linux-kernel - Re: [PATCH] printk: Add loglevel for "do not print to consoles".

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Message-ID: <579fbe97-9aae-2b67-03ff-01291b9cbb7d@i-love.sakura.ne.jp>
Date:   Fri, 24 Apr 2020 23:00:01 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        linux-kernel@...r.kernel.org, Dmitry Safonov <dima@...sta.com>,
        Michal Hocko <mhocko@...e.com>,
        Yafang Shao <laoar.shao@...il.com>
Subject: Re: [PATCH] printk: Add loglevel for "do not print to consoles".

On 2020/04/24 22:28, Steven Rostedt wrote:
> What I would suggest doing is create your own buffer to store the output.
> Then use the seq_buf() operations and such to print to that buffer, and
> then be able to read that output from something like a debug filesystem, or
> some user space daemon that will write to syslog.

Since KERN_NO_CONSOLES is for -ENOMEM situations (GFP_KERNEL allocation which
can sleep needs to invoke the OOM killer, or GFP_ATOMIC allocation which cannot
sleep has failed), we can't create buffer on demand. For process context, it
would be possible to create buffer upon fork() time. But for atomic context,
it is so difficult to create buffer on demand. We could allocate shared buffer
like logbuf but it means that we have to replicate what printk() is doing (too
much code), for when atomic memory allocation happens resembles when printk()
is called. Borrowing printk()'s logbuf is simpler.