Re: [RFC PATCH] mm/damon: reset thread status parameters upon kdamond termination

[Liew Rui Yan <aethernet65535@gmail.com>]

On Thu,  2 Apr 2026 06:54:39 -0700 SeongJae Park <sj@kernel.org> wrote:

> On Thu,  2 Apr 2026 13:34:58 +0800 Liew Rui Yan <aethernet65535@gmail.com> wrote:
> 
> > [...]
> > My plan is to add the error check directly in apply_parameters(), right
> > after damon_commit_ctx():
> > 
> >     err = damon_commit_ctx(ctx, param_ctx);
> >     if (err) {
> >         enabled = false;
> >         kdamond_pid = -1;
> >     }
> > 
> > This covers the reproducible case (addr_unit=3 + commit_inputs=Y).
> > 
> > For truly unexpected termination (e.g., memory allocation failure inside
> > kdamond), I'm considering a fallback mechanism in enabled_store():
> > 
> > - When the user writes 'N' to 'enabled', if damon_stop() fails but
> >   kdamond is actually terminated, we reset enabled and kdamond_pid.
> 
> Sounds good.
> 
> > 
> > - When the user writes 'Y' to 'enabled', if enabled is already 'Y' but
> >   kdamond is terminated, we treat this as a restart request.
> 
> This sounds bit odd to me.  User shows enabled=Y and kdamond_pid!=-1.  Why they
> would write 'Y' again?  Yes, they would notice kdamond is terminated in real,
> using 'ps' like tool.  But even in the case, I'd imagine users would write N to
> enabled first, and then write Y to enabled.  In other words, this could improve
> the user experience, but seems not really necessary to me, at least as a
> hotfix.  What do you think?

I see your point. I wasn't thinking of this as a hotfix at first, but
you're right - for a bug like this, simple and backportable is the way
to go.

So I will drop the "Write Y to Restart" logic in the next version. I
will only fix the state in the 'N' path and after commit failures.

> 
> > 
> > This way, even if kdamond terminates unexpectedly, the next user
> > interaction will recover the state automatically.
> > 
> > Does this approach sound reasonable?
> 
> Yes, the plan for the next version sounds good to me.

Great! Thank you for helping me scope this properly! :>

Best regards,
Rui Yan