Re: [PATCH 7.2.y] mm/damon/core: skip aging from repeated aggressive merging

[Greg KH <greg@kroah.com>]

On Tue, Sep 08, 2026 at 09:34:19PM -0700, SJ Park wrote:
> The number of DAMON regions could temporarily exceed the user-defined
> maximum number of regions limit for corner cases.  For example, users
> could lower the limit via runtime parameters update.  For such a case,
> kdamond_merge_regions() repeats merging regions in the case doubling the
> merge threshold.  The repeated merge operation could update the age of
> regions multiple times.  This corrupts the monitoring results.  Fix the
> issue by asking the merge operation to skip aging for the corner case.
> 
> The user impact is degradation of the monitoring quality.  The impact
> should be mild, since the degradation is only temporal, and it is not
> common to happen in realistic setups.
> 
> The issue was discovered [1,2] by Sashiko.
> 
> 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]
> Fixes: 310d6c15e910 ("mm/damon/core: merge regions aggressively when max_nr_regions is unmet")
> Signed-off-by: SJ Park <sj@kernel.org>
> Cc: <stable@vger.kernel.org> # 6.10
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> (cherry picked from commit 0250dbe08c730d003ef9f484da56ae09a1ea0c4c)
> Signed-off-by: SJ Park <sj@kernel.org>
> ---
>  mm/damon/core.c             | 21 +++++++++++++--------
>  mm/damon/tests/core-kunit.h |  2 +-
>  2 files changed, 14 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index cff932b3317d0..024a3a185bfe8 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -3115,18 +3115,20 @@ static void damon_verify_merge_regions_of(struct damon_region *r)
>   * sz_limit	size upper limit of each region
>   */
>  static void damon_merge_regions_of(struct damon_target *t, unsigned int thres,
> -				   unsigned long sz_limit)
> +		unsigned long sz_limit, bool count_age)
>  {
>  	struct damon_region *r, *prev = NULL, *next;
>  
>  	damon_for_each_region_safe(r, next, t) {
>  		damon_verify_merge_regions_of(r);
> -		if (abs(r->nr_accesses - r->last_nr_accesses) > thres)
> -			r->age = 0;
> -		else if ((r->nr_accesses == 0) != (r->last_nr_accesses == 0))
> -			r->age = 0;
> -		else
> -			r->age++;
> +		if (count_age) {
> +			if (abs(r->nr_accesses - r->last_nr_accesses) > thres)
> +				r->age = 0;
> +			else if ((r->nr_accesses == 0) != (r->last_nr_accesses == 0))
> +				r->age = 0;
> +			else
> +				r->age++;
> +		}
>  
>  		if (prev && prev->ar.end == r->ar.start &&
>  		    abs(prev->nr_accesses - r->nr_accesses) <= thres &&
> @@ -3160,15 +3162,18 @@ 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);
> diff --git a/mm/damon/tests/core-kunit.h b/mm/damon/tests/core-kunit.h
> index fcf7c7fadb5fe..59bba0552d88c 100644
> --- a/mm/damon/tests/core-kunit.h
> +++ b/mm/damon/tests/core-kunit.h
> @@ -256,7 +256,7 @@ static void damon_test_merge_regions_of(struct kunit *test)
>  		damon_add_region(r, t);
>  	}
>  
> -	damon_merge_regions_of(t, 9, 9999);
> +	damon_merge_regions_of(t, 9, 9999, true);
>  	/* 0-112, 114-130, 130-156, 156-170, 170-230, 230-10170 */
>  	KUNIT_EXPECT_EQ(test, damon_nr_regions(t), 6u);
>  	for (i = 0; i < 6; i++) {
> -- 
> 2.47.3
> 
> 

Does not apply to the latest 7.2.y queue :(