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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Mon, 25 Jun 2018 11:36:17 +0200
Message-ID: <CACT4Y+YXRD1RCV4jFyEaCOB8D17j3+bXG9qXJ5XrcmsznjidRg@mail.gmail.com>
Subject: Re: [PATCH] printk: inject caller information into the body of message
To:     Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc:     Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Fengguang Wu <fengguang.wu@intel.com>,
        Petr Mladek <pmladek@suse.com>,
        syzkaller <syzkaller@googlegroups.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>
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On Mon, Jun 25, 2018 at 3:41 AM, Sergey Senozhatsky
<sergey.senozhatsky.work@gmail.com> wrote:
> On (06/22/18 22:06), Tetsuo Handa wrote:
>> >
>> > Awesome. If you and Fengguang can combine forces and lead the
>> > whole thing towards "we couldn't care of pr_cont() less", it
>> > would be really huuuuuge. Go for it!
>>
>> Can't we have seq_printf()-like one which flushes automatically upon seeing '\n'
>> or buffer full? Printing memory information is using a lot of pr_cont(), even in
>> function names (e.g. http://lkml.kernel.org/r/20180622083949.GR10465@dhcp22.suse.cz ).
>> Since OOM killer code is serialized by oom_lock, we can use static buffer for
>> OOM killer messages.
>
> I'm not the right guy to answer this question. Sorry. We need to Cc MM
> people on this.
>
> Does OOM's pr_cont() usage cause too much disturbance to syzkaller? I thought
> that OOM was slightly out of sight.

Hard to tell. Nothing specific comes to mind.
We do see lines like these:

BUG: unable to handle kernel [ 110.NUM] device gre0 entered promiscuous mode
BUG:--------[ cut here ]------------

and frequently it's also required to look deep inside of crash message
to understand what they really mean. Hard to tell how random pr_cont's
contribute to the problem. We now throw away everything that looks any
corrupted right away.
I guess the main requirement is that the crash report itself does not
use pr_cont and provided we have task/cpu context we can separate the
crash report lines from everything else (assuming that random
pr_cont's on other CPUs won't glue to the report lines).