idea: when users call read() or readdir() to get contents of /proc or its
      children, these calls will invoke kernel provided read() or
      readdir(), and these functions will fill in the right contents
      

==========

when you do ls /proc to read the dir contents of /proc

proc_root_readdir

  proc_readdir // returns non-process directory entries; these were
               // created before using create_proc_entry

  proc_pid_readdir // returns fake dir entries, whose inode # encodes the
                   // process id.  no entry created for each process

  
==========
inode operations


proc_root_lookup 
   proc_pid_lookup