title: unshare --pid /bin/bash - fork cannot allocate memory
body: <p>I'm experimenting with linux namespaces.  Specifically the pid namespace.</p>
  
  <p>I thought I'd test something out with bash but run into this problem:</p>
  
  <pre><code>unshare -p /bin/bash
  bash: fork: Cannot allocate memory
  </code></pre>
  
  <p>Running ls from there gave a core dump.  Exit is the only thing possible.</p>
  
  <p>Why is it doing that?</p>