Re: INFO: rcu detected stall in shmem_fault

[Tetsuo Handa]

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.


syz-executor0 invoked oom-killer: gfp_mask=0x6200ca(GFP_HIGHUSER_MOVABLE), nodemask=(null), order=0, oom_score_adj=-1000
syz-executor0 cpuset=syz0 mems_allowed=0
CPU: 0 PID: 2050 Comm: syz-executor0 Not tainted 4.19.0-rc7-next-20181009+ #90
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
(...snipped...)
Memory limit reached of cgroup /syz0
memory: usage 205168kB, limit 204800kB, failcnt 6909
memory+swap: usage 0kB, limit 9007199254740988kB, failcnt 0
kmem: usage 0kB, limit 9007199254740988kB, failcnt 0
Memory cgroup stats for /syz0: cache:680KB rss:176336KB rss_huge:163840KB shmem:740KB mapped_file:660KB dirty:0KB writeback:0KB swap:0KB inactive_anon:712KB active_anon:176448KB inactive_file:0KB active_file:4KB unevictable:0KB
Out of memory and no killable processes...