Re: [PATCH 6.18.y 1/2] mm/damon/core: allow esz to be set to zero

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] mm/damon: removing min_sz_region clamp on esz breaks quota accounting
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commit f7906e90ad53cf62853b7b051dfd11b8012181f1
Author: Liew Rui Yan <aethernet65535@gmail.com>
Subject: mm/damon/core: allow esz to be set to zero

This patch modifies damos_set_effective_quota() to allow the effective
size quota (esz) to be reduced as much as possible when temporal quota
goals are achieved. It does this by removing the clamp against
min_sz_region when calculating esz.

Link: https://lore.kernel.org/20260908135413.97570-1-sj@kernel.org

> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index 6fa02025d2af1..facb1fa1f695f 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c

[ ... ]

> @@ -2212,8 +2213,8 @@ static void damos_set_effective_quota(struct damos_quota *quota,
>  							quota->total_charged_ns);
>  		else
>  			throughput = PAGE_SIZE * 1024;
> -		esz = min(throughput * quota->ms, esz);
> -		esz = max(ctx->min_sz_region, esz);
> +		esz_time = max(throughput * quota->ms, ctx->min_sz_region);
> +		esz = min(esz_time, esz);
>  	}

[Severity: Medium]
Will this break quota accounting when goals are over-achieved?

By removing the clamp against min_sz_region, esz can drop below the minimum
region size, for example to 1.

If esz becomes 1, does ALIGN_DOWN() in damos_apply_scheme() evaluate to 0?

mm/damon/core.c:damos_apply_scheme() {
        if (quota->esz && quota->charged_sz + sz > quota->esz) {
                sz = ALIGN_DOWN(quota->esz - quota->charged_sz,
                                c->min_sz_region);
                if (!sz)
                        goto update_stat;
                damon_split_region_at(t, r, sz);
        }
        ...
        quota->charged_sz += sz;
}

If sz evaluates to 0, it jumps to update_stat and bypasses incrementing
quota->charged_sz.

Because quota->charged_sz gets frozen at 0, wouldn't this prevent the quota
short-circuit from ever being taken in damon_do_apply_schemes()?

mm/damon/core.c:damon_do_apply_schemes() {
        /* Check the quota */
        if (quota->esz && quota->charged_sz >= quota->esz)
                continue;
}

Can this also corrupt the qt_exceeds statistic since the same check
fails in damos_adjust_quota()?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260929174456.9251-1-sj@kernel.org?part=1