The Devpts Filesystem

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