Re: [PATCH] mm/damon/core: fix quota could not be set to zero

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] The patch places the `if (esz)` check after the throughput calculation, inadvertently reverting a previous fix (commit 8bbde987c2b8) and causing schemes with low throughput to permanently deactivate.
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commit a1511f591a76665e55c325a03d3d47bf4ba87dc6
Author: Liew Rui Yan <aethernet65535@gmail.com>

mm/damon/core: fix quota could not be set to zero

This commit modifies the damos quota setting logic to skip clamping to
min_region_sz when the effective size quota is already zero. This is
intended to allow the temporal tuner's zero quota goal to be respected.

> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index 3f89cfdf5f022..fdec7c0fe913a 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -3122,7 +3122,8 @@ static void damos_set_effective_quota(struct damon_ctx *ctx, struct damos *s)
>  		else
>  			throughput = PAGE_SIZE * 1024;
>  		esz = min(throughput * quota->ms, esz);
> -		esz = max(ctx->min_region_sz, esz);
> +		if (esz)
> +			esz = max(ctx->min_region_sz, esz);

[Severity: High]
Does placing the if (esz) check after the throughput calculation inadvertently
revert the fix from commit 8bbde987c2b8 and cause a regression where slow
schemes permanently deactivate?

If a damos scheme with a time quota processes regions slowly, integer division
can cause throughput to truncate to 0. When this happens, the calculation
esz = min(throughput * quota->ms, esz) evaluates to 0.

Because the new if (esz) check is placed after this min() calculation, the 0
value causes the check to fail, skipping the max(ctx->min_region_sz, esz)
clamp.

With esz == 0, damos_quota_is_full() always returns true, bypassing all
scheme operations. Since no regions are processed, total_charged_ns and
total_charged_sz are never updated. This leaves throughput permanently at 0,
resulting in an unrecoverable starvation loop.

>  	}
>  
>  	if (quota->sz && quota->sz < esz)

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Sashiko AI review · https://sashiko.dev/#/patchset/20260901105951.106246-1-aethernet65535@gmail.com?part=1