AcpiOsMapMemory's NULL failure contract is dead code — pmap_mapdev panics on KVA exhaustion instead of returning NULL, converting ACPICA's graceful AE_NO_MEMORY degradation into an unrecoverable kernel panic with firmware/AML-controlled mapping volume

Summary

The NULL check is unreachable (pmap_mapdev panics on kmem_alloc_nofault==0) while ACPICA's exregion explicitly handles NULL as AE_NO_MEMORY - firmware-controlled SystemMemory opregion volume or KVA pressure bricks instead of degrading. Fallible-mapping fix sketched.