Re: [PATCH v2] mm/damon/core: reset invalid quota->charge_target_from

[sashiko-bot@kernel.org]

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

Pre-existing issues:
- [High] DAMOS scheme processing permanently stalls if a valid target has zero regions, reproducing the exact lockup the patch intended to fix.
--

commit be23af6d135629bfa902e51163989385be75b34b
Author: SJ Park <sj@kernel.org>
One-line subject: mm/damon/core: reset invalid quota->charge_target_from

This commit fixes an issue where DAMOS can suddenly stop working if a target
process that the quota is fully charged on is terminated. It handles this
corner case by catching the invalid target and resetting the charge target
pointer.

> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index 645cb367019ae..06cf2ab7e97d7 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -3393,8 +3393,15 @@ static void kdamond_apply_schemes(struct damon_ctx *c)
>  	max_region_sz = damon_region_sz_limit(c);
>  	mutex_lock(&c->walk_control_lock);
>  	damon_for_each_target(t, c) {
> -		if (c->ops.target_valid && c->ops.target_valid(t) == false)
> +		if (c->ops.target_valid && c->ops.target_valid(t) == false) {
> +			damon_for_each_scheme(s, c) {
> +				if (s->quota.charge_target_from != t)
> +					continue;
> +				s->quota.charge_target_from = NULL;
> +				s->quota.charge_addr_from = 0;
> +			}
>  			continue;
> +		}
>  		damos_apply_target(c, t, max_region_sz);

[Severity: High]
This is a pre-existing issue, but does this incomplete fix leave DAMOS
vulnerable to permanently stalling when a valid target has zero regions,
or when using operations where target_valid() is not implemented?

If a DAMOS quota is exhausted while applying actions to a target, saving its
pointer to s->quota.charge_target_from, and before the next apply interval
all regions of that target are dynamically removed but the target itself is
kept valid, kdamond_apply_schemes() will process the target here.

Because the target has zero regions, the loop inside damos_apply_target()
is bypassed:

mm/damon/core.c:damos_apply_target() {
	...
	damon_for_each_region(r, t) {
		...
		damon_do_apply_schemes(c, t, r);
		...
	}
}

Consequently, damos_skip_charged_region() is never executed to clear
charge_target_from.

When evaluating all other targets, damos_skip_charged_region() will check
the stale charge_target_from pointer and unconditionally return true,
skipping all actions and permanently stalling scheme processing.

Could we ensure charge_target_from is cleared even if the target has no
regions, or if the target_valid() operation is missing?

>  	}

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260910142846.172957-1-sj@kernel.org?part=1