"panic: uma_small_alloc: free page still has mappings!" on sparc

I got this on a 12 CPU sparc64 running 7.0 from a week or so ago:

panic: uma_small_alloc: free page still has mappings!

panic() at panic+0x164
uma_small_alloc() at uma_small_alloc+0x9c
slab_zalloc() at slab_zalloc+0x98
uma_zone_slab() at uma_zone_slab+0x1ac
uma_zalloc_bucket() at uma_zalloc_bucket+0x1b4
uma_zalloc_arg() at uma_zalloc_arg+0x398
malloc() at malloc+0xf0
sigacts_alloc() at sigacts_alloc+0x10
fork1() at fork1+0x708
fork() at fork+0x10
syscall() at syscall+0x2a0
-- syscall (2, FreeBSD ELF64, fork) %o7=0x110460 --

Any ideas?

Kris