tjh.dev
Linux kernel mirror (for testing)
git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
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1Each mount of the devpts filesystem is now distinct such that ptys
2and their indicies allocated in one mount are independent from ptys
3and their indicies in all other mounts.
4
5All mounts of the devpts filesystem now create a /dev/pts/ptmx node
6with permissions 0000.
7
8To retain backwards compatibility the a ptmx device node (aka any node
9created with "mknod name c 5 2") when opened will look for an instance
10of devpts under the name "pts" in the same directory as the ptmx device
11node.
12
13As an option instead of placing a /dev/ptmx device node at /dev/ptmx
14it is possible to place a symlink to /dev/pts/ptmx at /dev/ptmx or
15to bind mount /dev/ptx/ptmx to /dev/ptmx. If you opt for using
16the devpts filesystem in this manner devpts should be mounted with
17the ptmxmode=0666, or chmod 0666 /dev/pts/ptmx should be called.
18
19Total count of pty pairs in all instances is limited by sysctls:
20kernel.pty.max = 4096 - global limit
21kernel.pty.reserve = 1024 - reserved for filesystems mounted from the initial mount namespace
22kernel.pty.nr - current count of ptys
23
24Per-instance limit could be set by adding mount option "max=<count>".
25This feature was added in kernel 3.4 together with sysctl kernel.pty.reserve.
26In kernels older than 3.4 sysctl kernel.pty.max works as per-instance limit.