Re: [RFC PATCH] mm/damon/core: trace esz at first setup

[sashiko-bot@kernel.org]

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

Pre-existing issues:
- [High] The monotonic accumulators `total_charged_ns` and `total_charged_sz` will rapidly overflow on 32-bit architectures, breaking the throughput estimation logic.
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commit 4cb83c3ed801305fa15e94548668201c275a1037
Author: SeongJae Park <sj@kernel.org>

mm/damon/core: trace esz at first setup

This commit fixes an issue where the initial effective size quota is not
traced, which leaves tracepoint investigations incomplete when using the
temporal tuner. It updates the quota adjustment logic to unconditionally
trace the initial quota value.

> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index 4e223857a0f99..0db6530825d1d 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -2883,6 +2883,8 @@ static void damos_adjust_quota(struct damon_ctx *c, struct damos *s)
>  	if (!quota->total_charged_sz && !quota->charged_from) {

[Severity: High]
This isn't a bug introduced by this patch, but will total_charged_ns and
total_charged_sz overflow on 32-bit architectures?

Since unsigned long is 32 bits on those systems, total_charged_ns will
overflow after roughly 4.29 seconds of total active execution time.

Additionally, the throughput calculation in damos_set_effective_quota():

damos_set_effective_quota() {
    ...
    if (quota->total_charged_ns)
        throughput = mult_frac(quota->total_charged_sz,
                1000000, quota->total_charged_ns);
    ...
}

uses mult_frac() which evaluates (x % d) * n internally using the type of x.
Since the remainder could be up to total_charged_ns - 1, multiplying it by
1,000,000 will quickly overflow the 32-bit boundary. This produces incorrect
throughput estimates and impacts the effective size quota calculations on
32-bit systems.

>  		quota->charged_from = jiffies;
>  		damos_set_effective_quota(c, s);
> +		if (trace_damos_esz_enabled())
> +			damos_trace_esz(c, s, quota);
>  	}
>  
>  	/* New charge window starts */

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Sashiko AI review · https://sashiko.dev/#/patchset/20260520005940.92003-1-sj@kernel.org?part=1