Re: [RFC PATCH 06/13] mm/damon/core: remove damon_warn_fix_nr_accesses_corruption()

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Critical] damon_nr_accesses_mvsum() causes a divide-by-zero kernel panic if aggr_interval is configured to 0.
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commit 6a4a453ed1f9a5204defaea4d03c5096a0240cc8
Author: SeongJae Park <sj@kernel.org>

mm/damon/core: remove damon_warn_fix_nr_accesses_corruption()

This commit removes the damon_warn_fix_nr_accesses_corruption function
and its call site in kdamond_reset_aggregated. This removes the
corruption detection and fix logic for nr_accesses_bp since it is no
longer used.

> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index a7b45b3cad508..35a4f22a35f7e 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c

[ ... ]

> @@ -2046,7 +2033,6 @@ static void kdamond_reset_aggregated(struct damon_ctx *c)
>  			trace_damon_aggregated(ti, r, damon_nr_regions(t));
>  			trace_damon_region_aggregated(ti, r,
>  					damon_nr_regions(t), nr_probes);
> -			damon_warn_fix_nr_accesses_corruption(r);
>  			r->last_nr_accesses = r->nr_accesses;
>  			r->nr_accesses = 0;
>  			for (i = 0; i < DAMON_MAX_PROBES; i++)

[Severity: Critical]
This isn't a bug introduced by this patch, but by a previous patch in the
series (mm/damon: introduce damon_nr_accesses_mvsum). Does the
damon_nr_accesses_mvsum() function cause a divide-by-zero if aggr_interval is
configured to 0?

In damon_nr_accesses_mvsum(), window_len is calculated as:

    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);

If a user configures aggr_interval to 0 (and sample_interval to 0, which
passes the sample_interval > aggr_interval validation in damon_set_attrs()),
window_len evaluates to 0.

Because mult_frac() directly performs a division by this parameter, wouldn't
this result in a divide-by-zero exception and a kernel panic?

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Sashiko AI review · https://sashiko.dev/#/patchset/20260619193415.73833-1-sj@kernel.org?part=6