[Discardable Shared Memory] Adding Windows 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 1b50da260e09b566807fb40e72a1ab0638db11d3..dc5b114bdd0ca46d29e7d3cfa6c105c8f66c19a2 100644
--- a/base/memory/discardable_shared_memory.cc
+++ b/base/memory/discardable_shared_memory.cc
@@ -21,6 +21,10 @@
 #include "third_party/ashmem/ashmem.h"
 #endif
 
+#if defined(OS_WIN)
+#include "base/win/windows_version.h"
+#endif
+
 namespace base {
 namespace {
 
@@ -214,6 +218,7 @@ DiscardableSharedMemory::LockResult DiscardableSharedMemory::Lock(
   DCHECK_EQ(locked_pages_.size(), locked_page_count_);
 #endif
 
+// Pin pages if supported.
 #if defined(OS_ANDROID)
   SharedMemoryHandle handle = shared_memory_.handle();
   if (SharedMemory::IsHandleValid(handle)) {
@@ -222,6 +227,14 @@ 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)) {
+      return PURGED;
+    }
+  }
 #endif
 
   return SUCCESS;
@@ -240,6 +253,7 @@ void DiscardableSharedMemory::Unlock(size_t offset, size_t length) {
 
   DCHECK(shared_memory_.memory());
 
+// Unpin pages if supported.
 #if defined(OS_ANDROID)
   SharedMemoryHandle handle = shared_memory_.handle();
   if (SharedMemory::IsHandleValid(handle)) {
@@ -248,7 +262,15 @@ 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) {
+    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();
   size_t end = start + length / base::GetPageSize();
@@ -340,6 +362,16 @@ bool DiscardableSharedMemory::Purge(Time current_time) {
               AlignToPageSize(mapped_size_), MADV_PURGE_ARGUMENT)) {
     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.
+  // NOTE: this is the best that can be done from the browser, until the
+  // child/renderer notices the purge and releases its own handles.

[reveman, 2015/11/03 01:42:31]

nit: concepts such as browser and renderer process should not leak into base/.
if we need this comment here then maybe "purging process" instead of browser and
"client process" instead of child/renderer.

[penny, 2015/11/03 02:17:08]

Done.

+  if (!VirtualFree(reinterpret_cast<char*>(shared_memory_.memory()) +
+                       AlignToPageSize(sizeof(SharedState)),
+                   AlignToPageSize(mapped_size_), MEM_DECOMMIT)) {
+    DPLOG(ERROR) << "VirtualFree() MEM_DECOMMIT failed in Purge()";
+  }
 #endif
 
   last_known_usage_ = Time();