More checks for PG_ZERO pages

[kib]

Summary

vm_page_free_prep(): convert PG_ZERO zeroed page check to use sf_buf

Make the check MI by allocating sf_buf in non-blockable manner.  For
DMAP arches, this should be nop since sf_buf allocation cannot fail.
For non-DMAP arches, we get the checks activated unless there is serious
sf_buf pressure, which typically should be not.

The context for vm_page_free_prep() should be ready to block on some VM
mutexes, which should make it reasonable to block on sf_buf list lock.

Move the code to INVARIANTS build, and control it with the sysctl
debug.vm_check_pg_zero.


 vm_fault: under INVARIANTS, add the verifier that PG_ZERO page is indeed zeroed

Controlled by the same sysctl debug.vm_check_pg_zero.

[markj]

Would it be sensible to compile this code under INVARIANTS and leave it disabled by default with a sysctl?

[kib]

Move to INVARIANTS and sysctl debug.vm_check_pg_zero to control.

[markj]

sys/vm/vm_page.c

152

Maybe this should be INVARIANTS-only too.

[kib]

Move control var and sysctl under INVARIANTS.