(kgdb) bt
#0  doadump () at pcpu.h:223
#1  0xffffffff805afa29 in boot (howto=260)
    at /usr/home/wxs/freebsd/head/sys/kern/kern_shutdown.c:419
#2  0xffffffff805afe7c in panic (fmt=Variable "fmt" is not available.
	)
    at /usr/home/wxs/freebsd/head/sys/kern/kern_shutdown.c:575
#3  0xffffffff801f8d37 in db_panic (addr=Variable "addr" is not available.
	)
    at /usr/home/wxs/freebsd/head/sys/ddb/db_command.c:478
#4  0xffffffff801f9141 in db_command (last_cmdp=0xffffffff80c46620, cmd_table=Variable "cmd_table" is not available.
	)
    at /usr/home/wxs/freebsd/head/sys/ddb/db_command.c:445
#5  0xffffffff801f9390 in db_command_loop ()
	    at /usr/home/wxs/freebsd/head/sys/ddb/db_command.c:498
#6  0xffffffff801fb359 in db_trap (type=Variable "type" is not available.
		)
    at /usr/home/wxs/freebsd/head/sys/ddb/db_main.c:229
#7  0xffffffff805e11c5 in kdb_trap (type=3, code=0, tf=0xffffff803e74f690)
	    at /usr/home/wxs/freebsd/head/sys/kern/subr_kdb.c:534
#8  0xffffffff808a1368 in trap (frame=0xffffff803e74f690)
		    at /usr/home/wxs/freebsd/head/sys/amd64/amd64/trap.c:613
#9  0xffffffff80886ae7 in calltrap ()
			    at /usr/home/wxs/freebsd/head/sys/amd64/amd64/exception.S:224
#10 0xffffffff805e139d in kdb_enter (why=0xffffffff8099121c "panic", 
				    msg=0xa <Address 0xa out of bounds>) at cpufunc.h:63
#11 0xffffffff805afe8b in panic (fmt=Variable "fmt" is not available.
					)
    at /usr/home/wxs/freebsd/head/sys/kern/kern_shutdown.c:558
#12 0xffffffff8080552e in kmem_malloc (map=0xffffff00020000e8, size=131072, 
	    flags=258) at /usr/home/wxs/freebsd/head/sys/vm/vm_kern.c:305
#13 0xffffffff807fd8a8 in keg_alloc_slab (keg=0xffffff000291dd20, 
		    zone=0xffffff000291f300, wait=258)
    at /usr/home/wxs/freebsd/head/sys/vm/uma_core.c:846
#14 0xffffffff807fde48 in keg_fetch_slab (keg=0xffffff000291dd20, 
	    zone=0xffffff000291f300, flags=2)
    at /usr/home/wxs/freebsd/head/sys/vm/uma_core.c:2152
#15 0xffffffff807fdefe in zone_fetch_slab (zone=0xffffff000291f300, 
	    keg=0xffffff000291dd20, flags=2)
    at /usr/home/wxs/freebsd/head/sys/vm/uma_core.c:2212
#16 0xffffffff807ff32f in uma_zalloc_arg (zone=0xffffff000291f300, 
	    udata=0xffffff0002917080, flags=2)
    at /usr/home/wxs/freebsd/head/sys/vm/uma_core.c:2381
#17 0xffffffff80fc2564 in vdev_queue_io_to_issue () from /boot/kernel/zfs.ko
#18 0xffffffff80fc26d9 in vdev_queue_io_done () from /boot/kernel/zfs.ko
#19 0xffffff00117a5b40 in ?? ()
#20 0xffffffff81051d00 in ?? ()
#21 0xffffff00024745c0 in ?? ()
#22 0xffffff803e74fb10 in ?? ()
#23 0xffffffff80fd471a in zio_vdev_io_done () from /boot/kernel/zfs.ko
	Previous frame inner to this frame (corrupt stack?)
(kgdb)