resizing file size with ftruncate() after mmap()

Question

The code snippet works fine on my machine(Linux/x86-64)

int main()
{
  char* addr;
  int rc;
  int fd;
  const size_t PAGE_SIZE = 4096; // assuming the page size is 4096

  char buf[PAGE_SIZE];
  memset(buf, 'x', sizeof(buf));

  // error checking is ignored, for demonstration purpose

  fd = open("abc", O_RDWR | O_CREAT, S_IWUSR | S_IRUSR);

  ftruncate(fd, 0);

  write(fd, buf, 4090);

  // the file size is less than one page, but we allocate 2 page address space
  addr = mmap(NULL, PAGE_SIZE * 2, PROT_WRITE, MAP_SHARED, fd, 0);

  // it would crash if we read/write from addr[4096]

  // extend the size after mmap
  ftruncate(fd, PAGE_SIZE * 2);

  // now we can access(read/write) addr[4096]...addr[4096*2 -1]

  munmap(addr, PAGE_SIZE * 2);
  close(fd);

  exit(EXIT_SUCCESS);
}

But POSIX says:

If the size of the mapped file changes after the call to mmap() as a result of some other operation on the mapped file, the effect of references to portions of the mapped region that correspond to added or removed portions of the file is unspecified.

So I guess this is not a portable way. But is it guaranteed to work on Linux?