Extreme contigmalloc() slowness with mpt driver

I have an amd64 machine with 16GB of RAM that takes ages to boot (~40
minutes on 7.0).  This is because the mpt driver takes 20 minutes to
attach (with 2 instances).  This in turn is because the following code
from dev/mpt/mpt_pci.c:mpt_dma_mem_alloc() takes about 5 seconds to
execute, and it is run 256 times in a loop:

                error = bus_dmamap_create(mpt->buffer_dmat, 0, &req->dmap);

When I set vm.old_contigmalloc=1, the system boots without delay.

This points to a bug in contigmalloc.

Kris