When loopfiles are used mkfs has to target the file and not the containing device

To make it even safer one might also check for the existance of /var/lib/partman/devices/$dev/loop

loopfile=$(losetup $device 2>/dev/null|cut -f 3 -d ' ')
loopfile=${loopfile#\(}
loopfile=${loopfile%\)}
[ -z "$loopfile" ] && [ -f loop ] && loopfile=$(cat loop)
if [ -n "$loopfile" ]; then
   if log-output -t partman --pass-stdout mkfs.ext3 -F "$loopfile" >/dev/null; then
       sync
       status=OK
   else
       status=failed
   fi
else
    #run mkfs as usual
fi

This will need to be done to the individual filesystem support udebs for any filesystems we care about in this situation. Maintaining separate copies of the format scripts will create too much of a headache. Published for ext3:

 partman-ext3 (49ubuntu1) hardy; urgency=low

   * Special case loopmounted filesystems as it's safer to format the
     underlying file, not the device.

Evan,

Since the packages affected are a bit scattered, Colin suggest to submit one patch for a single package to be extended to the others. As far as I am concerned I need ext3 and swap