On 7/27/21 9:54 AM, Jiri Slaby wrote:

On 27. 07. 21, 7:34, Vasily Averin wrote:

quoted

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 <redacted>
Acked-by: Greg Kroah-Hartman <redacted>
---
  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.

Thank you very much,
	Vasily Averin

quoted

          if (tp == NULL)
              return;
          tty->driver->termios[idx] = tp;

@@ -3119,7 +3119,7 @@ struct tty_struct *alloc_tty_struct(struct tty_driver *driver, int idx)

  {
      struct tty_struct *tty;
  -    tty = kzalloc(sizeof(*tty), GFP_KERNEL);
+    tty = kzalloc(sizeof(*tty), GFP_KERNEL_ACCOUNT);
      if (!tty)
          return NULL;
 

thanks,