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

[SJ Park <sj@kernel.org>]

On Sun, 02 Aug 2026 16:43:14 +0000 sashiko-bot@kernel.org wrote:

> 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?

Good catch.  To handle this, the fix will be little bit complicated.  It may
not deserve stable@ backport.  I will drop this from the next version of this
series and work on more complete fix.

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


Thanks,
SJ