PG_private_2
u |= kpf_copy_bit(k, KPF_PRIVATE_2, PG_private_2);
1UL << PG_private | 1UL << PG_private_2 | \
(1UL << PG_private | 1UL << PG_private_2)
PG_fscache = PG_private_2, /* page backed by cache */
clear_bit_unlock(PG_private_2, folio_flags(folio, 0));
folio_wake_bit(folio, PG_private_2);
folio_wait_bit(folio, PG_private_2);
ret = folio_wait_bit_killable(folio, PG_private_2);