Index: llvm/include/llvm/Support/FileSystem.h
===================================================================
--- llvm/include/llvm/Support/FileSystem.h
+++ llvm/include/llvm/Support/FileSystem.h
@@ -1251,6 +1251,9 @@
   size_t size() const;
   char *data() const;
 
+  /// Write changes to disk and synchronize. Equivalent to POSIX msync.
+  std::error_code sync() const;
+
   /// Get a const view of the data. Modifying this memory has undefined
   /// behavior.
   const char *const_data() const;
Index: llvm/lib/Support/Unix/Path.inc
===================================================================
--- llvm/lib/Support/Unix/Path.inc
+++ llvm/lib/Support/Unix/Path.inc
@@ -860,6 +860,14 @@
   return Size;
 }
 
+std::error_code mapped_file_region::sync() const {
+  assert(Mapping && "Mapping failed but used anyway!");
+  assert(Mode == readwrite && "sync only valid for readwrite regions!");
+  if (::msync(Mapping, Size, MS_SYNC))
+    return std::error_code(errno, std::generic_category());
+  return std::error_code();
+}
+
 char *mapped_file_region::data() const {
   assert(Mapping && "Mapping failed but used anyway!");
   return reinterpret_cast<char*>(Mapping);
Index: llvm/lib/Support/Windows/Path.inc
===================================================================
--- llvm/lib/Support/Windows/Path.inc
+++ llvm/lib/Support/Windows/Path.inc
@@ -935,6 +935,16 @@
   return Size;
 }
 
+std::error_code mapped_file_region::sync() const {
+  assert(Mapping && "Mapping failed but used anyway!");
+  assert(Mode == readwrite && "sync only valid for readwrite regions!");
+  if (!::FlushViewOfFile(Mapping, Size))
+    return mapWindowsError(GetLastError());
+  if (!::FlushFileBuffers(FileHandle))
+    return mapWindowsError(GetLastError());
+  return std::error_code();
+}
+
 char *mapped_file_region::data() const {
   assert(Mapping && "Mapping failed but used anyway!");
   return reinterpret_cast<char*>(Mapping);
Index: llvm/unittests/Support/Path.cpp
===================================================================
--- llvm/unittests/Support/Path.cpp
+++ llvm/unittests/Support/Path.cpp
@@ -1368,6 +1368,33 @@
   ASSERT_NO_ERROR(fs::remove(TempPath));
 }
 
+TEST_F(FileSystemTest, FileMappingSync) {
+  // Create a temp file.
+  int FD;
+  SmallString<64> TempPath;
+  ASSERT_NO_ERROR(fs::createTemporaryFile("prefix", "temp", FD, TempPath));
+  StringRef Content("hello there");
+  ASSERT_NO_ERROR(fs::resize_file_before_mapping_readwrite(FD, Content.size()));
+
+  {
+    // Map in the file and write some content.
+    std::error_code EC;
+    fs::mapped_file_region mfr(fs::convertFDToNativeFile(FD),
+                               fs::mapped_file_region::readwrite,
+                               Content.size(), 0, EC);
+    ASSERT_NO_ERROR(EC);
+    ASSERT_EQ(close(FD), 0);
+    std::copy(Content.begin(), Content.end(), mfr.data());
+
+    // Synchronize and check the contents before unmapping.
+    mfr.sync();
+    auto Buffer = MemoryBuffer::getFile(TempPath);
+    ASSERT_TRUE((bool)Buffer);
+    ASSERT_EQ(Content, Buffer->get()->getBuffer());
+  }
+  ASSERT_NO_ERROR(fs::remove(TempPath));
+}
+
 TEST(Support, NormalizePath) {
   //                           Input,        Expected Win, Expected Posix
   using TestTuple = std::tuple<const char *, const char *, const char *>;