[$] A new filesystem for pidfds

The pidfd abstraction is a Linux-specific
way of referring to processes that avoids the race conditions inherent in
Unix process ID numbers. Since a pidfd is a file descriptor, it needs a
filesystem to implement the usual operations performed on files. As the
use of pidfds has grown, they have stressed the limits of the simple
filesystem that was created for them. Christian Brauner has created
a new filesystem for pidfds that seems likely to debut in the 6.9
kernel, but it ran into a little bump along the way, demonstrating that
things you cannot see can still hurt you.