Index: llvm/trunk/lib/Support/FileOutputBuffer.cpp
===================================================================
--- llvm/trunk/lib/Support/FileOutputBuffer.cpp
+++ llvm/trunk/lib/Support/FileOutputBuffer.cpp
@@ -120,7 +120,7 @@
   return llvm::make_unique<InMemoryBuffer>(Path, MB, Mode);
 }
 
-static Expected<std::unique_ptr<OnDiskBuffer>>
+static Expected<std::unique_ptr<FileOutputBuffer>>
 createOnDiskBuffer(StringRef Path, size_t Size, unsigned Mode) {
   Expected<fs::TempFile> FileOrErr =
       fs::TempFile::create(Path + ".tmp%%%%%%%", Mode);
@@ -144,10 +144,14 @@
   std::error_code EC;
   auto MappedFile = llvm::make_unique<fs::mapped_file_region>(
       File.FD, fs::mapped_file_region::readwrite, Size, 0, EC);
+
+  // mmap(2) can fail if the underlying filesystem does not support it.
+  // If that happens, we fall back to in-memory buffer as the last resort.
   if (EC) {
     consumeError(File.discard());
-    return errorCodeToError(EC);
+    return createInMemoryBuffer(Path, Size, Mode);
   }
+
   return llvm::make_unique<OnDiskBuffer>(Path, std::move(File),
                                          std::move(MappedFile));
 }