leaked swap?

First, a note that this was observed on a system that runs a fairly old current
(~ 1 year old) with a fairly long uptime (> 6 months).
I noticed that the system was nearly out of memory, 98% of swap was in use,
there was less than 1 GB of free memory, several GBs of each of active, inactive
and laundry memory, and many GBs of wired (mostly ZFS).
I decided to pro-actively reboot the system, but to speed that up I put the
system to the single-user mode (via shutdown) and then back to multi-user. So,
there was no real hardware reboot and the kernel kept running.  However, all
userland processes were terminated.

To my surprise, even while in the single-user mode the swap utilization didn't
go below 70%.  Also, laundry memory remained in multi-GB area, but let's ignore
this for now.

I think that the swap could be used only for anonymous memory, so I expected it
go to zero after the shutdown to the single user mode.
Does anyone have any ideas?
Maybe that's something that has already been fixed?
If not, any ideas on what to look for?
Thanks!

-- 
Andriy Gapon