9-BETA1 Fatal trap 9 in __mnt_vnode_next

I am running FreeBSD 9-Beta1 (csup from July 31) and I had a Fatal trap 9
in __mnt_vnode_next:

(transcribed from a picture)

Fatal trap 9: general protection fault while in kernel mode
cpuid = 0; apic id =00
instruction pointer     = 0x20:0xffffffff8086d790
stack pointer           = 0x20:0xffffff8340be0a40
frame pointer           = 0x28:0xffffff8340be0a60
code segment            = base 0x0, limit 0xfffff, type 0x1b
                        = DPL 0, pres 1, long 1, def32 0, gran 1
processor eflags        = interrupt enabled, resume, IOPL = 0
current process         = 18 (syncer)
[ thread pid 18 tid 100077 ]
Stopped at      __mnt_vnode_next+0xc0:   movq   %rax,0x30(%rdx)
db> bt
Tracing pid 18 tid 100077 td 0xfffffe00090378c0
__mnt_vnode_next() at __mnt_vnode_next+0xc0
vfs_msync() at vfs_msync+0x1c7
sync_fsync() at sync_fsync+0x13f
VOP_FSYNC_APV() at VOP_FSYN_APV+0x49
sync_vnode() at sync_vnode+0x156
sched_sync() at sched_sync+0x270
fork_exit() at fork_exit+0x120
fork_trampoline() at fork_trampoline+0xe
--- trap 0, rip = 0, rsp = 0xffffff8340be0d00, rbp = 0 ---
db>

I have compiled the world and kernel myself with /etc/malloc.conf -> aj,
make.conf is:

WRKDIRPREFIX=/usr/obj
CFLAGS= -O2 -fno-strict-aliasing -pipe
CPUTYPE?=athlon64
COPTFLAGS= -O -pipe
# added by use.perl 2011-08-01 19:47:48
PERL_VERSION=5.12.4

I removed the WITNESS and INVARIANT options from the kernel config.

/boot/loader.conf:

geom_eli_load="YES"
geom_mirror_load="YES"
zfs_load="YES"
vm.kmem_size=12G

I have the root filesystem on ZFS, and swap in a mirrored GELI.

At the moment of the crash I also had a UFS filesystem mounted.

Hopefully this is useful to someone.

Best,
Edwin