Tracking dirty page rate in Linux using python

Question

I'm trying to write a piece of code in Python which tracks page dirtying of a specific process in Linux. I got the solution description in the Linux Kernel documents as follow:

The soft-dirty is a bit on a PTE which helps to track which pages a task writes to. In order to do this tracking one should

1. Clear soft-dirty bits from the task's PTEs.

   This is done by writing "4" into the /proc/PID/clear_refs file of the task in question.

2. Wait some time.

3. Read soft-dirty bits from the PTEs.

   This is done by reading from the /proc/PID/pagemap. The bit 55 of the 64-bit qword is the soft-dirty one. If set, the respective PTE was written to since step 1.

I have a problem in performing step 3. I wrote a code in python which always returns a zero. I do not know how I can read bit 55 in each PTE. I appreciate any help in this regards.

Here is I wrote in Python:

    #!/usr/bin/python

    import sys
    import os
    import struct

    def read_entry(pid, size=8):
        maps_path = "/proc/{0}/pagemap".format(pid)
        if not os.path.isfile(maps_path):
            print "Process {0} doesn't exist.".format(pid)[0]
            return
        with open(maps_path, 'rb') as f:
            try:
                f.seek(0)
                while True:
                    return struct.unpack('Q', f.read(size))

            finally:
                f.close()

if __name__ == "__main__":
    pid = sys.argv[1]
    print (read_entry(pid))