Question about dm target size

[Josef Bacik <jbacik@fb.com>]

Hello,

I'm creating a dm target to better test power fail situations and I'm 
having trouble figuring out how to make the dm device appear as a 
different size.  So for example you do the normal dm table

offset size power-fail /dev/whatever args

I want to use the entire size of /dev/whatever, but I want my dm device 
to show up as size/2.  So right now I'm doing this in my ->ctr function

ti->len /= 2;

Is that acceptable, or will this have some side-effect that's going to 
bite me in the ass?  I can't see any other target that does something 
similar and there appears to be no helper function.  This seems to work 
as far as blockdev --getsz is concerned, but I'm worried I need to do 
more.  Thanks,

Josef