Index: include/clang/Basic/FileManager.h
===================================================================
--- include/clang/Basic/FileManager.h
+++ include/clang/Basic/FileManager.h
@@ -239,7 +239,7 @@
   getBufferForFile(const FileEntry *Entry, bool isVolatile = false,
                    bool ShouldCloseOpenFile = true);
   llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>>
-  getBufferForFile(StringRef Filename);
+  getBufferForFile(StringRef Filename, bool isVolatile = false);
 
   /// Get the 'stat' information for the given \p Path.
   ///
Index: lib/Basic/FileManager.cpp
===================================================================
--- lib/Basic/FileManager.cpp
+++ lib/Basic/FileManager.cpp
@@ -450,13 +450,13 @@
 }
 
 llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>>
-FileManager::getBufferForFile(StringRef Filename) {
+FileManager::getBufferForFile(StringRef Filename, bool isVolatile) {
   if (FileSystemOpts.WorkingDir.empty())
-    return FS->getBufferForFile(Filename);
+    return FS->getBufferForFile(Filename, -1, true, isVolatile);
 
   SmallString<128> FilePath(Filename);
   FixupRelativePath(FilePath);
-  return FS->getBufferForFile(FilePath.c_str());
+  return FS->getBufferForFile(FilePath.c_str(), -1, true, isVolatile);
 }
 
 /// getStatValue - Get the 'stat' information for the specified path,
Index: lib/Frontend/ASTUnit.cpp
===================================================================
--- lib/Frontend/ASTUnit.cpp
+++ lib/Frontend/ASTUnit.cpp
@@ -156,7 +156,8 @@
 static std::unique_ptr<llvm::MemoryBuffer>
 getBufferForFileHandlingRemapping(const CompilerInvocation &Invocation,
                                   vfs::FileSystem *VFS,
-                                  StringRef FilePath) {
+                                  StringRef FilePath,
+                                  bool isVolatile) {
   const auto &PreprocessorOpts = Invocation.getPreprocessorOpts();
 
   // Try to determine if the main file has been remapped, either from the
@@ -176,7 +177,7 @@
         llvm::sys::fs::UniqueID MID = MPathStatus->getUniqueID();
         if (MainFileID == MID) {
           // We found a remapping. Try to load the resulting, remapped source.
-          BufferOwner = valueOrNull(VFS->getBufferForFile(RF.second));
+          BufferOwner = valueOrNull(VFS->getBufferForFile(RF.second, -1, true, isVolatile));
           if (!BufferOwner)
             return nullptr;
         }
@@ -201,7 +202,7 @@
 
   // If the main source file was not remapped, load it now.
   if (!Buffer && !BufferOwner) {
-    BufferOwner = valueOrNull(VFS->getBufferForFile(FilePath));
+    BufferOwner = valueOrNull(VFS->getBufferForFile(FilePath, -1, true, isVolatile));
     if (!BufferOwner)
       return nullptr;
   }
@@ -707,7 +708,7 @@
 std::unique_ptr<llvm::MemoryBuffer>
 ASTUnit::getBufferForFile(StringRef Filename, std::string *ErrorStr) {
   assert(FileMgr);
-  auto Buffer = FileMgr->getBufferForFile(Filename);
+  auto Buffer = FileMgr->getBufferForFile(Filename, UserFilesAreVolatile);
   if (Buffer)
     return std::move(*Buffer);
   if (ErrorStr)
@@ -1278,7 +1279,7 @@
       PreambleInvocationIn.getFrontendOpts().Inputs[0].getFile();
   std::unique_ptr<llvm::MemoryBuffer> MainFileBuffer =
       getBufferForFileHandlingRemapping(PreambleInvocationIn, VFS.get(),
-                                        MainFilePath);
+                                        MainFilePath, UserFilesAreVolatile);
   if (!MainFileBuffer)
     return nullptr;
 
