On Thu, 20 Sep 2012, Michal Hocko wrote:

Yes printk_once is an alternative but I really wanted to have this as
much visible as possible. People tend to react to stack traceces more
and this one will trigger only if somebody is either doing something
wrong or the configuration is the one we are looking for.

That's the complete opposite of what Linus has said he wants, he said very 
specifically that he doesn't want WARN_ONCE() or WARN_ON_ONCE() for 
deprecation of tunables.  If you want to have this merged, then please get 
him to ack it.

Comparing to oom_adj, that one was used much more often so the WARN_ONCE
was too verbose especially when you usually had to wait for an userspace
update which is not the case here.

How is WARN_ONCE() too verbose for oom_adj?  It's printed once!  And how 
can you claim that userspace doesn't need to change if it's creating a 
hierarchy while use_hierarchy == 0?