please read PID

6.9 introduces pidfdfs, a pseudo filesystem similar to nsfs. This is not visible from userspace, and is not mounted. It facilitates system calls which demand a filesystem backing: statx() now works, as do LSMs (Linux Security Modules).^[1]

Introduction of PIDFDFS

The pidfdfs filesystem can now create file handles (when requested by a name_to_handle_at(); these can be used to create a system-wide unique identifier for processes even on 32-bit systems. It is also now possible to bind-mount pidfds.^[2]

^1]: [Pidfd - dankwiki, the wiki of nick black

^2]: [The first part of the 6.14 merge window [LWN.net]