linux-kernel - Re: [PATCH printk v3 6/6] printk: introduce console_prepend

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Message-ID: <Y7VWDUUQ4f4EX/8v@alley>
Date:   Wed, 4 Jan 2023 11:33:49 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH printk v3 6/6] printk: introduce
 console_prepend_dropped() for dropped messages

On Wed 2023-01-04 10:12:01, John Ogness wrote:
> On 2023-01-03, Petr Mladek <pmladek@...e.com> wrote:
> >> Unless you think it is OK to kmalloc 8KB instead of 1KB for the
> >> syslog calls. Then yes, we do not need SYSLOG_MESSAGE_MAX.
> >
> > IMHO, it is acceptable and even correct. syslog uses the same
> > prefixes as console. It would make sense to use the same
> > buffers for formatting.
> >
> > That said, 8kB looks non-necessary big to me.
> >
> > It seems that it comes from devkmsg interface, see the commit
> > d43ff430f434d862db59582 ("printk: guard the amount written
> > per line by devkmsg_read()"). It was supposed to include
> > the message, the extended prefix and dictionary, where
> >
> >    + message is limited by LOG_LINE_MAX
> >    + prefix includes few well defined fields (should be < 128B)
> >    + dictionary comes from dev_printk() => ( < 128B as well)
> >
> > I believe that 2kB or 4kB would be perfectly fine.
> 
> The main issue is multi-line records. Normal messages become _much_
> larger than extended messages in this case because they add a prefix per
> '\n', whereas extended messages just use "\x0a". Extended messages
> really could only end up being significantly longer than normal messages
> if there are many non-printable characters in the message. But AFAIK
> non-printables are not really used in printk messages.

Right.

> So IMHO it does not make sense that normal messages are limited to 1KB
> but extended messages can use 8KB. I agree that a universal limit of 2KB
> for normal/extended/syslog would be a nice compromise. Normal messages
> will have more space available and it will reduce the overall static
> buffer usage. It would mean that syslog calls will kmalloc 2KB instead
> of 1KB, but I expect that should be acceptable since, generally
> speaking, overall we are reducing memory usage.

I agree that 2kB are a good compromise and the allocation should be acceptable.

Best Regards,
Petr