Each mount of the devpts filesystem is now distinct such that ptys
  and their indicies allocated in one mount are independent from ptys
  and their indicies in all other mounts.

  All mounts of the devpts filesystem now create a /dev/pts/ptmx node
  with permissions 0000.

  To retain backwards compatibility the a ptmx device node (aka any node
  created with "mknod name c 5 2") when opened will look for an instance
  of devpts under the name "pts" in the same directory as the ptmx device
  node.

  As an option instead of placing a /dev/ptmx device node at /dev/ptmx
  it is possible to place a symlink to /dev/pts/ptmx at /dev/ptmx or
  to bind mount /dev/ptx/ptmx to /dev/ptmx.  If you opt for using
  the devpts filesystem in this manner devpts should be mounted with
  the ptmxmode=0666, or chmod 0666 /dev/pts/ptmx should be called.

  Total count of pty pairs in all instances is limited by sysctls:
  kernel.pty.max = 4096		- global limit

  kernel.pty.reserve = 1024	- reserved for filesystems mounted from the initial mount namespace

  kernel.pty.nr			- current count of ptys
  
  Per-instance limit could be set by adding mount option "max=<count>".
  This feature was added in kernel 3.4 together with sysctl kernel.pty.reserve.
  In kernels older than 3.4 sysctl kernel.pty.max works as per-instance limit.