Re: [RFC PATCH v1.1 01/13] mm/damon: introduce damon_nr_accesses_mvsum()

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Integer overflow in `mult_frac` within `damon_nr_accesses_mvsum()` corrupts DAMON moving sum access score on 32-bit architectures.
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commit 33b6652e4ab7390d9a1e1c6a292edc9a0fec212a
Author: SeongJae Park <sj@kernel.org>

mm/damon: introduce damon_nr_accesses_mvsum()

This commit introduces damon_nr_accesses_mvsum() which returns a pseudo
moving sum of a given region's nr_accesses for the last aggregation interval.

> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index 7e4b9affc5b06..af95783d3402a 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c

[ ... ]

> @@ -210,6 +210,63 @@ static struct damon_probe *damon_nth_probe(int n, struct damon_ctx *ctx)
>  	return NULL;
>  }
>  
> +/*
> + * damon_mvsum() - Returns pseudo moving sum value for a time window.
> + * @current_nr:		The value of the current aggregation window.

[ ... ]

> +unsigned int damon_nr_accesses_mvsum(struct damon_region *r,
> +		struct damon_ctx *ctx)
> +{
> +	unsigned long sample_interval, aggr_interval;
> +	unsigned long window_len, left_window, left_window_bp;
> +
> +	sample_interval = ctx->attrs.sample_interval ? : 1;
> +	aggr_interval = ctx->attrs.aggr_interval ? : 1;
> +	window_len = aggr_interval / sample_interval;
> +	left_window = ctx->next_aggregation_sis - ctx->passed_sample_intervals;
> +	left_window_bp = mult_frac(left_window, 10000, window_len);

[Severity: High]
Can the intermediate multiplication inside mult_frac() overflow on 32-bit
architectures?

The mult_frac(x, n, d) macro computes (x / d) * n + (x % d) * n / d. Since
left_window is generally less than window_len, the remainder (x % d) evaluates
directly to left_window.

On 32-bit architectures, both left_window and the resulting product are 32-bit
unsigned long values. If window_len exceeds 429,496 (for example, a 5ms
sample_interval coupled with a ~36 minute aggr_interval), the intermediate
calculation of left_window * 10000 can exceed ULONG_MAX (4,294,967,295) and
silently wrap around.

This wrap around would cause left_window_bp to calculate as a truncated,
artificially low value, leading to an incorrect access frequency score that
might affect memory reclaim or protection decisions.

> +
> +	return damon_mvsum(r->nr_accesses, r->last_nr_accesses,
> +			left_window_bp);
> +}

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Sashiko AI review · https://sashiko.dev/#/patchset/20260620172244.90953-1-sj@kernel.org?part=1