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@xxxxxxxxxxxxx>
> >> Acked-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> >> ---
> >> 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
