linux-kernel - Re: [PATCH 1/6] mm/damon/core: reset age if nr_accesses changes between non-zero and zero

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Message-ID: <20250915145158.1425390-1-joshua.hahnjy@gmail.com>
Date: Mon, 15 Sep 2025 07:51:57 -0700
From: Joshua Hahn <joshua.hahnjy@...il.com>
To: SeongJae Park <sj@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	damon@...ts.linux.dev,
	kernel-team@...a.com,
	linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH 1/6] mm/damon/core: reset age if nr_accesses changes between non-zero and zero

On Sun, 14 Sep 2025 18:58:02 -0700 SeongJae Park <sj@...nel.org> wrote:

> DAMON resets the age of a region if its nr_accesses value has
> significantly changed. Specifically, the threshold is calculated as 20%
> of largest nr_accesses of the current snapshot.  This means that regions
> changing the nr_accesses from zero to small non-zero value or from a
> small non-zero value to zero will keep the age.  Since many users treat
> zero nr_accesses regions special, this can be confusing.  Kernel code
> including DAMOS' regions priority calculation and DAMON_STAT's idle time
> calculation also treat zero nr_accesses regions special.  Make it
> unconfusing by resetting the age when the nr_accesses changes between
> zero and a non-zero value.

Hi SJ,

Thank you for the patch, I think the goal of the patch makes sesne to me.
I have a small nit / idea which I think makes the code a bit clearer, at least
for me. It seems that we basically want to XOR the two values's zero-ness, so
maybe something like 

(!!r->nr_accesses) ^ (!!r->last_nr_access) or
(r->nr_accesses == 0) ^ (r->last_nr_access == 0)

Can achieve the goal? I know bitwise operations are sometimes harder to
understand, so I am just throwing the idea out there : -) 


Anyways, the rest of it looks good to me, please feel free to add my review!

Reviewed-by: Joshua Hahn <joshua.hahnjy@...il.com>

> Signed-off-by: SeongJae Park <sj@...nel.org>
> ---
>  mm/damon/core.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index be5942435d78..996647caca02 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -2261,6 +2261,9 @@ static void damon_merge_regions_of(struct damon_target *t, unsigned int thres,
>  	damon_for_each_region_safe(r, next, t) {
>  		if (abs(r->nr_accesses - r->last_nr_accesses) > thres)
>  			r->age = 0;
> +		else if ((!r->nr_accesses && r->last_nr_accesses) ||
> +				(r->nr_accesses && !r->last_nr_accesses))
> +			r->age = 0;
>  		else
>  			r->age++;
>  
> -- 
> 2.39.5

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