Re: [RFC PATCH 1/6] livepatch: Support scoped atomic replace using replace set

[Petr Mladek <pmladek@suse.com>]

On Mon 2026-05-18 14:25:04, Song Liu wrote:
> On Wed, May 13, 2026 at 7:34 AM Yafang Shao <laoar.shao@gmail.com> wrote:
> >
> > Convert the replace attribute from a boolean to a u32 to function as a
> > "replace set." A newly loaded livepatch will now atomically replace
> > existing patches that belong to the same set.
> >
> > This change currently supports function replacement only; support for
> > state and shadow variables will be introduced in subsequent patches.
> >
> > Suggested-by: Song Liu <song@kernel.org>
> > Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> > ---
> >  .../livepatch/cumulative-patches.rst          | 17 ++++++++------
> >  Documentation/livepatch/livepatch.rst         | 23 +++++++++++--------
> >  include/linux/livepatch.h                     |  5 ++--
> >  kernel/livepatch/core.c                       | 16 ++++++++-----
> >  kernel/livepatch/state.c                      | 17 +++++++-------
> >  kernel/livepatch/transition.c                 | 10 ++++----
> >  scripts/livepatch/init.c                      |  7 +-----
> >  scripts/livepatch/klp-build                   | 14 +++++------
> >  8 files changed, 59 insertions(+), 50 deletions(-)
> >
> > diff --git a/Documentation/livepatch/cumulative-patches.rst b/Documentation/livepatch/cumulative-patches.rst
> > index 1931f318976a..6ef49748110e 100644
> > --- a/Documentation/livepatch/cumulative-patches.rst
> > +++ b/Documentation/livepatch/cumulative-patches.rst
> > @@ -17,18 +17,20 @@ from all older livepatches and completely replace them in one transition.
> >  Usage
> >  -----
> >
> > -The atomic replace can be enabled by setting "replace" flag in struct klp_patch,
> > -for example::
> > +The "replace_set" attribute in ``struct klp_patch`` acts as a **replace set**,
> > +defining the scope of the replacement. By default, the replace set is 1.
> > +
> > +For example::
> >
> >         static struct klp_patch patch = {
> >                 .mod = THIS_MODULE,
> >                 .objs = objs,
> > -               .replace = true,
> > +               .replace_set = 1,
> >         };
> 
> I wonder whether we should have "replace_set = 0" means no replace.
> This will simplify the transition for users of the existing replace=false
> option. I would like to hear other folks' thoughts on this.

I would find this confusing. Also it would complicate the code.

I always considered the "replace" and "no replace" mode as two
separate worlds:

    + people using many "no replace" livepatches
    + people always using atomic replace

But the code had to handle also the world where:

    + people might combine "no replace" and "replace all" livepatches

which looked like a clash of the two worlds. And different people might
have different expectations about the behavior.

The "replace_set" allows to remove this clash. It looks like a win-win.
And it makes the change acceptable for me.

Best Regards,
Petr