Lockless truncate and the buffered lockless i/o on contention have been broken pretty much since they were first implemented, which was a long time ago (~Lustre 2.0, maybe earlier).

It's past time to remove them both.  The only reason the lockless i/o on contention hasn't been more of a problem is that contention detection on the server is also mostly broken.

One note:
I am going to leave the server side code in place for now, because I have a rewrite of it here:
https://review.whamcloud.com/#/c/35287/

And while I do not expect to pick up automatic lockahead, this contention detection work has a role to play in the "switched buffered i/o to direct i/o" patch series.

It is much easier to do that patch if the server side contention detection is not removed entirely.