mm/oom_kill.c: avoid attempting to kill init sharing same memory

It's possible that an oom killed victim shares an ->mm with the init
process and thus oom_kill_process() would end up trying to kill init as
well.

This has been shown in practice:

	Out of memory: Kill process 9134 (init) score 3 or sacrifice child
	Killed process 9134 (init) total-vm:1868kB, anon-rss:84kB, file-rss:572kB
	Kill process 1 (init) sharing same memory
	...
	Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000009

And this will result in a kernel panic.

If a process is forked by init and selected for oom kill while still
sharing init_mm, then it's likely this system is in a recoverable state.
However, it's better not to try to kill init and allow the machine to
panic due to unkillable processes.

[[email protected]: rewrote changelog]
[[email protected]: fix inverted test, per Ben]
Signed-off-by: Chen Jie <[email protected]>
Signed-off-by: David Rientjes <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Acked-by: Hillf Danton <[email protected]>
Cc: Ben Hutchings <[email protected]>
Cc: Li Zefan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/oom_kill.c

@@ -608,6 +608,8 @@ void oom_kill_process(struct oom_control *oc, struct task_struct *p,
 			continue;
 		if (unlikely(p->flags & PF_KTHREAD))
 			continue;
+		if (is_global_init(p))
+			continue;
 		if (p->signal->oom_score_adj == OOM_SCORE_ADJ_MIN)
 			continue;