panic: vm_page_insert: page already inserted

Whilst loading a device driver I'm working on, I got an instant panic
during the attach.  I haven't seen this before and doing the kldload
immediately after a reboot was successful.  This is -current from a
7th Jan, about 1200UT.  contigmalloc_fn() is a patched version of
contigmalloc() that tracks file and line number information.

I've got a crashdump.  Anyone got any ideas for investigation?

panic: vm_page_insert: page already inserted
KDB: stack backtrace:
kdb_backtrace(c070a1cf,c0772e80,c071eee4,d755d9b4,100) at kdb_backtrace+0x2e
panic(c071eee4,1,c071ee29,21e,0) at panic+0xb7
vm_page_insert(c190e9c0,c07c5c60,17735,0,d7634000) at vm_page_insert+0x47
contigmalloc2(c190e9c0,7,0,102,1000) at contigmalloc2+0x160
contigmalloc_fn(7000,c074b9e0,102,0,ffffffff) at contigmalloc_fn+0xe2
bus_dmamem_alloc_fn(c3a91000,c5976034,c,c5976030,c598e25d) at bus_dmamem_alloc_fn+0xe9
saa_alloc_pta(c5976000,c3372180,c597601c,0,ffffffff) at saa_alloc_pta+0xd7
saa_attach(c3372180,c3ec584c,c0748218,c3372180,c3372180) at saa_attach+0x192
device_attach(c3372180,c3372180,c070bd04,90c,24) at device_attach+0x6a
device_probe_and_attach(c3372180,c3345800,d755db5c,0,c4a8ac40) at device_probe_and_attach+0x10b
pci_driver_added(c3372580,c598f928,c0748088,c32f4380,c598f940) at pci_driver_added+0xf5
devclass_add_driver(c32f4380,c598f928,1,c4296380,c598f954) at devclass_add_driver+0xe0
driver_module_handler(c4296380,0,c598f940,c07716e0,0) at driver_module_handler+0x8c
module_register_init(c598f954,c07078c8,d755dbf0,d755dbf4,0) at module_register_init+0x81
linker_file_sysinit(c5986a00,c5c70060,d755dc18,1,c5986a00) at linker_file_sysinit+0x90
linker_load_file(c5c70060,d755dc3c,0,c104ea80,0) at linker_load_file+0xe9
linker_load_module(0,c3326000,0,0,d755dc68) at linker_load_module+0xc9
kldload(c3e80c00,d755dd04,4,c3e80c00,c353fc60) at kldload+0x11a

-- 
Peter Jeremy