Line-accurate confirmation (runtime demo = guaranteed allocation
stall of all contigmalloc callers - a guest-wide wedge, not run for
Info tier):
sys/vm/vm_contig.c:268-271 'if ((alignment & (alignment-1)) != 0)'
   accepts alignment==0; the scan then requires (phys & ULONG_MAX)==0
   i.e. physical page 0 - never free -> deterministic failure after
   three full vm_contig_pg_clean passes over all 1024 per-cpu page
   queues (with M_WAITOK it also loops 'very very slow')
sys/vm/vm_contig.c:594 contigfree zero-size panic names the wrong
   function ('vm_contig_pg_kmap: size must not be 0' inside
   contigfree)
No in-tree caller passes 0.