diff --git a/lld/ELF/Config.h b/lld/ELF/Config.h
--- a/lld/ELF/Config.h
+++ b/lld/ELF/Config.h
@@ -165,6 +165,7 @@
   bool ltoNewPassManager;
   bool mergeArmExidx;
   bool mipsN32Abi = false;
+  bool mmapOutputFile;
   bool nmagic;
   bool noinhibitExec;
   bool nostdlib;
diff --git a/lld/ELF/Driver.cpp b/lld/ELF/Driver.cpp
--- a/lld/ELF/Driver.cpp
+++ b/lld/ELF/Driver.cpp
@@ -886,6 +886,8 @@
   config->mipsGotSize = args::getInteger(args, OPT_mips_got_size, 0xfff0);
   config->mergeArmExidx =
       args.hasFlag(OPT_merge_exidx_entries, OPT_no_merge_exidx_entries, true);
+  config->mmapOutputFile =
+      args.hasFlag(OPT_mmap_output_file, OPT_no_mmap_output_file, true);
   config->nmagic = args.hasFlag(OPT_nmagic, OPT_no_nmagic, false);
   config->noinhibitExec = args.hasArg(OPT_noinhibit_exec);
   config->nostdlib = args.hasArg(OPT_nostdlib);
diff --git a/lld/ELF/Options.td b/lld/ELF/Options.td
--- a/lld/ELF/Options.td
+++ b/lld/ELF/Options.td
@@ -232,6 +232,10 @@
     "Enable merging .ARM.exidx entries (default)",
     "Disable merging .ARM.exidx entries">;
 
+defm mmap_output_file: B<"mmap-output-file",
+    "Mmap the output file for writing (default)",
+    "Do not mmap the output file for writing">;
+
 def nmagic: F<"nmagic">, MetaVarName<"<magic>">,
   HelpText<"Do not page align sections, link against static libraries.">;
 
@@ -566,7 +570,6 @@
 def: F<"no-copy-dt-needed-entries">;
 def: F<"no-ctors-in-init-array">;
 def: F<"no-keep-memory">;
-def: F<"no-mmap-output-file">;
 def: F<"no-pipeline-knowledge">;
 def: F<"no-warn-mismatch">;
 def: Flag<["-"], "p">;
diff --git a/lld/ELF/Writer.cpp b/lld/ELF/Writer.cpp
--- a/lld/ELF/Writer.cpp
+++ b/lld/ELF/Writer.cpp
@@ -2594,6 +2594,8 @@
   unsigned flags = 0;
   if (!config->relocatable)
     flags = FileOutputBuffer::F_executable;
+  if (!config->mmapOutputFile)
+    flags |= FileOutputBuffer::F_no_mmap;
   Expected<std::unique_ptr<FileOutputBuffer>> bufferOrErr =
       FileOutputBuffer::create(config->outputFile, fileSize, flags);
 
diff --git a/lld/test/ELF/mmap-output-file.s b/lld/test/ELF/mmap-output-file.s
new file mode 100644
--- /dev/null
+++ b/lld/test/ELF/mmap-output-file.s
@@ -0,0 +1,12 @@
+# REQUIRES: x86
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
+# RUN: ld.lld %t.o -o %t
+# RUN: ld.lld %t.o -mmap-output-file -o %t1
+# RUN: cmp %t %t1
+# RUN: ld.lld %t.o -no-mmap-output-file -o %t2
+# RUN: cmp %t %t2
+
+.globl _start
+_start:
+  nop
diff --git a/lld/test/ELF/silent-ignore.test b/lld/test/ELF/silent-ignore.test
--- a/lld/test/ELF/silent-ignore.test
+++ b/lld/test/ELF/silent-ignore.test
@@ -6,7 +6,6 @@
 RUN:   -no-copy-dt-needed-entries \
 RUN:   -no-ctors-in-init-array \
 RUN:   -no-keep-memory \
-RUN:   -no-mmap-output-file \
 RUN:   -no-pipeline-knowledge \
 RUN:   -no-warn-mismatch \
 RUN:   -p \
diff --git a/llvm/include/llvm/Support/FileOutputBuffer.h b/llvm/include/llvm/Support/FileOutputBuffer.h
--- a/llvm/include/llvm/Support/FileOutputBuffer.h
+++ b/llvm/include/llvm/Support/FileOutputBuffer.h
@@ -32,6 +32,10 @@
   enum {
     /// set the 'x' bit on the resulting file
     F_executable = 1,
+
+    /// Don't use mmap and instead write an in-memory buffer to a file when this
+    /// buffer is closed.
+    F_no_mmap = 2,
   };
 
   /// Factory method to create an OutputBuffer object which manages a read/write
diff --git a/llvm/lib/Support/FileOutputBuffer.cpp b/llvm/lib/Support/FileOutputBuffer.cpp
--- a/llvm/lib/Support/FileOutputBuffer.cpp
+++ b/llvm/lib/Support/FileOutputBuffer.cpp
@@ -189,7 +189,10 @@
   case fs::file_type::regular_file:
   case fs::file_type::file_not_found:
   case fs::file_type::status_error:
-    return createOnDiskBuffer(Path, Size, Mode);
+    if (Flags & F_no_mmap)
+      return createInMemoryBuffer(Path, Size, Mode);
+    else
+      return createOnDiskBuffer(Path, Size, Mode);
   default:
     return createInMemoryBuffer(Path, Size, Mode);
   }
diff --git a/llvm/unittests/Support/FileOutputBufferTest.cpp b/llvm/unittests/Support/FileOutputBufferTest.cpp
--- a/llvm/unittests/Support/FileOutputBufferTest.cpp
+++ b/llvm/unittests/Support/FileOutputBufferTest.cpp
@@ -118,6 +118,28 @@
   EXPECT_TRUE(IsExecutable);
   ASSERT_NO_ERROR(fs::remove(File4.str()));
 
+  // TEST 5: In-memory buffer works as expected.
+  SmallString<128> File5(TestDirectory);
+  File5.append("/file5");
+  {
+    Expected<std::unique_ptr<FileOutputBuffer>> BufferOrErr =
+        FileOutputBuffer::create(File5, 8000, FileOutputBuffer::F_no_mmap);
+    ASSERT_NO_ERROR(errorToErrorCode(BufferOrErr.takeError()));
+    std::unique_ptr<FileOutputBuffer> &Buffer = *BufferOrErr;
+    // Start buffer with special header.
+    memcpy(Buffer->getBufferStart(), "AABBCCDDEEFFGGHHIIJJ", 20);
+    ASSERT_NO_ERROR(errorToErrorCode(Buffer->commit()));
+    // Write to end of buffer to verify it is writable.
+    memcpy(Buffer->getBufferEnd() - 20, "AABBCCDDEEFFGGHHIIJJ", 20);
+    // Commit buffer.
+    ASSERT_NO_ERROR(errorToErrorCode(Buffer->commit()));
+  }
+
+  // Verify file is correct size.
+  uint64_t File5Size;
+  ASSERT_NO_ERROR(fs::file_size(Twine(File5), File5Size));
+  ASSERT_EQ(File5Size, 8000ULL);
+  ASSERT_NO_ERROR(fs::remove(File5.str()));
   // Clean up.
   ASSERT_NO_ERROR(fs::remove(TestDirectory.str()));
 }