quoted

 	struct mem_cgroup *memcg = mem_cgroup_from_cont(cgrp);
+	int ret = -EAGAIN;
 
 	if (val >= (1 << NR_MOVE_TYPE))
 		return -EINVAL;

@@ -4123,10 +4130,13 @@ static int mem_cgroup_move_charge_write(struct cgroup *cgrp,
 	 * inconsistent.
 	 */
 	cgroup_lock();
+	if (memcg->attach_in_progress)
+		goto out;
 	memcg->move_charge_at_immigrate = val;
+	ret = 0;
+out:
 	cgroup_unlock();
-
-	return 0;
+	return ret;

Unsure whether this is a good behavior. 

to be honest, I am not so sure myself. I kinda leaned towards this after
some consideration, because the other solution I saw was basically busy
waiting...

It's a bit nasty to fail for
internal temporary reasons like this.  If it ever causes a problem,
the occurrences are likely to be far and between making it difficult
to debug.  Can't you determine to immigrate or not in ->can_attach(),
record whether to do that or not on the css, and finish it in
->attach() according to that.  There's no need to consult the config
multiple times.

Well, yeah... that is an option too, and indeed better. Good call.

I will send it again soon



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