[PATCH] TTY, devpts: document pty count limiting

[PATCH] TTY, devpts: document pty count limiting

[Konstantin Khlebnikov]

Logic was changed in kernel 3.4 by commit e9aba5158a80 ("tty: rework pty
count limiting") but still isn't documented. Better late than never.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
---
 Documentation/filesystems/devpts.txt |    9 +++++++++
 Documentation/sysctl/kernel.txt      |    7 +++++++
 2 files changed, 16 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 6fccb69c03e7..01a42021c2e5 100644
--- a/Documentation/sysctl/kernel.txt
+++ b/Documentation/sysctl/kernel.txt
@@ -63,6 +63,7 @@ show up in /proc/sys/kernel:
 - printk_delay
 - printk_ratelimit
 - printk_ratelimit_burst
+- pty/
 - randomize_va_space
 - real-root-dev               ==> Documentation/initrd.txt
 - reboot-cmd                  [ SPARC only ]
@@ -683,6 +684,12 @@ send before ratelimiting kicks in.
 
 ==============================================================
 
+pty/:
+
+See Documentation/filesystems/devpts.txt.
+
+==============================================================
+
 randomize_va_space:
 
 This option can be used to select the type of process address

Re: [PATCH] TTY, devpts: document pty count limiting

[Greg KH]

On Wed, Oct 21, 2015 at 01:08:54PM +0300, Konstantin Khlebnikov wrote:
> Logic was changed in kernel 3.4 by commit e9aba5158a80 ("tty: rework pty
> count limiting") but still isn't documented. Better late than never.
> 
> Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
> ---
>  Documentation/filesystems/devpts.txt |    9 +++++++++
>  Documentation/sysctl/kernel.txt      |    7 +++++++
>  2 files changed, 16 insertions(+)

Any reason you didn't cc: the tty maintainer on this?

Re: [PATCH] TTY, devpts: document pty count limiting

[Eric W. Biederman]

Greg KH <gregkh@linuxfoundation.org> writes:

> On Wed, Oct 21, 2015 at 01:08:54PM +0300, Konstantin Khlebnikov wrote:
>> Logic was changed in kernel 3.4 by commit e9aba5158a80 ("tty: rework pty
>> count limiting") but still isn't documented. Better late than never.
>> 
>> Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
>> ---
>>  Documentation/filesystems/devpts.txt |    9 +++++++++
>>  Documentation/sysctl/kernel.txt      |    7 +++++++
>>  2 files changed, 16 insertions(+)
>
> Any reason you didn't cc: the tty maintainer on this?

It looks like he attempted to and managed to get your old suse.de
address.

Eric

Re: [PATCH] TTY, devpts: document pty count limiting

[Greg KH]

On Wed, Oct 21, 2015 at 11:08:52PM -0500, Eric W. Biederman wrote:
> Greg KH <gregkh@linuxfoundation.org> writes:
> 
> > On Wed, Oct 21, 2015 at 01:08:54PM +0300, Konstantin Khlebnikov wrote:
> >> Logic was changed in kernel 3.4 by commit e9aba5158a80 ("tty: rework pty
> >> count limiting") but still isn't documented. Better late than never.
> >> 
> >> Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
> >> ---
> >>  Documentation/filesystems/devpts.txt |    9 +++++++++
> >>  Documentation/sysctl/kernel.txt      |    7 +++++++
> >>  2 files changed, 16 insertions(+)
> >
> > Any reason you didn't cc: the tty maintainer on this?
> 
> It looks like he attempted to and managed to get your old suse.de
> address.

Ah, you are right, my apologies, that address is long dead :)

greg k-h