Index: llvm/include/llvm/Support/FileSystem.h
===================================================================
--- llvm/include/llvm/Support/FileSystem.h
+++ llvm/include/llvm/Support/FileSystem.h
@@ -464,6 +464,32 @@
   return !equivalent(A, B, result) && result;
 }
 
+/// @brief Is the file mounted on a local filesystem?
+///
+/// @param path Input path.
+/// @param result Set to true if \a path is on fixed media such as a hard disk,
+///               false if it is not.
+/// @returns errc::success if result has been successfully set, otherwise a
+///          platform specific error_code.
+std::error_code is_local(const Twine &path, bool &result);
+
+/// @brief Version of is_local accepting an open file descriptor.
+std::error_code is_local(int FD, bool &result);
+
+/// @brief Simpler version of is_local for clients that don't need to
+///        differentiate between an error and false.
+inline bool is_local(const Twine &Path) {
+  bool Result;
+  return !is_local(Path, Result) && Result;
+}
+
+/// @brief Simpler version of is_local accepting an open file descriptor for
+///        clients that don't need to differentiate between an error and false.
+inline bool is_local(int FD) {
+  bool Result;
+  return !is_local(FD, Result) && Result;
+}
+
 /// @brief Does status represent a directory?
 ///
 /// @param status A file_status previously returned from status.
Index: llvm/lib/Support/MemoryBuffer.cpp
===================================================================
--- llvm/lib/Support/MemoryBuffer.cpp
+++ llvm/lib/Support/MemoryBuffer.cpp
@@ -297,6 +297,16 @@
   if (MapSize < 4 * 4096 || MapSize < (unsigned)PageSize)
     return false;
 
+  // Don't use mmap on files which are not on locally mounted file systems, as
+  // this can lead to instability if the filesystem is unmounted or a network
+  // connection is interrupted.  If we read the entire contents from disk, we
+  // could still fail if the NFS goes down during the short window where the
+  // file contents are being read, but mmaps can be left open for very long
+  // periods of time, significantly increasing the likelihood of such an error
+  // occuring in practice.
+  if (!sys::fs::is_local(FD))
+    return false;
+
   if (!RequiresNullTerminator)
     return true;
 
Index: llvm/lib/Support/Unix/Path.inc
===================================================================
--- llvm/lib/Support/Unix/Path.inc
+++ llvm/lib/Support/Unix/Path.inc
@@ -65,23 +65,31 @@
 #endif
 
 #include <sys/types.h>
-#if !defined(__APPLE__) && !defined(__OpenBSD__) && !defined(__ANDROID__)
+#if !defined(__APPLE__) && !defined(__OpenBSD__) && !defined(__FreeBSD__) &&   \
+    !defined(__ANDROID__) && !defined(__linux__)
 #include <sys/statvfs.h>
 #define STATVFS statvfs
+#define FSTATVFS fstatvfs
 #define STATVFS_F_FRSIZE(vfs) vfs.f_frsize
 #else
-#ifdef __OpenBSD__
+#if defined(__OpenBSD__) || defined(__FreeBSD__)
 #include <sys/param.h>
 #include <sys/mount.h>
-#elif defined(__ANDROID__)
+#elif defined(__ANDROID__) || defined(__linux__)
 #include <sys/vfs.h>
 #else
 #include <sys/mount.h>
 #endif
 #define STATVFS statfs
+#define FSTATVFS fstatfs
 #define STATVFS_F_FRSIZE(vfs) static_cast<uint64_t>(vfs.f_bsize)
 #endif
 
+#if defined(__NetBSD__)
+#define STATVFS_F_FLAG(vfs) (vfs).f_flag
+#else
+#define STATVFS_F_FLAG(vfs) (vfs).f_flags
+#endif
 
 using namespace llvm;
 
@@ -335,6 +343,40 @@
   return std::error_code();
 }
 
+static bool is_local_impl(struct STATVFS &Vfs) {
+#if defined(__linux__)
+  constexpr uint32_t CIFS_MAGIC_NUMBER = 0xFF534D42;
+  switch ((uint32_t)Vfs.f_type) {
+  case NFS_SUPER_MAGIC:
+  case SMB_SUPER_MAGIC:
+  case CIFS_MAGIC_NUMBER:
+    return false;
+  default:
+    return true;
+  }
+#else
+  return !!(STATVFS_F_FLAG(Vfs) & MNT_LOCAL);
+#endif
+}
+
+std::error_code is_local(const Twine &Path, bool &Result) {
+  struct STATVFS Vfs;
+  if (::STATVFS(Path.str().c_str(), &Vfs))
+    return std::error_code(errno, std::generic_category());
+
+  Result = is_local_impl(Vfs);
+  return std::error_code();
+}
+
+std::error_code is_local(int FD, bool &Result) {
+  struct STATVFS Vfs;
+  if (::FSTATVFS(FD, &Vfs))
+    return std::error_code(errno, std::generic_category());
+
+  Result = is_local_impl(Vfs);
+  return std::error_code();
+}
+
 std::error_code rename(const Twine &from, const Twine &to) {
   // Get arguments.
   SmallString<128> from_storage;
@@ -491,6 +533,23 @@
 
