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

[SeongJae Park <sj@kernel.org>]

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!

> 
> 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...


Thanks,
SJ

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