size_t length) {
       DPLOG(ERROR) << "ashmem_unpin_region() failed";
     }
   }
-#elif defined(OS_WIN)
-  if (base::win::GetVersion() >= base::win::VERSION_WIN8) {
-    // Note: MEM_RESET is not technically gated on Win8.  However, this Unlock
-    // function needs to match the Lock behaviour (MEM_RESET_UNDO) to properly
-    // implement memory pinning.  It needs to bias towards preserving the
-    // contents of memory between an Unlock and next Lock.
-    if (!VirtualAlloc(reinterpret_cast<char*>(shared_memory_.memory()) +
-                          AlignToPageSize(sizeof(SharedState)) + offset,
-                      length, MEM_RESET, PAGE_READWRITE)) {
-      DPLOG(ERROR) << "VirtualAlloc() MEM_RESET failed in Unlock()";
-    }
-  }
 #endif
 
   size_t start = offset / base::GetPageSize();