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

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
- [High] Kernel panic due to division by zero in `damon_nr_accesses_mvsum()` when `aggr_interval` is configured to 0.
- [Medium] Integer overflow in `mult_frac()` leading to severely incorrect access scoring on 32-bit systems when `window_len` is large.
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commit 0c6de91631a66fe41c1e81f68011cf31d86b4825
Author: SeongJae Park <sj@kernel.org>

mm/damon: introduce damon_nr_accesses_mvsum()

This commit introduces damon_nr_accesses_mvsum() to calculate a pseudo
moving sum of a given region's nr_accesses for the last aggregation
interval. The function is designed to be executed on-demand rather than
at every sampling interval.

> 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, window_len, left_window, left_window_bp;
> +
> +	sample_interval = ctx->attrs.sample_interval ? : 1;
> +	window_len = ctx->attrs.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]
What happens here if aggr_interval is configured to 0?

If userspace configures both aggr_interval and sample_interval to 0
via sysfs, sample_interval evaluates to 1 through the fallback, but
window_len becomes 0.

When window_len is passed as the denominator to mult_frac(), does this
result in a divide-by-zero kernel panic?

[Severity: Medium]
Is there a risk of an integer overflow here on 32-bit systems?

The mult_frac(x, n, d) macro evaluates the fractional part using the type
of x, which is an unsigned long here. On 32-bit architectures, this has
a maximum value of ~4.29 billion.

If a 5-second aggregation interval and 1-microsecond sample interval are
configured, window_len and left_window could reach 5,000,000.

The intermediate multiplication in mult_frac():
(left_window % window_len) * 10000

could evaluate to around 50,000,000,000, which overflows a 32-bit
unsigned long. Could this lead to inaccurate access frequency calculations?

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

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Sashiko AI review · https://sashiko.dev/#/patchset/20260619193415.73833-1-sj@kernel.org?part=1