LOR with page queue mutex and vm object

I got this on alpha just now (kernel updated 2 days ago, filesystems
created with -b 32768 -f 4096 to test reports of panics with these
settings, which may or may not be relevant).  I don't think I've seen
this trace before...apologies if it's another false positive.

Kris

 1st 0xfffffc00006e48d0 vm page queue mutex (vm page queue mutex) _at_ /a/asami/portbuild/alpha/src-client/sys/vm/vm_pageout.c:405
 2nd 0xfffffc0005102e00 vm object (vm object) _at_ /a/asami/portbuild/alpha/src-client/sys/kern/vfs_bio.c:2280
Stack backtrace:
db_print_backtrace() at db_print_backtrace+0x18
backtrace() at backtrace+0x2c
witness_lock() at witness_lock+0x6fc
_mtx_lock_flags() at _mtx_lock_flags+0xc8
vfs_setdirty() at vfs_setdirty+0x7c
bdwrite() at bdwrite+0x3b8
ffs_update() at ffs_update+0x388
ufs_inactive() at ufs_inactive+0x254
ufs_vnoperate() at ufs_vnoperate+0x2c
vput() at vput+0x170
vm_pageout_scan() at vm_pageout_scan+0x7ac
vm_pageout() at vm_pageout+0x3ac
fork_exit() at fork_exit+0x100
exception_return() at exception_return
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