It is possible for the kernel mapping to be created with superpage by
directly installing pde using pmap_enter_2mpage() without filling the
corresponding page table page. This can happen e.g. if the range is
already backed by reservation and vm_fault_soft_fast() conditions are
satisfied, which was observed on the pipe_map.
In this case, demotion must fill the page obtained from the pmap
radix, same as if the page is newly allocated. Use PG_PROMOTED bit as
an indicator that the page is valid, instead of the wire count of the
page table page.
Since the PG_PROMOTED bit is set on pde when we leave TLB entries for
4k pages around, which in particular means that the ptes were filled,
it provides more correct indicator. Note that pmap_protect_pde()
clears PG_PROMOTED, which handles the case when protection was changed
on the superpage without adjusting ptes.
Reported by: pho
In collaboration with: alc
Tested by: alc, pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D20380
a9c7546a1d