Odd SIGSEGV issue introduced by commit 6b31d5955cb29 ("mm, oom: fix potential data corruption when oom_reaper races with writer")

[Christophe LEROY]

Hello,

I have an odd issue on my powerpc 8xx board.

I am running latest 4.14 and get the following SIGSEGV which appears 
more or less randomly.

[    9.190354] touch[91]: unhandled signal 11 at 67807b58 nip 777cf114 
lr 777cf100 code 30001
[   24.634810] ifconfig[160]: unhandled signal 11 at 67ae7b58 nip 
77aaf114 lr 77aaf100 code 30001
[   30.383737] default.deconfi[231]: unhandled signal 11 at 67c8bb58 nip 
77c53114 lr 77c53100 code 30001
[   37.655588] S15syslogd[251]: unhandled signal 11 at 6784fb58 nip 
77817114 lr 77817100 code 30001
[   40.974649] snmpd[315]: unhandled signal 11 at 67e0bb58 nip 77dd3114 
lr 77dd3100 code 30001
[   43.220964] exe[338]: unhandled signal 11 at 67cd3b58 nip 77c9b114 lr 
77c9b100 code 30001
[   44.191494] exe[348]: unhandled signal 11 at 67c1fb58 nip 77be7114 lr 
77be7100 code 30001
[   59.175022] sleep[655]: unhandled signal 11 at 67ca3b58 nip 77c6b114 
lr 77c6b100 code 30001
[   61.853406] smcroute[705]: unhandled signal 11 at 6789bb58 nip 
77863114 lr 77863100 code 30001
[   64.662431] smcroute[778]: unhandled signal 11 at 67e03b58 nip 
77dcb114 lr 77dcb100 code 30001
[   65.623103] smcroute[795]: unhandled signal 11 at 67bdbb58 nip 
77ba3114 lr 77ba3100 code 30001
[   66.579416] exe[825]: unhandled signal 11 at 67edbb58 nip 77ea3114 lr 
77ea3100 code 30001
[   68.382941] exe[864]: unhandled signal 11 at 6789bb58 nip 77863114 lr 
77863100 code 30001
[   95.187346] exe[1147]: unhandled signal 11 at 67e83b58 nip 77e4b114 
lr 77e4b100 code 30001
[  105.238218] exe[1158]: unhandled signal 11 at 67ca3b58 nip 77c6b114 
lr 77c6b100 code 30001
[  127.556731] exe[1181]: unhandled signal 11 at 67cc3b58 nip 77c8b114 
lr 77c8b100 code 30001
[  135.558982] exe[1195]: unhandled signal 11 at 678d7b58 nip 7789f114 
lr 7789f100 code 30001
[  147.579142] exe[1216]: unhandled signal 11 at 67c6bb58 nip 77c33114 
lr 77c33100 code 30001
[  175.538747] exe[1262]: unhandled signal 11 at 67e2fb58 nip 77df7114 
lr 77df7100 code 30001
[  186.552670] exe[1275]: unhandled signal 11 at 6781fb58 nip 777e7114 
lr 777e7100 code 30001
[  230.629786] exe[1344]: unhandled signal 11 at 67cb3b58 nip 77c7b114 
lr 77c7b100 code 30001
[  249.640396] repair-service.[1369]: unhandled signal 11 at 67e5fb58 
nip 77e27114 lr 77e27100 code 30001
[  378.003410] exe[1593]: unhandled signal 11 at 678d7b58 nip 7789f114 
lr 7789f100 code 30001
[  414.060661] exe[1656]: unhandled signal 11 at 67cc7b58 nip 77c8f114 
lr 77c8f100 code 30001

The problem is present in 3.13, 3.14 and 3.15.

I bisected its appearance with commit 6b31d5955cb29 ("mm, oom: fix 
potential data corruption when oom_reaper races with writer")

And I bisected its disappearance with commit 99cd1302327a2 ("powerpc: 
Deliver SEGV signal on pkey violation")

Looking at those two commits, especially the one which makes it 
dissapear, I'm quite sceptic. Any idea on what could be the cause and/or 
how to investigate further ?

Thanks
Christophe