diff --git a/lib/Support/FileOutputBuffer.cpp b/lib/Support/FileOutputBuffer.cpp
index e6e4da34652..df4ae9322bc 100644
--- a/lib/Support/FileOutputBuffer.cpp
+++ b/lib/Support/FileOutputBuffer.cpp
@@ -77,6 +77,10 @@ FileOutputBuffer::create(StringRef FilePath, size_t Size,
   if (EC)
     return EC;
 
+  EC = sys::fs::resize_file(FD, Size);
+  if (EC)
+    return EC;
+
   auto MappedFile = llvm::make_unique<mapped_file_region>(
       FD, mapped_file_region::readwrite, Size, 0, EC);
   int Ret = close(FD);
diff --git a/lib/Support/Unix/Path.inc b/lib/Support/Unix/Path.inc
index 75bcc03bbd3..bd6a605a93d 100644
--- a/lib/Support/Unix/Path.inc
+++ b/lib/Support/Unix/Path.inc
@@ -414,19 +414,7 @@ std::error_code setLastModificationAndAccessTime(int FD, TimeValue Time) {
 
 std::error_code mapped_file_region::init(int FD, uint64_t Offset,
                                          mapmode Mode) {
-  // Figure out how large the file is.
-  struct stat FileInfo;
-  if (fstat(FD, &FileInfo) == -1)
-    return std::error_code(errno, std::generic_category());
-  uint64_t FileSize = FileInfo.st_size;
-
-  if (Size == 0)
-    Size = FileSize;
-  else if (FileSize < Size) {
-    // We need to grow the file.
-    if (ftruncate(FD, Size) == -1)
-      return std::error_code(errno, std::generic_category());
-  }
+  assert(Size != 0);
 
   int flags = (Mode == readwrite) ? MAP_SHARED : MAP_PRIVATE;
   int prot = (Mode == readonly) ? PROT_READ : (PROT_READ | PROT_WRITE);
diff --git a/unittests/Support/Path.cpp b/unittests/Support/Path.cpp
index 88baadeca02..0d661c8ae6c 100644
--- a/unittests/Support/Path.cpp
+++ b/unittests/Support/Path.cpp
@@ -654,12 +654,15 @@ TEST_F(FileSystemTest, FileMapping) {
   SmallString<64> TempPath;
   ASSERT_NO_ERROR(
       fs::createTemporaryFile("prefix", "temp", FileDescriptor, TempPath));
+  unsigned Size = 4096;
+  ASSERT_NO_ERROR(fs::resize_file(FileDescriptor, Size));
+
   // Map in temp file and add some content
   std::error_code EC;
   StringRef Val("hello there");
   {
     fs::mapped_file_region mfr(FileDescriptor,
-                               fs::mapped_file_region::readwrite, 4096, 0, EC);
+                               fs::mapped_file_region::readwrite, Size, 0, EC);
     ASSERT_NO_ERROR(EC);
     std::copy(Val.begin(), Val.end(), mfr.data());
     // Explicitly add a 0.
@@ -671,14 +674,14 @@ TEST_F(FileSystemTest, FileMapping) {
   int FD;
   EC = fs::openFileForRead(Twine(TempPath), FD);
   ASSERT_NO_ERROR(EC);
-  fs::mapped_file_region mfr(FD, fs::mapped_file_region::readonly, 0, 0, EC);
+  fs::mapped_file_region mfr(FD, fs::mapped_file_region::readonly, Size, 0, EC);
   ASSERT_NO_ERROR(EC);
 
   // Verify content
   EXPECT_EQ(StringRef(mfr.const_data()), Val);
 
   // Unmap temp file
-  fs::mapped_file_region m(FD, fs::mapped_file_region::readonly, 0, 0, EC);
+  fs::mapped_file_region m(FD, fs::mapped_file_region::readonly, Size, 0, EC);
   ASSERT_NO_ERROR(EC);
   ASSERT_EQ(close(FD), 0);
 }