Re: [PATCH 6.6.y] mm/damon/core: skip aging from repeated aggressive merging

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] Infinite loop in `kdamond_merge_regions` when `max_thres` > 2^30 due to `unsigned int` overflow.
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commit 2beb7e632c69db2ccfb8ea5cced50b76ca2b931f
Author: SJ Park <sj@kernel.org>

mm/damon/core: skip aging from repeated aggressive merging

This commit modifies the DAMON core to skip aging when repeatedly merging
regions aggressively. It prevents corrupting monitoring results when the user
temporarily lowers the maximum number of regions limit.

Link: https://lore.kernel.org/20260712165432.87609-1-sj@kernel.org
Link: https://lore.kernel.org/20260621203548.10718-1-sj@kernel.org [1]
Link: https://lore.kernel.org/20260709145425.96247-1-sj@kernel.org [2]

> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index 734ab958ec576..0713fda22682c 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c

[ ... ]

> @@ -1234,15 +1236,18 @@ static void kdamond_merge_regions(struct damon_ctx *c, unsigned int threshold,
>  	struct damon_target *t;
>  	unsigned int nr_regions;
>  	unsigned int max_thres;
> +	bool count_age = true;
>  
>  	max_thres = c->attrs.aggr_interval /
>  		(c->attrs.sample_interval ?  c->attrs.sample_interval : 1);
>  	do {
>  		nr_regions = 0;
>  		damon_for_each_target(t, c) {
> -			damon_merge_regions_of(t, threshold, sz_limit);
> +			damon_merge_regions_of(t, threshold, sz_limit,
> +					count_age);
>  			nr_regions += damon_nr_regions(t);
>  		}
> +		count_age = false;
>  		threshold = max(1, threshold * 2);
>  	} while (nr_regions > c->attrs.max_nr_regions &&
>  			threshold / 2 < max_thres);

[Severity: High]
This isn't a bug introduced by this patch, but could this lead to an
infinite loop if max_thres is sufficiently large?

If a user configures DAMON parameters such that the aggregation interval
divided by the sample interval results in a max_thres greater than 2^30,
threshold (an unsigned int) will strictly double each iteration. Once
threshold reaches 2^31, threshold * 2 wraps around to 0, which max() sets
to 1.

At that point, threshold / 2 becomes 0, and the loop condition
threshold / 2 < max_thres will permanently evaluate to true, causing an
unrecoverable hang of the kdamond kernel thread if the region limit is
not met.

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Sashiko AI review · https://sashiko.dev/#/patchset/20260909044707.221421-1-sj@kernel.org?part=1