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Date: Thu, 11 Jan 2018 14:15:24 +0900
From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Subject: Re: [PATCH v5 0/2] printk: Console owner and waiter logic cleanup
Message-ID: <20180111051524.GC494@jagdpanzerIV>
References: <20180110132418.7080-1-pmladek@suse.com>
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To: Tejun Heo <tj@kernel.org>, Peter Zijlstra <peterz@infradead.org>
Cc: Petr Mladek <pmladek@suse.com>, Linus Torvalds <torvalds@linux-foundation.org>, akpm@linux-foundation.org, Steven Rostedt <rostedt@goodmis.org>, Sergey Senozhatsky <sergey.senozhatsky@gmail.com>, linux-mm@kvack.org, Cong Wang <xiyou.wangcong@gmail.com>, Dave Hansen <dave.hansen@intel.com>, Johannes Weiner <hannes@cmpxchg.org>, Mel Gorman <mgorman@suse.de>, Michal Hocko <mhocko@kernel.org>, Vlastimil Babka <vbabka@suse.cz>, Jan Kara <jack@suse.cz>, Mathieu Desnoyers <mathieu.desnoyers@efficios.com>, Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>, rostedt@home.goodmis.org, Byungchul Park <byungchul.park@lge.com>, Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>, Pavel Machek <pavel@ucw.cz>, linux-kernel@vger.kernel.org

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

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