+.BR pivot_root ()
+allows the caller to switch to a new root filesystem while at the same time
+placing the old root mount at a location under
+.I new_root
+from where it can subsequently be unmounted.
+(The fact that it moves all processes that have a root directory
+or current working directory on the old root filesystem to the
+new root filesystem frees the old root filesystem of users,
+allowing it to be unmounted more easily.)
+A typical use of
+.BR pivot_root ()
+is during system startup, when the
+system mounts a temporary root filesystem (e.g., an \fBinitrd\fP), then
+mounts the real root filesystem, and eventually turns the latter into
+the current root of all relevant processes or threads.
+A modern use is to set up a root filesystem during
+the creation of a container.
+.PP