'swap_pager: indefinite wait buffer' with swapfile

I configured a vnode-backed md and enabled swapping on it.  A few
hours later after moderate swap use the console showed:

swap_pager: indefinite wait buffer: bufobj: 0, blkno: 889347, size: 8192
[...repeated...]

The backing store was a sparse file, but there was ample space:

# ls -l /data2/swapfile
-rw-r--r--  1 root  wheel  17179869184 Sep 11 16:50 /data2/swapfile
# df /data2
Filesystem       1K-blocks     Used    Avail Capacity  Mounted on
/dev/stripe/data  51666218 27042730 20490192    57%    /data2
# swapinfo
Device          1K-blocks     Used    Avail Capacity
/dev/da0b         6297480   949304  6297480    15%
/dev/md41        16777216   842544 16777216     5%
Total            23074696  1791848 21282848     8%

Kris