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Date: Tue, 26 Aug 2025 13:54:31 +0000
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: Pratyush Yadav <pratyush@...nel.org>
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Subject: Re: [PATCH v3 00/30] Live Update Orchestrator

> > https://github.com/googleprodkernel/linux-liveupdate/tree/luo/v3
> >
> > Changelog from v2:
> > - Addressed comments from Mike Rapoport and Jason Gunthorpe
> > - Only one user agent (LiveupdateD) can open /dev/liveupdate
> > - With the above changes, sessions are not needed, and should be
> >   maintained by the user-agent itself, so removed support for
> >   sessions.
>
> If all the FDs are restored in the agent's context, this assigns all the
> resources to the agent. For example, if the agent restores a memfd, all
> the memory gets charged to the agent's cgroup, and the client gets none
> of it. This makes it impossible to do any kind of resource limits.
>
> This was one of the advantages of being able to pass around sessions
> instead of FDs. The agent can pass on the right session to the right
> client, and then the client does the restore, getting all the resources
> charged to it.
>
> If we don't allow this, I think we will make LUO/LiveupdateD unsuitable
> for many kinds of workloads. Do you have any ideas on how to do proper
> resource attribution with the current patches? If not, then perhaps we
> should reconsider this change?

Hi Pratyush,

That's an excellent point, and you're right that we must have a
solution for correct resource charging.

I'd prefer to keep the session logic in the userspace agent (luod
https://tinyurl.com/luoddesign).

For the charging problem, I believe there's a clear path forward with
the current ioctl-based API. The design of the ioctl commands (with a
size field in each struct) is intentionally extensible. In a follow-up
patch, we can extend the liveupdate_ioctl_fd_restore struct to include
a target pid field. The luod agent, would then be able to restore an
FD on behalf of a client and instruct the kernel to charge the
associated resources to that client's PID.

This keeps the responsibilities clean: luod manages sessions and
authorization, while the kernel provides the specific mechanism for
resource attribution. I agree this is a must-have feature, but I think
it can be cleanly added on top of the current foundation.

Pasha

>
> [...]
>
> --
> Regards,
> Pratyush Yadav

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