The most suspicious part from me is, vm_physseg allocation code I made.
I've confirmed:
#define VM_PHYSSEG_MAX 1
#define VM_PHYSSEG_STRAT VM_PSTRAT_BSEARCH /* XXX */
#define VM_NFREELIST 1
#define VM_FREELIST_DEFAULT 0
works on powerpc/ibm40x.
I can help to bisect if you tell me the config.