swp_tb_to_folio
swp_tb_to_folio(old_tb) != folio);
WARN_ON_ONCE(!swp_tb_is_folio(old_tb) || swp_tb_to_folio(old_tb) != old);
WARN_ON_ONCE(swp_tb_to_folio(__swap_table_get(ci, ci_off)) != old);
folio = swp_tb_to_folio(swp_tb);