According to the slabinfo line, 12 bytes. The revoke_table is 12 bytes, too, and I'm not aware of any problems with slab debugging enabled.
at the previous try, with slab debugging enabled, it was spawning tons
of errors but I suspect it's a bug in the slab debugging, it was
complaining with red zone memory corruption, could be due the tiny size
of this object (only 8 bytes).
andrea@xeon:~> grep anon_vma /proc/slabinfo
anon_vma 1230 1500 12 250 1 : tunables 120 60 8 : slabdata 6 6 0