Re: [PATCH 04/18] mm/damon/core: handle unreset nr_accesses in damon_nr_accesses_mvsum()

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] damon_nr_accesses_mvsum() incorrectly returns stale data (r->last_nr_accesses) when damon_set_attrs() is called exactly at the end of an aggregation window.
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commit ebe6cc65ba2de465cab83d7ffe913bb9f8304f66
Author: SJ Park <sj@kernel.org>

mm/damon/core: handle unreset nr_accesses in damon_nr_accesses_mvsum()

This commit adds logic to damon_nr_accesses_mvsum() to return
last_nr_accesses when the remaining window is full. It aims to avoid
unnecessarily counting unreset nr_accesses when damon_set_attrs() is
called at the end of an aggregation window.

> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index 94dd425ee4353..df79f4d10bdc3 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -266,6 +266,9 @@ unsigned int damon_nr_accesses_mvsum(struct damon_region *r,
>  			ctx->passed_sample_intervals;
>  	left_window_bp = mult_frac(left_window, 10000, window_len);
>  
> +	if (left_window_bp == 10000)
> +		return r->last_nr_accesses;
> +

[Severity: Medium]
Will this early return cause the function to return stale data from the 
previous window instead of freshly accumulated data?

If damon_set_attrs() is called at the end of an aggregation window,
damon_update_monitoring_result() processes this with aggregating = false. 
This intentionally preserves the fully accumulated r->nr_accesses for the
newly completed window. 

After damon_set_attrs() pushes ctx->next_aggregation_sis forward, a
subsequent call to damon_nr_accesses_mvsum() will calculate left_window_bp
as 10000. 

Because of this early return, won't it discard the freshly accumulated 
r->nr_accesses and incorrectly return the older r->last_nr_accesses instead?

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

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Sashiko AI review · https://sashiko.dev/#/patchset/20260630040812.149729-1-sj@kernel.org?part=4