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

[Chenghao Duan]

On Sat, Oct 10, 2026 at 03:05:40PM +0200, Luca Boccassi wrote:
> On Sat, 10 Oct 2026 at 04:11, Chenghao Duan <duanchenghao@xxxxxxxxxx> wrote:
> >
> > 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@xxxxxxxxxx>
> 
> Hi,
> 
> At some point up in the stack, everything runs as a systemd unit, so
> it's only a question of figuring out in your setup how to pass the
> FD(s) up and down the stack.

Hi Luca,

Thanks for your reply.

The `LUOSession=` configuration only takes effect for services managed
by the system manager. If I run the CRIU command from an SSH login shell
as I am doing now, the environment belongs to a transient unit under the
user manager. In this case, we cannot use `LUOSession=` to obtain the
LUO session fd via systemd. Does this mean the process needs to open
`/dev/liveupdate` by itself to create the session fd?

Below is the attached service of the process found via `systemctl status`.

[root@localhost criu]# ps -ef |grep criu
root       57589    7729  0 18:25 pts/2    00:00:00 ./criu/criu dump -D ../test -t 52922 -j --memfd-images -v4
root       57784    7571  0 18:27 pts/0    00:00:00 grep --color=auto criu
[root@localhost criu]# systemctl status 57589
● session-5.scope - Session 5 of User root
     Loaded: loaded (/run/systemd/transient/session-5.scope; transient)
  Transient: yes
     Active: active (running) since Tue 2026-10-13 10:03:55 CST; 8h ago
      Tasks: 4
     Memory: 148.9M (peak: 1.3G)
        CPU: 47.000s
     CGroup: /user.slice/user-0.slice/session-5.scope
             ├─ 7712 "sshd: root [priv]"
             ├─ 7724 "sshd: root@pts/2"
             ├─ 7729 -bash
             └─57589 ./criu/criu dump -D ../test -t 52922 -j --memfd-images -v4

I'm not sure whether my understanding is correct. Could you please
provide your feedback?


BRs
Chenghao