mutex lock assertion panic in vm_page_free() on sparc64

Hi,

I'm repeatedly getting the following panic (three-four times a day)
on sparc64 running a fresh -CURRENT, building world:

db> show msgbuf
panic: mutex vm object not owned at /usr/src/sys/vm/vm_page.c:608
cpuid = 0
KDB: enter: panic
exclusive sleep mutex vm page queue mutex r = 0 (0xc04be700) locked _at_ /usr/src/sys/sparc64/sparc64/vm_machdep.c:483
exclusive sleep mutex UMA lock r = 0 (0xc04bdcc0) locked _at_ /usr/src/sys/vm/uma_core.c:1485
exclusive sleep mutex Giant r = 0 (0xc0422728) locked _at_ /usr/src/sys/vm/vm_pageout.c:666
db> where
Tracing pid 7 tid 100016 td 0xfffff80067c0c720
panic() at panic+0x16c
_mtx_assert() at _mtx_assert+0x6c
vm_page_remove() at vm_page_remove+0x34
vm_page_free_toq() at vm_page_free_toq+0xb8
vm_page_free() at vm_page_free+0x10
uma_small_free() at uma_small_free+0x54
zone_drain() at zone_drain+0x2e8
zone_foreach() at zone_foreach+0x3c
uma_reclaim() at uma_reclaim+0x10
vm_pageout_scan() at vm_pageout_scan+0x13c
vm_pageout() at vm_pageout+0x3e8
fork_exit() at fork_exit+0x9c
fork_trampoline() at fork_trampoline+0x8


Cheers,
-- 
Ruslan Ermilov
ru_at_FreeBSD.org
FreeBSD committer