panic du jour: vnode_pager_getpages: unexpected missing page

I'm debugging a program making heavy us of mmap(). Contrary to what
man-page says, I expected it to be possible to mmap() regions larger
than 2Gb on amd64, and mmap() did, indeed succeeded for a 3.7Gb file.

However some time into running the program exited on signal 6 (ABRT).

A minute or so after that the whole system panic-ed:

vnode_pager_getpages: unexpected missing page: first addr: -1, foff 
0x080006000, vnp_size 0x0b5d20000

The machine runs yesterday's -current, has a single Opteron CPU and 2Gb
of RAM. There was very little IO at the time of death (according to the
frozen `systat 1 -vm').

 1) Am I wrong, and mmap-ing more than 2Gb is not supposed
    to work even on a 64-bit system?
 2) How can I help in eliminating this panic?
 3) Should I go back to the latest 5.3-beta, or is the -current
    kernel not dramatically more unstable?

Thanks,

 -mi