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

Sergey Senozhatsky
2018-01-11T05:15:30Z

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