   int flags = (Mode == readwrite) ? MAP_SHARED : MAP_PRIVATE;
   int prot = (Mode == readonly) ? PROT_READ : (PROT_READ | PROT_WRITE);
+#if defined(__APPLE__)
+//----------------------------------------------------------------------
+// Newer versions of MacOSX have a flag that will allow us to read from
+// binaries whose code signature is invalid without crashing by using
+// the MAP_RESILIENT_CODESIGN flag. Also if a file from removable media
+// is mapped we can avoid crashing and return zeroes to any pages we try
+// to read if the media becomes unavailable by using the
+// MAP_RESILIENT_MEDIA flag.
+//----------------------------------------------------------------------
+#if defined(MAP_RESILIENT_CODESIGN)
+  flags |= MAP_RESILIENT_CODESIGN;
+#endif
+#if defined(MAP_RESILIENT_MEDIA)
+  flags |= MAP_RESILIENT_MEDIA;
+#endif
+#endif // #if defined (__APPLE__)
+
   Mapping = ::mmap(nullptr, Size, prot, flags, FD, Offset);
   if (Mapping == MAP_FAILED)
     return std::error_code(errno, std::generic_category());
Index: llvm/lib/Support/Windows/Path.inc
===================================================================
--- llvm/lib/Support/Windows/Path.inc
+++ llvm/lib/Support/Windows/Path.inc
@@ -277,6 +277,80 @@
   return std::error_code();
 }
 
+static std::error_code is_local_internal(SmallVectorImpl<wchar_t> &Path,
+                                         bool &Result) {
+  SmallVector<wchar_t, 128> VolumePath;
+  size_t Len = 128;
+  while (true) {
+    VolumePath.resize(Len);
+    BOOL Success =
+        ::GetVolumePathNameW(Path.data(), VolumePath.data(), VolumePath.size());
+
+    if (Success)
+      break;
+
+    DWORD Err = ::GetLastError();
+    if (Err != ERROR_INSUFFICIENT_BUFFER)
+      return mapWindowsError(Err);
+
+    Len *= 2;
+  }
+  // If the output buffer has exactly enough space for the path name, but not
+  // the null terminator, it will leave the output unterminated.  Push a null
+  // terminator onto the end to ensure that this never happens.
+  VolumePath.push_back(L'\0');
+  VolumePath.set_size(wcslen(VolumePath.data()));
+  const wchar_t *P = VolumePath.data();
+
+  UINT Type = ::GetDriveTypeW(P);
+  switch (Type) {
+  case DRIVE_FIXED:
+    Result = true;
+    return std::error_code();
+  case DRIVE_REMOTE:
+  case DRIVE_CDROM:
+  case DRIVE_RAMDISK:
+  case DRIVE_REMOVABLE:
+    Result = false;
+    return std::error_code();
+  default:
+    return make_error_code(errc::no_such_file_or_directory);
+  }
+  llvm_unreachable("Unreachable!");
+}
+
+std::error_code is_local(const Twine &path, bool &result) {
+  if (!llvm::sys::fs::exists(path) || !llvm::sys::path::has_root_path(path))
+    return make_error_code(errc::no_such_file_or_directory);
+
+  SmallString<128> Storage;
+  StringRef P = path.toStringRef(Storage);
+
+  // Convert to utf-16.
+  SmallVector<wchar_t, 128> WidePath;
+  if (std::error_code ec = widenPath(P, WidePath))
+    return ec;
+  return is_local_internal(WidePath, result);
+}
+
+std::error_code is_local(int FD, bool &Result) {
+  SmallVector<wchar_t, 128> FinalPath;
+  HANDLE Handle = reinterpret_cast<HANDLE>(_get_osfhandle(FD));
+
+  size_t Len = 128;
+  do {
+    FinalPath.reserve(Len);
+    Len = ::GetFinalPathNameByHandleW(Handle, FinalPath.data(),
+                                      FinalPath.capacity() - 1, VOLUME_NAME_NT);
+    if (Len == 0)
+      return mapWindowsError(::GetLastError());
+  } while (Len > FinalPath.capacity());
+
+  FinalPath.set_size(Len);
+
+  return is_local_internal(FinalPath, Result);
+}
+
 std::error_code rename(const Twine &from, const Twine &to) {
   // Convert to utf-16.
   SmallVector<wchar_t, 128> wide_from;
Index: llvm/unittests/Support/Path.cpp
===================================================================
--- llvm/unittests/Support/Path.cpp
+++ llvm/unittests/Support/Path.cpp
@@ -1136,6 +1136,28 @@
   ::close(FileDescriptor);
 }
 
+TEST_F(FileSystemTest, is_local) {
+  SmallString<128> CurrentPath;
+  ASSERT_NO_ERROR(fs::current_path(CurrentPath));
+
+  bool Result;
+  ASSERT_NO_ERROR(fs::is_local(CurrentPath, Result));
+  EXPECT_TRUE(Result);
+  EXPECT_TRUE(fs::is_local(CurrentPath));
+
+  int FD;
+  SmallString<64> TempPath;
+  ASSERT_NO_ERROR(fs::createTemporaryFile("prefix", "temp", FD, TempPath));
+  FileRemover Cleanup(TempPath);
+
+  // Make sure it exists.
+  ASSERT_TRUE(sys::fs::exists(Twine(TempPath)));
+
+  ASSERT_NO_ERROR(fs::is_local(FD, Result));
+  EXPECT_TRUE(Result);
+  EXPECT_TRUE(fs::is_local(FD));
+}
+
 TEST_F(FileSystemTest, set_current_path) {
   SmallString<128> path;