Re: [RFC PATCH v1.2 01/17] 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:
- [Medium] Integer overflow in left_window_bp calculation on 32-bit architectures
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commit c5893684a7c356bd0f4693c2ef6895c46e83d332
Author: SeongJae Park <sj@kernel.org>

mm/damon: introduce damon_nr_accesses_mvsum()

Introduce a new DAMON core function, damon_nr_accesses_mvsum(), to return a
pseudo moving sum value of a given region's nr_accesses for the last
aggregation interval. This value acts as a relative access frequency score
that is updated only when requested.

> diff --git a/mm/damon/core.c b/mm/damon/core.c
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c

[ ... ]

> +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;
> +	if (time_after_eq(ctx->passed_sample_intervals,
> +				ctx->next_aggregation_sis))
> +		left_window = 0;
> +	else
> +		left_window = ctx->next_aggregation_sis -
> +			ctx->passed_sample_intervals;
> +	left_window_bp = mult_frac(left_window, 10000, window_len);

[Severity: Medium]
Can this cause an integer overflow on 32-bit architectures?

Looking at the mult_frac macro, the remainder is multiplied by the numerator:
r * n_ / d_

If user-configurable parameters result in a large ratio, such as a 10
microsecond sample interval and a 5 second aggregation interval, window_len
becomes 500,000.

If left_window is near that maximum, for example 499,999, the intermediate
multiplication inside the macro would be 499,999 * 10000. This evaluates to
4,999,990,000, which exceeds the 32-bit unsigned long maximum of
4,294,967,295.

Could this wrap around and result in an incorrectly small remainder,
producing an inaccurate left_window_bp percentage? Passing 10000ULL might
enforce 64-bit math to avoid this.

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

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Sashiko AI review · https://sashiko.dev/#/patchset/20260621155715.87932-1-sj@kernel.org?part=1