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[[!tag open_issue_hurd]]

As soon as one has accessed /proc/mounts, /proc can't be umount: 

    # settrans -ga /proc
    settrans: /proc: Device or resource busy

This is because mtab is still mounted on /proc/mounts:

    # settrans -ga /proc/mounts
    # settrans -ga /proc

settrans -R is supposed to be doing this, but unfortunately this is not
implemented yet in libnetfs, see #ifdef NOTYET in netfs_shutdown.