kvm_getswapinfo() function returns smaller device sizes than the actual swap device sizes configured available.  "systat -swap" uses this API and shows problem.  "sysctl -c" uses a different API and displays correct sizes.

While "usesd" swap device sizes are reported correctly, when a swap device is used fully, it looks like system haven overwritten next partition.

The root of problem is libkvm subtracts vm.dmmax, maximum VOP I/O allowed, form size of device erroneously.

The following shows that 1 MB swap device is prepared, attached, and then fully used.  Given FreeBSD swap skips 2 4k blocks, 1016 is the correct size available for swapping. 

# dd if=/dev/zero of=1MB bs=1M count=1
1+0 records in
1+0 records out
1048576 bytes transferred in 0.035823 secs (29271150 bytes/sec)
# mdconfig -a -t vnode -f 1MB
md0
# swapon /dev/md0
# swapctl -l
Device:       1024-blocks     Used:
/dev/ada0s1b    2097144         0
/dev/md0           1016         0
# mount -t tmpfs tmpfs /mnt/tmp
# dd if=/dev/zero of=/mnt/tmp/fill bs=1M count=2000
2000+0 records in
2000+0 records out
2097152000 bytes transferred in 36.267636 secs (57824337 bytes/sec)
# swapctl -l
Device:       1024-blocks     Used:
/dev/ada0s1b    2097144     144220
/dev/md0           1016      1016
# systat -swap
Disk   1K-blocks   Used /0%  /10  /20  /30  /40  /50  /60  /70  /80  /90  /100
ada0s1b  2097016 144216 XXXX
md0          888   1016 XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
XX       2097904 145232 XXXX