Re: [PATCH v4] mm/damon/core: allow esz to be set to zero

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:

Pre-existing issues:
- [High] Integer overflow in temporal goal tuner calculating `esz_bp` and `ULONG_MAX` capping severely limits DAMOS effective size quota on 32-bit systems.
- [High] Integer overflow when calculating time quota constraints (`throughput * quota->ms`) on 32-bit systems limits time-based quota.
--

commit 05338618157ff96b3145359ae173701a5ed4f699
Author: Liew Rui Yan <aethernet65535@gmail.com>

mm/damon/core: allow esz to be set to zero

When the temporal quota goal tuner achieves its goal, it sets esz_bp to
zero, but damos_set_effective_quota() previously clamped this to
min_region_sz. This patch fixes this by keeping the effective size quota
(esz) as minimum as possible when the goal is achieved.

> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index ce8c6f99106ed..3e9c9b6385d4e 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -3239,6 +3239,7 @@ static void damos_set_effective_quota(struct damon_ctx *ctx, struct damos *s)
>  	struct damos_quota *quota = &s->quota;
>  	unsigned long throughput;
>  	unsigned long esz = ULONG_MAX;
> +	unsigned long esz_time;
>  
>  	if (!quota->ms && list_empty(&quota->goals)) {
>  		quota->esz = quota->sz;

[ ... ]

> @@ -3259,8 +3260,8 @@ static void damos_set_effective_quota(struct damon_ctx *ctx, struct damos *s)
>  					1000000, quota->total_charged_ns);
>  		else
>  			throughput = PAGE_SIZE * 1024;
> -		esz = min(throughput * quota->ms, esz);
> -		esz = max(ctx->min_region_sz, esz);
> +		esz_time = max(throughput * quota->ms, ctx->min_region_sz);

[Severity: High]
This isn't a bug introduced by this patch, but does this inline multiplication
wrap around on 32-bit systems?

Both throughput (calculated as bytes per millisecond) and quota->ms are
unsigned long. On a 32-bit architecture, a realistic throughput like 2 MB/ms
combined with a quota limit of 3000 ms yields a product of 6,000,000,000. 

Since this exceeds the 32-bit unsigned long limit of 4.29 billion, it silently
wraps around to a significantly smaller size limit, which unintentionally bounds
the effective quota and breaks the intended time-based logic.

[Severity: High]
This is a pre-existing issue, but I also noticed a similar overflow problem
regarding the quota->esz_bp calculation in this same path.

When a user configures quota->sz via sysfs, it is scaled here:

mm/damon/core.c:damos_goal_tune_esz_bp_temporal() {
    ...
		quota->esz_bp = quota->sz * 10000;
	else
		quota->esz_bp = ULONG_MAX;
    ...
}

If quota->sz is unset (0), esz_bp defaults to ULONG_MAX (4,294,967,295 on
32-bit systems). When this is later scaled down:

mm/damon/core.c:damos_set_effective_quota() {
    ...
		esz = quota->esz_bp / 10000;
    ...
}

The resulting esz becomes 429,496 bytes (~419 KB). Does this artificially
throttle DAMOS region actions on 32-bit platforms?

Additionally, if quota->sz is set larger than ~429 KB, quota->sz * 10000
silently overflows the 32-bit unsigned long, creating a much smaller wrapped
quota. Could we address this ULONG_MAX limitation and multiplication overflow
on 32-bit architectures?

> +		esz = min(esz_time, esz);
>  	}
>  
>  	if (quota->sz && quota->sz < esz)

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260908135413.97570-1-sj@kernel.org?part=1