Re: INFO: rcu detected stall in shmem_fault

[David Rientjes]

On Wed, 10 Oct 2018, Tetsuo Handa wrote:

> syzbot is hitting RCU stall due to memcg-OOM event.
> https://syzkaller.appspot.com/bug?id=4ae3fff7fcf4c33a47c1192d2d62d2e03efffa64
> 
> What should we do if memcg-OOM found no killable task because the allocating task
> was oom_score_adj == -1000 ? Flooding printk() until RCU stall watchdog fires 
> (which seems to be caused by commit 3100dab2aa09dc6e ("mm: memcontrol: print proper
> OOM header when no eligible victim left") because syzbot was terminating the test
> upon WARN(1) removed by that commit) is not a good behavior.
> 

Not printing anything would be the obvious solution but the ideal solution 
would probably involve

 - adding feedback to the memcg oom killer that there are no killable 
   processes,

 - adding complete coverage for memcg_oom_recover() in all uncharge paths
   where the oom memcg's page_counter is decremented, and

 - having all processes stall until memcg_oom_recover() is called so 
   looping back into try_charge() has a reasonable expectation to succeed.