Re: [RFC PATCH v3 2/4] mm/damon: fix esz=0 quota bypass allowing unlimited migration

[Ravi Jonnalagadda <ravis.opensrc@gmail.com>]

On Mon, Feb 23, 2026 at 5:54 PM SeongJae Park <sj@kernel.org> wrote:
>
> On Mon, 23 Feb 2026 12:32:30 +0000 Ravi Jonnalagadda <ravis.opensrc@gmail.com> wrote:
>
> > When the TEMPORAL goal tuner sets esz_bp=0 to signal that a goal has
> > been achieved, the quota check was not actually stopping migration.
> >
> > The condition:
> >     if (quota->esz && quota->charged_sz >= quota->esz)
> >
> > When esz=0, this evaluates to (false && ...) = false, so the continue
> > is never executed and migration proceeds without limit.
>
> Nice finding, thank you for sharing this!

Thanks!

>
> >
> > Change the logic to:
> >     if (!quota->esz || quota->charged_sz >= quota->esz)
> >
> > Now when esz=0, (!0 = true) causes the continue to execute, properly
> > stopping migration when the goal is achieved.
>
> But this code is written in the way because the current code assumes zero 'esz'
> means it is not being used and therefore be ignored.  This change should be ok
> for your use case, but could introduce an unexpected behavioral change for
> other users.
>
> One easy workaround would be setting esz_bp with a value smaller than 40960000
> instead of 0.  That is, esz_bp is the bytes in bp, so setting it smaller than
> 40960000 will result in making it effectively zero, e.g., like below.
>
> '''
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -2623,7 +2623,7 @@ static void damos_goal_tune_esz_bp_temporal(struct damos_quota *quota)
>         unsigned long score = damos_quota_score(quota);
>
>         if (score >= 10000)
> -               quota->esz_bp = 0;
> +               quota->esz_bp = 10000;
>         else if (quota->sz)
>                 quota->esz_bp = quota->sz * 10000;
>         else
> '''
>
> But maybe there is a better way to cleanly fix this.  Let me take a time to
> think more...
>

I understand. Please take your time. I'm happy to test whatever approach
you come up with.

>
> Thanks,
> SJ
>
> [...]