[PATCH 1/2] mm/damon/core: Use only installed probe in damon_merge_two_regions()

[Liew Rui Yan <aethernet65535@gmail.com>]

Hi Suhaas,

Thank you for your patches series!

On Wed, 07 Oct 2026 21:18:45 +0530 Suhaas Joshi <suhaas@s-joshi.in> wrote:

> While merging 2 regions, we iterate over the entire probe_hits[] array,
> whose size is determined by the DAMON_MAX_PROBES macro. It is possible,
> however, that we have fewer probes installed than DAMON_MAX_PROBES. In such

I find this a bit hard to read.  Could we change it to this?

    "However, it is possible that we have fewer probes installed than
    DAMON_MAX_PROBES."

> cases, we end up making redundant iterations. Therefore, to remedy this,
> iterate over the list of installed probes instead of iterating over the
> entire array. For doing this, start accepting a struct damon_ctx in
> damon_merge_two_regions(), and update calling functions to pass this
> argument.
> 
> Update the damon_test_merge_two() test to use this new signature for
> damon_merge_two_regions() as well.
> 
> Signed-off-by: Suhaas Joshi <suhaas@s-joshi.in>
> ---
>  mm/damon/core.c             | 14 +++++++++-----
>  mm/damon/tests/core-kunit.h | 19 +++++++++++++++++--
>  2 files changed, 26 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index 733025b36745..a3af5ee685e1 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -3499,20 +3499,24 @@ static void damon_verify_merge_two_regions(
>  /*
>   * Merge two adjacent regions into one region
>   */
> -static void damon_merge_two_regions(struct damon_target *t,
> -		struct damon_region *l, struct damon_region *r)
> +static void damon_merge_two_regions(struct damon_ctx *ctx, struct damon_target *t,
> +				    struct damon_region *l, struct damon_region *r)
>  {
>  	unsigned long sz_l = damon_sz_region(l), sz_r = damon_sz_region(r);
>  	int i;
> +	struct damon_probe *p;
>  
>  	l->nr_accesses = (l->nr_accesses * sz_l + r->nr_accesses * sz_r) /
>  			(sz_l + sz_r);
>  	l->age = (l->age * sz_l + r->age * sz_r) / (sz_l + sz_r);
>  	l->ar.end = r->ar.end;
> -	/* todo: do this for only installed probes */
> -	for (i = 0; i < DAMON_MAX_PROBES; i++)
> +
> +	i = 0;
> +	damon_for_each_probe(p, ctx) {
>  		l->probe_hits[i] = (l->probe_hits[i] * sz_l + r->probe_hits[i]
>  				* sz_r) / (sz_l + sz_r);
> +		++i;
> +	}
>  	damon_verify_merge_two_regions(l, r);
>  	damon_destroy_region(r, t);
>  }
> @@ -3565,7 +3569,7 @@ static void damon_merge_regions_of(struct damon_target *t, unsigned int thres,
>  			goto set_prev_continue;
>  		if (damon_sz_region(prev) + damon_sz_region(r) > sz_limit)
>  			goto set_prev_continue;
> -		damon_merge_two_regions(t, prev, r);
> +		damon_merge_two_regions(ctx, t, prev, r);
>  		continue;
>  set_prev_continue:
>  		prev = r;
> diff --git a/mm/damon/tests/core-kunit.h b/mm/damon/tests/core-kunit.h
> index df84d9cc7d20..5ad89c1d9fc2 100644
> --- a/mm/damon/tests/core-kunit.h
> +++ b/mm/damon/tests/core-kunit.h
> @@ -182,13 +182,27 @@ static void damon_test_merge_two(struct kunit *test)
>  {
>  	struct damon_target *t;
>  	struct damon_region *r, *r2, *r3;
> +	struct damon_probe *p;
> +	struct damon_ctx *ctx;
>  	int i;
>  
> +	p = damon_new_probe();
> +	if (!p)
> +		kunit_skip(test, "probe alloc fail");
> +	ctx = damon_new_ctx();
> +	if (!ctx) {
> +		damon_destroy_probe(p);
> +		kunit_skip(test, "context alloc fail");
> +	}
> +	damon_add_probe(ctx, p);
>  	t = damon_new_target();
> -	if (!t)
> +	if (!t) {
> +		damon_destroy_ctx(ctx);
>  		kunit_skip(test, "target alloc fail");
> +	}
>  	r = damon_new_region(0, 100);
>  	if (!r) {
> +		damon_destroy_ctx(ctx);
>  		damon_free_target(t);
>  		kunit_skip(test, "region alloc fail");
>  	}
> @@ -206,7 +220,7 @@ static void damon_test_merge_two(struct kunit *test)
>  	r2->age = 21;
>  	damon_add_region(r2, t);
>  
> -	damon_merge_two_regions(t, r, r2);
> +	damon_merge_two_regions(ctx, t, r, r2);
>  	KUNIT_EXPECT_EQ(test, r->ar.start, 0ul);
>  	KUNIT_EXPECT_EQ(test, r->ar.end, 300ul);
>  	KUNIT_EXPECT_EQ(test, r->nr_accesses, 16u);
> @@ -220,6 +234,7 @@ static void damon_test_merge_two(struct kunit *test)
>  	}
>  	KUNIT_EXPECT_EQ(test, i, 1);
>  
> +	damon_destroy_ctx(ctx);
>  	damon_free_target(t);
>  }
>  
> -- 
> 2.55.0

Other than the commit message I just mentioned, and the issue reported
[1] by Sashiko, LGTM.

Reviewed-by: Liew Rui Yan <aethernet65535@gmail.com>

[1] https://lore.kernel.org/damon/sashiko-outbox-163177@kernel.org

Best regards,
Rui Yan