src = buf->ops->map(sp->s_pipe, buf, 1);
dst = kmap_atomic(wc->w_this_page, KM_USER1);
memcpy(dst + from, src + src_from, bytes);
src = buf->ops->map(sp->s_pipe, buf, 1);
dst = kmap_atomic(wc->w_this_page, KM_USER1);
memcpy(dst + from, src + src_from, bytes);