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Subject: Re: [PATCH v7 09/10] memcg: enable accounting for tty-related objects
To:     Jiri Slaby <jirislaby@kernel.org>
Cc:     cgroups@vger.kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-kernel@vger.kernel.org
References: <6f21a0e0-bd36-b6be-1ffa-0dc86c06c470@virtuozzo.com>
 <cover.1627362057.git.vvs@virtuozzo.com>
 <b8baa04f-e789-0321-b39d-07c5696ff755@virtuozzo.com>
 <1eef95fe-6172-796e-edd1-095545da6e74@kernel.org>
From:   Vasily Averin <vvs@virtuozzo.com>
Message-ID: <d42bd2a3-74a0-163f-6e3a-ad702f6d2817@virtuozzo.com>
Date:   Tue, 27 Jul 2021 11:02:31 +0300
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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@virtuozzo.com>
>> Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.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.

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,