writecache_find_entry
old = writecache_find_entry(wc, read_original_sector(wc, e), 0);
e = writecache_find_entry(wc, bio->bi_iter.bi_sector, WFE_RETURN_FOLLOWING);
e = writecache_find_entry(wc, bio->bi_iter.bi_sector, 0);
e = writecache_find_entry(wc, bio->bi_iter.bi_sector, WFE_RETURN_FOLLOWING);
e = writecache_find_entry(wc, start, WFE_RETURN_FOLLOWING | WFE_LOWEST_SEQ);