title: mmap with offset and length maps the whole file
body: <p>Is it possible to <code>mmap</code> just part of a file using Python/mmap?</p>
  
  <p>I do it like this:</p>
  
  <pre><code>fn_from = 'test20mil.dat'
  print os.stat(fn_from).st_size
  8896332925
  f2= open(fn_from, 'r+')
  mm = mmap.mmap(f2.fileno(), 40, access=mmap.ACCESS_COPY, offset=2*mmap.ALLOCATIONGRANULARITY)
  </code></pre>
  
  <p>Should <code>mm</code> size be 40?</p>
  
  <pre><code>print mm.size()
  8896332925
  </code></pre>
  
  <p>Then when I read data it passes the boundary without error</p>
  
  <pre><code>while True:
    b= mm.read(256*1024)
    if not b: break;
  </code></pre>
  
  <p>What's the point of <code>length</code> then?</p>