Commit 8b253b07 authored by Konstantin Khlebnikov's avatar Konstantin Khlebnikov Committed by Greg Kroah-Hartman

TTY, devpts: document pty count limiting

Logic has been changed in kernel 3.4 by commit e9aba515
("tty: rework pty count limiting") but still not documented.

Sysctl kernel.pty.max works as global limit, kernel.pty.reserve ptys
are reserved for initial devpts instance (mounted without "newinstance").
Per-instance limit also could be set by mount option "max=%d".
Signed-off-by: default avatarKonstantin Khlebnikov <>
Signed-off-by: default avatarGreg Kroah-Hartman <>
parent 465893e1
......@@ -51,6 +51,15 @@ where 'ns_exec -cm /bin/bash' calls clone() with CLONE_NEWNS flag and execs
/bin/bash in the child process. A pty created by the sshd is not visible in
the original mount of /dev/pts.
Total count of pty pairs in all instances is limited by sysctls:
kernel.pty.max = 4096 - global limit
kernel.pty.reserve = 1024 - reserve for initial instance - 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.
User-space changes
......@@ -64,6 +64,7 @@ show up in /proc/sys/kernel:
- printk_delay
- printk_ratelimit
- printk_ratelimit_burst
- pty ==> Documentation/filesystems/devpts.txt
- randomize_va_space
- real-root-dev ==> Documentation/initrd.txt
- reboot-cmd [ SPARC only ]
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