Unowned vm object mutex in swp_pager_meta_ctl

I got this panic on a 12 CPU sparc64 system running current from about
a month ago:

panic: trap: fast data access mmu miss
cpuid = 8
KDB: enter: panic
[thread pid 68865 tid 100379 ]
Stopped at      kdb_enter+0x68: ta              %xcc, 1
db> wh
Tracing pid 68865 tid 100379 td 0xfffff800eeb91710
panic() at panic+0x248
trap() at trap+0x4e8
-- fast data access mmu miss tar=0 %o7=0xc01c9b94 --
swp_pager_meta_ctl() at swp_pager_meta_ctl+0x8
swap_pager_getpages() at swap_pager_getpages+0x1c
mdstart_swap() at mdstart_swap+0x2b8
md_kthread() at md_kthread+0x19c
fork_exit() at fork_exit+0x84
fork_trampoline() at fork_trampoline+0x8

#10 0x00000000c01d5b30 in panic (fmt=0xc04bbe08 "trap: %s") at ../../../kern/kern_shutdown.c:551
#11 0x00000000c03b0510 in trap (tf=0xecebd2f0) at ../../../sparc64/sparc64/trap.c:378
#12 0x00000000c0060fe0 in tl1_trap ()
#13 0x00000000c034abe8 in swp_pager_meta_ctl (object=0x0, pindex=0, flags=-1068777864) at ../../../vm/swap_pager.c:1906
#14 0x00000000c01c9b9c in _mtx_unlock_flags (m=0x0, opts=130487, file=0x0, line=421) at ../../../kern/kern_mutex.c:163
#15 0x00000000c03492a4 in swap_pager_getpages (object=0xfffff800e2aaee88, m=0xecebd628, count=1, reqpage=0)
    at ../../../vm/swap_pager.c:995
#16 0x00000000c0104b20 in mdstart_swap (sc=0xfffff800cf641800, bp=0xfffff8008271a798) at vm_pager.h:130
#17 0x00000000c0104ee4 in md_kthread (arg=0xfffff800cf641800) at ../../../dev/md/md.c:720
#18 0x00000000c01b9f8c in fork_exit (callout=0xc0104d40 <md_kthread>, arg=0xfffff800cf641800, frame=0xecebd880)
    at ../../../kern/kern_fork.c:816
#19 0x00000000c00611d0 in fork_trampoline ()
#20 0x00000000c00611d0 in fork_trampoline ()
Previous frame identical to this frame (corrupt stack?)

kgdb seems to have corrupted the trace (spurious extra frame and
different arguments to frame 13), but if you believe the arguments in
frame 15:

995             blk = swp_pager_meta_ctl(mreq->object, mreq->pindex, 0);

(kgdb) print mreq->object
$8 = 0xfffff800aeb21930

(kgdb) print *((vm_object_t)0xfffff800aeb21930)
$7 = {mtx = {mtx_object = {lo_name = 0xc04b3b58 "vm object", lo_type = 0xc04b4dd0 "standard object", lo_flags = 21168128,
      lo_profile_obj = {lpo_acqtime = 0, lpo_waittime = 0, lpo_filename = 0x0, lpo_namehash = 4063449288, lpo_lineno = 0,
        lpo_type = 0xc049f940 "sleep mutex", lpo_contest_holding = 0, lpo_contest_locking = 119}, lo_witness_data = {lod_list = {
          stqe_next = 0x0}, lod_witness = 0x0}}, mtx_lock = 4, mtx_recurse = 0}, object_list = {tqe_next = 0xfffff800f3281360,
    tqe_prev = 0xfffff800d706e250}, shadow_head = {lh_first = 0x0}, shadow_list = {le_next = 0x0, le_prev = 0xfffff800d7cb8168}, memq = {
    tqh_first = 0xfffff801387bf950, tqh_last = 0xfffff80139c06cb8}, root = 0xfffff80139c06ca8, size = 10, generation = 23, ref_count = 1,
  shadow_count = 0, type = 0 '\0', flags = 8448, pg_color = 8, paging_in_progress = 0, resident_page_count = 10, backing_object = 0x0,
  backing_object_offset = 0, pager_object_list = {tqe_next = 0x0, tqe_prev = 0x0}, handle = 0x0, un_pager = {vnp = {vnp_size = 0},
    devp = {devp_pglist = {tqh_first = 0x0, tqh_last = 0x0}}, swp = {swp_bcount = 0}}}

(kgdb) print ((vm_object_t)0xfffff800aeb21930)->mtx
$10 = {mtx_object = {lo_name = 0xc04b3b58 "vm object", lo_type = 0xc04b4dd0 "standard object", lo_flags = 21168128, lo_profile_obj = {
      lpo_acqtime = 0, lpo_waittime = 0, lpo_filename = 0x0, lpo_namehash = 4063449288, lpo_lineno = 0,
      lpo_type = 0xc049f940 "sleep mutex", lpo_contest_holding = 0, lpo_contest_locking = 119}, lo_witness_data = {lod_list = {
        stqe_next = 0x0}, lod_witness = 0x0}}, mtx_lock = 4, mtx_recurse = 0}
(kgdb) print ((vm_object_t)0xfffff800aeb21930)->mtx->mtx_lock
$11 = 4

This means the mutex is unowned and the assertion here failed:

(kgdb) list 1913
1908            struct swblock *swap;
1909            daddr_t r1;
1910            int idx;
1911
1912            VM_OBJECT_LOCK_ASSERT(object, MA_OWNED);
1913            /*
1914             * The meta data only exists of the object is OBJT_SWAP
1915             * and even then might not be allocated yet.
1916             */
1917            if (object->type != OBJT_SWAP)

Kris