Replace VirtualFree(DECOMMIT) with DiscardVirtualMemory in Purge().

Windows does not allow page-file backed memory-mapped file pages to be
de-committed, so the call to VirtualFree(MEM_DECOMMIT) always fails.

Windows 8.1 and above provide DiscardVirtualMemory(), which releases
the underlying storage for pages without decommitting them. This is
equivalent to swapping-out the pages, freeing up the physical memory
they occupied, but without any actual paging activity, since their
contents are being lost. The pages continue to be accounted against
the system's total commit charge, so this won't prevent apps OOMing
if the available commit charge is low.

Bug: 747657
Change-Id: Icf6d4b16c1adcc545b09bad207a9abd4a97e4726
Reviewed-on: https://chromium-review.googlesource.com/615062
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Penny MacNeil <pennymac@chromium.org>
Commit-Queue: Penny MacNeil <pennymac@chromium.org>
Cr-Commit-Position: refs/heads/master@{#495691}

base/memory/discardable_shared_memory.cc

@@ -24,6 +24,10 @@
 #include "third_party/ashmem/ashmem.h"
 #endif
 
+#if defined(OS_WIN)
+#include "base/win/windows_version.h"
+#endif
+
 namespace base {
 namespace {
 
@@ -383,12 +387,24 @@ bool DiscardableSharedMemory::Purge(Time current_time) {
     DPLOG(ERROR) << "madvise() failed";
   }
 #elif defined(OS_WIN)
-  // MEM_DECOMMIT the purged pages to release the physical storage,
-  // either in memory or in the paging file on disk.  Pages remain RESERVED.
-  if (!VirtualFree(reinterpret_cast<char*>(shared_memory_.memory()) +
-                       AlignToPageSize(sizeof(SharedState)),
-                   AlignToPageSize(mapped_size_), MEM_DECOMMIT)) {
-    DPLOG(ERROR) << "VirtualFree() MEM_DECOMMIT failed in Purge()";
+  if (base::win::GetVersion() >= base::win::VERSION_WIN8_1) {
+    // Discard the purged pages, which releases the physical storage (resident
+    // memory, compressed or swapped), but leaves them reserved & committed.
+    // This does not free commit for use by other applications, but allows the
+    // system to avoid compressing/swapping these pages to free physical memory.
+    static const auto discard_virtual_memory =
+        reinterpret_cast<decltype(&::DiscardVirtualMemory)>(GetProcAddress(
+            GetModuleHandle(L"kernel32.dll"), "DiscardVirtualMemory"));
+    if (discard_virtual_memory) {
+      DWORD discard_result = discard_virtual_memory(
+          reinterpret_cast<char*>(shared_memory_.memory()) +
+              AlignToPageSize(sizeof(SharedState)),
+          AlignToPageSize(mapped_size_));
+      if (discard_result != ERROR_SUCCESS) {
+        DLOG(FATAL) << "DiscardVirtualMemory() failed in Purge(): "
+                    << logging::SystemErrorCodeToString(discard_result);
+      }
+    }
   }
 #endif