Re: [RFC PATCH 3/4] livepatch: Add "replaceable" attribute to klp_patch

Yafang Shao
2026-04-06T11:08:46Z

On Sat, Apr 4, 2026 at 5:36 AM Song Liu <song@kernel.org> wrote:
>
> On Fri, Apr 3, 2026 at 1:55 PM Dylan Hatch <dylanbhatch@google.com> wrote:
> [...]
> > > IIRC, the use case for this change is when multiple users load various
> > > livepatch modules on the same system. I still don't believe this is the
> > > right way to manage livepatches. That said, I won't really NACK this
> > > if other folks think this is a useful option.
> >
> > In our production fleet, we apply exactly one cumulative livepatch
> > module, and we use per-kernel build "livepatch release" branches to
> > track the contents of these cumulative livepatches. This model has
> > worked relatively well for us, but there are some painpoints.
> >
> > We are often under pressure to selectively deploy a livepatch fix to
> > certain subpopulations of production. If the subpopulation is running
> > the same build of everything else, this would require us to introduce
> > another branching factor to the "livepatch release" branches --
> > something we do not support due to the added toil and complexity.
> >
> > However, if we had the ability to build "off-band" livepatch modules
> > that were marked as non-replaceable, we could support these selective
> > patches without the additional branching factor. I will have to
> > circulate the idea internally, but to me this seems like a very useful
> > option to have in certain cases.
>
> IIUC, the plan is:
>
> - The regular livepatches are cumulative, have the replace flag; and
> are replaceable.
> - The occasional "off-band" livepatches do not have the replace flag,
> and are not replaceable.
>
> With this setup, for systems with off-band livepatches loaded, we can
> still release a cumulative livepatch to replace the previous cumulative
> livepatch. Is this the expected use case?

That matches our expected use case.

>
> I think there are a few issues with this:
> 1. The "off-band" livepatches cannot be replaced atomically. To upgrade
> "off-band' livepatches, we will have to unload the old version and load
> the new version later.

Right. That is how the non-atomic-replace patch works.

> 2. Any conflict with the off-band livepatches and regular livepatches will
> be difficult to manage.

We need to manage this conflict with a complex user script. That said,
everything can be controlled from userspace.

> IOW, we kind removed the benefit of cumulative
> livepatches. For example, what shall we do if we really need two fixes
> to the same kernel functions: one from the original branch, the other
> from the off-band branch?

We run tens of livepatches on our production servers and have never
run into this issue. It's an extremely rare case — and if it does
happen, a user script should be able to handle it just fine.

--
Regards
Yafang