question about file mapping

Hi,

I create a file on tmpfs, and it read/write works fine. However, it cannot 
mmap, and returns EINVAL.
I found the reason is that dataspace of the file is not valid. 

        L4::Cap<L4Re::Dataspace> fds = fi->data_space()
       if (!fds.is_valid())
        {
          return -EINVAL;
        }

How to make it valid?  Is there another method to do file mapping?


Here is an example code snippet:
==========================

const char* filename = "/tmp/testfile";
ret = mount("/", "tmp", "tmpfs", 0, 0);

fd = open(filename, O_RDWR|O_CREAT);

write 10K data into file

addr = mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0); //return 
-1, and errno is EINVAL


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