It sounds like you don't want to support traditional discard at all, but only WRITE ZEROES. So in many ways this series is the right way forward. It would be nice if we could do a full blown REQ_OP_WRITE_ZEROES for dm_think that zeroes out partial blocks, similar to what hardware that implements WRITE SAME of zeroes or WRITE ZEROES would do. I'll see if I could include that in my series.
