linux-kernel - Re: [PATCH v7 09/10] memcg: enable accounting for tty-related objects

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Message-ID: <YP/SUlucImu11zT7@kroah.com>
Date:   Tue, 27 Jul 2021 11:30:58 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Vasily Averin <vvs@...tuozzo.com>
Cc:     Jiri Slaby <jirislaby@...nel.org>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 09/10] memcg: enable accounting for tty-related objects

On Tue, Jul 27, 2021 at 11:02:31AM +0300, Vasily Averin wrote:
> On 7/27/21 9:54 AM, Jiri Slaby wrote:
> > On 27. 07. 21, 7:34, Vasily Averin wrote:
> >> At each login the user forces the kernel to create a new terminal and
> >> allocate up to ~1Kb memory for the tty-related structures.
> >>
> >> By default it's allowed to create up to 4096 ptys with 1024 reserve for
> >> initial mount namespace only and the settings are controlled by host admin.
> >>
> >> Though this default is not enough for hosters with thousands
> >> of containers per node. Host admin can be forced to increase it
> >> up to NR_UNIX98_PTY_MAX = 1<<20.
> >>
> >> By default container is restricted by pty mount_opt.max = 1024,
> >> but admin inside container can change it via remount. As a result,
> >> one container can consume almost all allowed ptys
> >> and allocate up to 1Gb of unaccounted memory.
> >>
> >> It is not enough per-se to trigger OOM on host, however anyway, it allows
> >> to significantly exceed the assigned memcg limit and leads to troubles
> >> on the over-committed node.
> >>
> >> It makes sense to account for them to restrict the host's memory
> >> consumption from inside the memcg-limited container.
> >>
> >> Signed-off-by: Vasily Averin <vvs@...tuozzo.com>
> >> Acked-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> >> ---
> >>   drivers/tty/tty_io.c | 4 ++--
> >>   1 file changed, 2 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
> >> index 26debec..e787f6f 100644
> >> --- a/drivers/tty/tty_io.c
> >> +++ b/drivers/tty/tty_io.c
> >> @@ -1493,7 +1493,7 @@ void tty_save_termios(struct tty_struct *tty)
> >>       /* Stash the termios data */
> >>       tp = tty->driver->termios[idx];
> >>       if (tp == NULL) {
> >> -        tp = kmalloc(sizeof(*tp), GFP_KERNEL);
> >> +        tp = kmalloc(sizeof(*tp), GFP_KERNEL_ACCOUNT);
> > 
> > termios are not saved for PTYs (TTY_DRIVER_RESET_TERMIOS). Am I missing something?
> 
> No, you are right, I've missed this.
> Typical terminals inside containers use TTY_DRIVER_RESET_TERMIOS flag and therefore do not save termios.
> So its accounting have near-to-zero impact in real life.
> I'll prepare fixup to drop GFP_KERNEL_ACCOUNT here.

I'll go drop this patch from my tree.

thanks,

greg k-h