PG_dirty
u |= kpf_copy_bit(k, KPF_DIRTY, PG_dirty);
PG_savepinned = PG_dirty,
flags |= 1UL << PG_uptodate | 1UL << PG_dirty | 1UL << PG_lru
(1L << PG_dirty) |
#define dirty (1UL << PG_dirty)
page_flags |= (p->flags.f & (1UL << PG_dirty));