linux-kernel - Re: [PATCH v5 0/2] printk: Console owner and waiter logic cleanup

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Date:   Thu, 11 Jan 2018 14:15:24 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Tejun Heo <tj@...nel.org>, Peter Zijlstra <peterz@...radead.org>
Cc:     Petr Mladek <pmladek@...e.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        akpm@...ux-foundation.org, Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        linux-mm@...ck.org, Cong Wang <xiyou.wangcong@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Mel Gorman <mgorman@...e.de>, Michal Hocko <mhocko@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>, Jan Kara <jack@...e.cz>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        rostedt@...e.goodmis.org, Byungchul Park <byungchul.park@....com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Pavel Machek <pavel@....cz>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 0/2] printk: Console owner and waiter logic cleanup

On (01/10/18 19:21), Peter Zijlstra wrote:
> 
> On Wed, Jan 10, 2018 at 09:02:23AM -0800, Tejun Heo wrote:
> > 2. System runs out of memory, OOM triggers.
> > 3. OOM handler is printing out OOM debug info.
> > 4. While trying to emit the messages for netconsole, the network stack
> >    / driver tries to allocate memory and then fail, which in turn
> >    triggers allocation failure or other warning messages.  printk was
> >    already flushing, so the messages are queued on the ring.
> > 5. OOM handler keeps flushing but 4 repeats and the queue is never
> >    shrinking.  Because OOM handler is trapped in printk flushing, it
> >    never manages to free memory and no one else can enter OOM path
> >    either, so the system is trapped in this state.
> 
> Why not kill recursive OOM (msgs) ?

hm... do I understand it correctly that there is a

console_unlock()->call_console_drivers()->FOO_write()->kmalloc()->printk() recursion?

we call console drivers from printk-safe context now. so those printks
from kmalloc are redirected to per-CPU printk-safe buffer, which is
limited in size (we probably might start losing some of those OOM
messages) and which is flushed (log_store()) from another context.

	-ss