101478b83SMauro Carvalho Chehab.. SPDX-License-Identifier: GPL-2.0 201478b83SMauro Carvalho Chehab 301478b83SMauro Carvalho Chehab===================== 401478b83SMauro Carvalho ChehabThe Devpts Filesystem 501478b83SMauro Carvalho Chehab===================== 601478b83SMauro Carvalho Chehab 701478b83SMauro Carvalho ChehabEach mount of the devpts filesystem is now distinct such that ptys 8*d56b699dSBjorn Helgaasand their indices allocated in one mount are independent from ptys 9*d56b699dSBjorn Helgaasand their indices in all other mounts. 1001478b83SMauro Carvalho Chehab 1101478b83SMauro Carvalho ChehabAll mounts of the devpts filesystem now create a ``/dev/pts/ptmx`` node 1201478b83SMauro Carvalho Chehabwith permissions ``0000``. 1301478b83SMauro Carvalho Chehab 1401478b83SMauro Carvalho ChehabTo retain backwards compatibility the a ptmx device node (aka any node 1501478b83SMauro Carvalho Chehabcreated with ``mknod name c 5 2``) when opened will look for an instance 1601478b83SMauro Carvalho Chehabof devpts under the name ``pts`` in the same directory as the ptmx device 1701478b83SMauro Carvalho Chehabnode. 1801478b83SMauro Carvalho Chehab 1901478b83SMauro Carvalho ChehabAs an option instead of placing a ``/dev/ptmx`` device node at ``/dev/ptmx`` 2001478b83SMauro Carvalho Chehabit is possible to place a symlink to ``/dev/pts/ptmx`` at ``/dev/ptmx`` or 2101478b83SMauro Carvalho Chehabto bind mount ``/dev/ptx/ptmx`` to ``/dev/ptmx``. If you opt for using 2201478b83SMauro Carvalho Chehabthe devpts filesystem in this manner devpts should be mounted with 2301478b83SMauro Carvalho Chehabthe ``ptmxmode=0666``, or ``chmod 0666 /dev/pts/ptmx`` should be called. 2401478b83SMauro Carvalho Chehab 2501478b83SMauro Carvalho ChehabTotal count of pty pairs in all instances is limited by sysctls:: 2601478b83SMauro Carvalho Chehab 2701478b83SMauro Carvalho Chehab kernel.pty.max = 4096 - global limit 2801478b83SMauro Carvalho Chehab kernel.pty.reserve = 1024 - reserved for filesystems mounted from the initial mount namespace 2901478b83SMauro Carvalho Chehab kernel.pty.nr - current count of ptys 3001478b83SMauro Carvalho Chehab 3101478b83SMauro Carvalho ChehabPer-instance limit could be set by adding mount option ``max=<count>``. 3201478b83SMauro Carvalho Chehab 3301478b83SMauro Carvalho ChehabThis feature was added in kernel 3.4 together with 3401478b83SMauro Carvalho Chehab``sysctl kernel.pty.reserve``. 3501478b83SMauro Carvalho Chehab 3601478b83SMauro Carvalho ChehabIn kernels older than 3.4 sysctl ``kernel.pty.max`` works as per-instance limit. 37