Re: [RFC PATCH 06/10] mm/damon/core: add damon_merge_regions_of() debug_sanity check

[SeongJae Park <sj@kernel.org>]

On Sat, 21 Feb 2026 11:36:23 -0800 SeongJae Park <sj@kernel.org> wrote:

> damon_merge_regions_of() should be called only after aggregation is
> finished and therefore each region's nr_accesses and nr_accesses_bp
> match.  There were bugs that broke the assumption, during development of
> online DAMON parameter updates and monitoring results handling changes.
> Add a sanity check for that under CONFIG_DAMON_DEBUG_SANITY.
> 
> Signed-off-by: SeongJae Park <sj@kernel.org>
> ---
>  mm/damon/core.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index 291a847a8c9bb..94be598cafef1 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -2545,6 +2545,22 @@ static void damon_merge_two_regions(struct damon_target *t,
>  	damon_destroy_region(r, t);
>  }
>  
> +#ifdef CONFIG_DAMON_DEBUG_SANITY
> +static void damon_verify_merge_regions_of(struct damon_region *r)
> +{
> +	if (r->nr_accesses == r->nr_accesses_bp / 10000)
> +		return;
> +	pr_err("nr_accesses (%u) != nr_accesses_bp (%u)\n",
> +			r->nr_accesses, r->nr_accesses_bp);
> +	WARN_ONCE();
> +}
> +#else
> +static void damon_verify_merge_regions_of(struct damon_region *r)
> +{
> +}
> +#endif
> +
> +
>  /*
>   * Merge adjacent regions having similar access frequencies
>   *
> @@ -2565,6 +2581,8 @@ static void damon_merge_regions_of(struct damon_target *t, unsigned int thres,
>  		else
>  			r->age++;
>  
> +		damon_verify_merge_regions_of(r);
> +

This verification doesn't need to be here, after the above 'age' updates.  I'll
move this to the beginning of this scope in the next spin.

>  		if (prev && prev->ar.end == r->ar.start &&
>  		    abs(prev->nr_accesses - r->nr_accesses) <= thres &&
>  		    damon_sz_region(prev) + damon_sz_region(r) <= sz_limit)
> -- 
> 2.47.3


Thanks,
SJ