oom: avoid killing kthreads if they assume the oom killed thread's mm

After selecting a task to kill, the oom killer iterates all processes and
kills all other threads that share the same mm_struct in different thread
groups.  It would not otherwise be helpful to kill a thread if its memory
would not be subsequently freed.

A kernel thread, however, may assume a user thread's mm by using
use_mm().  This is only temporary and should not result in sending a
SIGKILL to that kthread.

This patch ensures that only user threads and not kthreads are sent a
SIGKILL if they share the same mm_struct as the oom killed task.
Signed-off-by: David Rientjes <rientjes@google.com>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/oom_kill.c

@@ -439,7 +439,7 @@ static int oom_kill_task(struct task_struct *p, struct mem_cgroup *mem)
 	task_unlock(p);
 
 	/*
-	 * Kill all processes sharing p->mm in other thread groups, if any.
+	 * Kill all user processes sharing p->mm in other thread groups, if any.
 	 * They don't get access to memory reserves or a higher scheduler
 	 * priority, though, to avoid depletion of all memory or task
 	 * starvation.  This prevents mm->mmap_sem livelock when an oom killed
@@ -449,7 +449,8 @@ static int oom_kill_task(struct task_struct *p, struct mem_cgroup *mem)
 	 * signal.
 	 */
 	for_each_process(q)
-		if (q->mm == mm && !same_thread_group(q, p)) {
+		if (q->mm == mm && !same_thread_group(q, p) &&
+		    !(q->flags & PF_KTHREAD)) {
 			task_lock(q);	/* Protect ->comm from prctl() */
 			pr_err("Kill process %d (%s) sharing same memory\n",
 				task_pid_nr(q), q->comm);