linux-kernel - [PATCH RESEND] TTY, devpts: document pty count limiting

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Message-ID: <20160221070614.9582.93900.stgit@buzz>
Date:	Sun, 21 Feb 2016 10:06:14 +0300
From:	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.com>
Cc:	linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org
Subject: [PATCH RESEND] TTY, devpts: document pty count limiting

Logic has been changed in kernel 3.4 by commit e9aba5158a80
("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: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
---
 Documentation/filesystems/devpts.txt |    9 +++++++++
 Documentation/sysctl/kernel.txt      |    1 +
 2 files changed, 10 insertions(+)

diff --git a/Documentation/filesystems/devpts.txt b/Documentation/filesystems/devpts.txt
index 68dffd87f9b7..30d2fcb32f72 100644
--- a/Documentation/filesystems/devpts.txt
+++ b/Documentation/filesystems/devpts.txt
@@ -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
+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.
+
 User-space changes
 ------------------
 
diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
index a93b414672a7..d05e70b7d8dd 100644
--- a/Documentation/sysctl/kernel.txt
+++ b/Documentation/sysctl/kernel.txt
@@ -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 ]