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.
Thank you very much,
Vasily Averin
>> 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,
