Comment # 21 on bug 1178797 from Andreas Herrmann

(In reply to Anton Smorodskyi from comment #20)
> (In reply to Andreas Herrmann from comment #19)
> > (In reply to Franck Bui from comment #18)
> > > (In reply to Andreas Herrmann from comment #17)
> > > > Depends on whether the rules file is used also with
> > > > an older kernel still supporting the legacy block layer.
> > > > (E.g. someone installing an old kernel)
> > > 
> > > Well as a kernel guy, you probably know better than us ;)
> > 
> > Thought more about it.
> > I think keeping the legacy rules (for CFQ and deadline) in there and moving
> > the elevator check after blk-mq rule is the safer approach. In case someone
> > boots an old kernel with legacy block layer support it works as usual.
> 
> maybe we could at least add some informational comment which would warn user
> that this check make sense only till kernel version X ?

Good idea. Will do that.
FYI, kernel v5.0 removed legacy IO path (and elevator parameter stopped
working), with kernel v5.4 elevator parameter was removed, kernel v5.5 added a
warning if elevator parameter was still specified.