Index: unittests/Frontend/ASTUnitTest.cpp
===================================================================
--- unittests/Frontend/ASTUnitTest.cpp
+++ unittests/Frontend/ASTUnitTest.cpp
@@ -18,6 +18,33 @@
 #include "llvm/Support/ToolOutputFile.h"
 #include "gtest/gtest.h"
 
+#define CREATE_AST_UNIT(isVolatile)                                            \
+  int FD;                                                                      \
+  llvm::SmallString<256> InputFileName;                                        \
+  ASSERT_FALSE(llvm::sys::fs::createTemporaryFile("ast-unit", "cpp", FD,       \
+                                                  InputFileName));             \
+  ToolOutputFile input_file(InputFileName, FD);                                \
+  input_file.os() << "";                                                       \
+                                                                               \
+  const char *Args[] = {"clang", "-xc++", InputFileName.c_str()};              \
+                                                                               \
+  IntrusiveRefCntPtr<DiagnosticsEngine> Diags =                                \
+      CompilerInstance::createDiagnostics(new DiagnosticOptions());            \
+                                                                               \
+  std::shared_ptr<CompilerInvocation> CInvok =                                 \
+      createInvocationFromCommandLine(Args, Diags);                            \
+                                                                               \
+  if (!CInvok)                                                                 \
+    FAIL() << "could not create compiler invocation";                          \
+                                                                               \
+  FileManager *FileMgr =                                                       \
+      new FileManager(FileSystemOptions(), vfs::getRealFileSystem());          \
+  auto PCHContainerOps = std::make_shared<PCHContainerOperations>();           \
+                                                                               \
+  std::unique_ptr<ASTUnit> AST = ASTUnit::LoadFromCompilerInvocation(          \
+      CInvok, PCHContainerOps, Diags, FileMgr, false, false, 0, TU_Complete,   \
+      false, false, isVolatile);
+
 using namespace llvm;
 using namespace clang;
 
@@ -38,29 +65,7 @@
     EXPECT_TRUE(PolicyWithDefaultLangOpt.UseVoidForZeroParams);
   }
 
-  int FD;
-  llvm::SmallString<256> InputFileName;
-  ASSERT_FALSE(llvm::sys::fs::createTemporaryFile("ast-unit", "cpp", FD, InputFileName));
-  ToolOutputFile input_file(InputFileName, FD);
-  input_file.os() << "";
-
-  const char* Args[] = {"clang", "-xc++", InputFileName.c_str()};
-
-  IntrusiveRefCntPtr<DiagnosticsEngine> Diags =
-      CompilerInstance::createDiagnostics(new DiagnosticOptions());
-
-  std::shared_ptr<CompilerInvocation> CInvok =
-      createInvocationFromCommandLine(Args, Diags);
-
-  if (!CInvok)
-    FAIL() << "could not create compiler invocation";
-
-  FileManager *FileMgr =
-      new FileManager(FileSystemOptions(), vfs::getRealFileSystem());
-  auto PCHContainerOps = std::make_shared<PCHContainerOperations>();
-
-  std::unique_ptr<ASTUnit> AST = ASTUnit::LoadFromCompilerInvocation(
-      CInvok, PCHContainerOps, Diags, FileMgr);
+  CREATE_AST_UNIT(false);
 
   if (!AST)
     FAIL() << "failed to create ASTUnit";
@@ -84,4 +89,17 @@
   EXPECT_FALSE(AU->getASTContext().getPrintingPolicy().UseVoidForZeroParams);
 }
 
+TEST(ASTUnit, GetBufferForFileMemoryMapping) {
+    CREATE_AST_UNIT(true);
+
+    if (!AST)
+      FAIL() << "failed to create ASTUnit";
+
+    std::unique_ptr<llvm::MemoryBuffer> memoryBuffer =
+        AST->getBufferForFile(InputFileName);
+
+    EXPECT_NE(memoryBuffer->getBufferKind(),
+              llvm::MemoryBuffer::MemoryBuffer_MMap);
+}
+
 } // anonymous namespace