Question about obtaining LUO session FD for non-systemd-managed service

[Chenghao Duan]

Hi Luca,

I watched your excellent presentation from the LPC conference in Prague.

I have a question I’d like to consult with you.

I am currently working on the `memfd_luo` feature in CRIU. My goal is to
leverage the LUO capability to store checkpoint images inside memfds.
After a kexec reboot, the image data will be retrieved directly from
memory instead of from disk.

Here is my pull request on the CRIU GitHub repository:
https://github.com/checkpoint-restore/criu/pull/3152

In the PR discussion, avagin suggested using systemd as the CRIU daemon
to centrally manage LUO sessions and access to `/dev/liveupdate`. I have
watched your talk and read parts of the systemd source code.

>From my understanding, systemd currently provides a unit‑based solution
where child processes can inherit LUO session file descriptors. However,
this approach requires the target service to run as a child of a systemd
unit. As far as I know, CRIU does not run as a systemd‑managed service.

So my question is: for services such as CRIU that are **not systemd‑managed
child services**, how can they obtain LUO‑session‑related file descriptors
managed by systemd?

Thank you very much for reading this email. My knowledge of systemd is still
limited, so please feel free to correct me if I have misunderstood anything.

Thanks & best regards,
Chenghao Duan <duanchenghao at kylinos.cn>