One further possible type is a mount via the loop device. For
example, the command
mount /tmp/disk.img /mnt -t vfat -o loop=/dev/loop3
will set up the loop device /dev/loop3 to correspond to the file
/tmp/disk.img, and then mount this device on /mnt.
If no explicit loop device is mentioned (but just an option '-o
loop' is given), then mount will try to find some unused loop
device and use that, for example
mount /tmp/disk.img /mnt -o loop
The mount command automatically creates a loop device from a
regular file if a filesystem type is not specified or the
filesystem is known for libblkid, for example:
mount /tmp/disk.img /mnt
mount -t ext4 /tmp/disk.img /mnt
This type of mount knows about three options, namely loop, offset
and sizelimit, that are really options to losetup(8). (These
options can be used in addition to those specific to the
filesystem type.)
Since Linux 2.6.25 auto-destruction of loop devices is supported,
meaning that any loop device allocated by mount will be freed by
umount independently of /etc/mtab.
You can also free a loop device by hand, using losetup -d or
umount -d.
Since util-linux v2.29, mount re-uses the loop device rather than
initializing a new device if the same backing file is already
used for some loop device with the same offset and sizelimit.
This is necessary to avoid a filesystem corruption.
