kernel panic of usb2

Hi ,
    i can't write anything in usb flash devices.i can mount device well 
with usb2_storage_mss.
db>bt
Tracing pid 1367 tid 100147 td 0xffffff0004e756e0
kdb_enter() at kdb_enter+0x3d
panic() at panic+0x176
vm_pfault() at vm_pfault+0x19ce
trap_pfault() at trap_pfault+0x2a9
trap() at trap+0c327
calltrap()  at calltrap+0x8
--- trap 0xc, rip = 0xffffffff80511166, rsp = 0xfffffffee962bac0, rdp = 
0xffffff
fee962bb00 ---
bcopy() at bcopy+0x16
usb2_bdma_per_sync() at usb2_bdma_per_sync+0x35
usb2_bdma_work_loop() at usb2_bdma_work_loop+0x346
usb2_command_wrapper() at usb2_command_wrapper+0x76
usb2_callback_wrapper() at usb2_callback_wrapper+0x2d4
usb2_command_wrapper() at usb2_command_wrapper+0x76
usb2_callback_proc() at usb2_callback_proc+0x90
usb2_process() at usb2_process+0xd2
fork_exit() at fork_exit+0x118
fork_trampoline() at fork_trampoline+0xe
--- trap 0, rip =0, rsp=0xfffffffee962bd40,  rbp = 0 ---

Thanks,
kevin