oblock_to_dblock
clear_discard(cache, oblock_to_dblock(cache, op->oblock));
r = test_bit(from_dblock(oblock_to_dblock(cache, b)),
clear_discard(cache, oblock_to_dblock(cache, oblock));
clear_discard(cache, oblock_to_dblock(cache, oblock));
clear_discard(cache, oblock_to_dblock(cache, oblock));