Re: [RFC PATCH v1.1 2/9] mm/damon/core: initialize damos_quota_goal->last_psi_total

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [Medium] The patch incompletely fixes the uninitialized `last_psi_total` variable usage. While it initializes `last_psi_total` when the `kdamond` thread starts, it fails to initialize it for schemes or quota goals added dynamically at runtime via sysfs.
--

--- Patch [2]: [RFC PATCH v1.1 2/9] mm/damon/core: initialize damos_quota_goal->last_psi_total ---
commit ca3c4694fe111f676d0de56148cf986c02532428
Author: SJ Park <sj@kernel.org>

mm/damon/core: initialize damos_quota_goal->last_psi_total

When DAMOS_QUOTA_SOME_MEM_PSI_US metric damos quota goal is set, the PSI
delta for the feedback loop is calculated using
damos_quota_goal->last_psi_total. It is not initialized at the beginning.
This commit initializes the value at the beginning of kdamond.

Link: https://lore.kernel.org/20260718005316.89585-1-sj@kernel.org

> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index e2900d0c984c9..3bdbf4fbf7147 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c

[ ... ]

> @@ -3744,6 +3755,7 @@ static void kdamond_init_ctx(struct damon_ctx *ctx)
>  	damon_for_each_scheme(scheme, ctx) {
>  		damos_set_next_apply_sis(scheme, ctx);
>  		damos_set_filters_default_reject(scheme);
> +		damos_init_quota_goal_last_psi(scheme);

[Severity: Medium]
This is a pre-existing issue, but does this completely fix the uninitialized
variable usage?

While this initializes last_psi_total when the kdamond thread starts, it
appears goals added dynamically at runtime via sysfs will still be
uninitialized.

When a goal is parsed from sysfs, memory is allocated without zeroing the
structure, leaving last_psi_total filled with slab garbage:

mm/damon/sysfs-schemes.c:damos_sysfs_add_quota_score() {
    ...
    goal = damos_new_quota_goal(sysfs_goal->metric,
            sysfs_goal->target_value);
    ...
}

During the context commit phase, the goal is dynamically allocated but
last_psi_total is intentionally bypassed:

mm/damon/core.c:damos_commit_quota_goals() {
    ...
    new_goal = damos_new_quota_goal(
            src_goal->metric, src_goal->target_value);
    ...
}

When the feedback loop executes, this uninitialized memory is consumed:

mm/damon/core.c:damos_set_quota_goal_current_value() {
    ...
    goal->current_value = now_psi_total - goal->last_psi_total;
    ...
}

Will this cause a random, extremely large or small initial quota adjustment
when a user adds a new DAMON scheme or quota goal with the
DAMOS_QUOTA_SOME_MEM_PSI_US metric to a running context via sysfs?

>  	}
>  }

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260802162631.90304-1-sj@kernel.org?part=2