mm, oom: show process exiting information in __oom_kill_process()

When the OOM killer finds a victim and tryies to kill it, if the victim is
already exiting, the task mm will be NULL and no process will be killed.
But the dump_header() has been already executed, so it will be strange to
dump so much information without killing a process. We'd better show some
helpful information to indicate why this happens.

Suggested-by: David Rientjes <[e-mail]>
Signed-off-by: Yafang Shao <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Acked-by: Michal Hocko <[e-mail]>
Cc: Tetsuo Handa <[e-mail]>
Cc: Qian Cai <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Linus Torvalds <[e-mail]>

mm/oom_kill.c

···
 
 	p = find_lock_task_mm(victim);
 	if (!p) {
+		pr_info("%s: OOM victim %d (%s) is already exiting. Skip killing the task\n",
+			message, task_pid_nr(victim), victim->comm);
 		put_task_struct(victim);
 		return;
 	} else if (victim != p) {