panic: bundirty: buffer 0xdf4ac490 still on queue 1

I raced mount/umount -f of a nfs volume with two fsx processes writing
into a directory on the nfs mount.  I get a lot of expected faults
like:

vm_fault: pager read error, pid 19302 (fsx)
nfs_getpages: error 70
vm_fault: pager read error, pid 19317 (fsx)
nfs_getpages: error 70
vm_fault: pager read error, pid 19576 (fsx)
nfs_getpages: error 70
vm_fault: pager read error, pid 19679 (fsx)

but then a panic:

panic: bundirty: buffer 0xdf4ac490 still on queue 1
cpuid = 2
KDB: enter: panic
[thread pid 20040 tid 100111 ]
Stopped at      kdb_enter+0x2b: nop
db> wh
Tracing pid 20040 tid 100111 td 0xc6a5fa20
kdb_enter(c06c2470) at kdb_enter+0x2b
panic(c06ca947,df4ac490,1,df4ac490,ee1f1990) at panic+0x127
bundirty(df4ac490) at bundirty+0x35
brelse(df4ac490,c6bdb218,df4ac490,2022,ee1f19bc) at brelse+0x758
nfs_writebp(df4ac490,1,c6a5fa20,ee1f19e8,c055ee45) at nfs_writebp+0x13e
nfs_bwrite(df4ac490) at nfs_bwrite+0x14
getblk(c6bdb150,3,0,8000,0) at getblk+0x31d
nfs_getcacheblk(c6bdb150,3,0,8000,c6a5fa20) at nfs_getcacheblk+0xb1
nfs_bioread(c6bdb150,ee1f1c64,0,c6a57a80,ee1f1b74) at nfs_bioread+0x68e
nfs_read(ee1f1b9c,c6b153a8,c6bdb150,ee1f1be8,c0578512) at nfs_read+0x2b
VOP_READ_APV(c0711880,ee1f1b9c) at VOP_READ_APV+0x9b
vn_read(c6b153a8,ee1f1c64,c6a57a80,0,c6a5fa20) at vn_read+0x19a
dofileread(c6a5fa20,3,c6b153a8,ee1f1c64,ffffffff) at dofileread+0x89
kern_readv(c6a5fa20,3,ee1f1c64,821723d,62da) at kern_readv+0x36
read(c6a5fa20,ee1f1d04,c0776068,ee1f1cc4,c0539c74) at read+0x45
syscall(1003b,3b,bfbf003b,d517,10dc3) at syscall+0x27e
Xint0x80_syscall() at Xint0x80_syscall+0x1f
--- syscall (3, FreeBSD ELF32, read), eip = 0x2814a083, esp = 0xbfbfe37c, ebp = 0xbfbfe3a8 ---
db>

Core available.

Kris