+	if (blk_rq_nr_phys_segments(req) != 2)
+		return BLK_STS_IOERR;

The magic number of segments adding up source and dest really need
constants and helpers to make the code understandable.

+	/* +1 shift as dst+src length is added in request merging, we send copy
+	 * for half the length.
+	 */
+	n_lba = blk_rq_bytes(req) >> (ns->lba_shift + 1);

I do not understand the logic and comment here.

+	if (WARN_ON(!n_lba))

WARN_ON_ONCE

+		return BLK_STS_NOTSUPP;

and BLK_STS_NOTSUPP seems like the wrong error here, this is an
invalid argument.