A loop device is a kernel abstraction that allows a file to be presented as
if it were a physical block device. The typical use for a loop device is to mount a
filesystem image stored in a file. Loop devices are global and shared between
users, which causes a number of problems for container workloads where the
instances are expected to be isolated from each other. Christian Brauner
has been working on this problem; he has posted a patch
set solving it by adding a small virtual filesystem called loopfs.

Source: LWN.net – [$] Private loop devices with loopfs