Re: [PATCH 6.12.y 3/3] mm/damon/core: fix unconditionally skip last region

[Greg KH <greg@kroah.com>]

On Tue, Sep 29, 2026 at 10:34:01PM -0400, Sasha Levin wrote:
> From: Liew Rui Yan <aethernet65535@gmail.com>
> 
> [ Upstream commit b3723b596b548c837a766aae3553c14a7b15af2b ]
> 
> Once quota set, the charge_{target,addr}_from unconditionally skips and
> resets at the last region of the tracked target, so the last region can be
> skipped even when it has not been processed.
> 
> Example:
> 
>     1. Target has 2 regions: R1 (0-100 bytes) and R2 (100-200 bytes).
>     2. Quota is configured to process only 100 bytes per window.
>     3. Window 1: Processes R1 (0-100).  Quota is full.  charge_{target,
>        addr}_from is saved at (Target, 100).
>     4. Window 2: The loop reaches R2.  Because R2 is
>        damon_last_region(t), the old code unconditionally returns true,
>        skipping R2 entirely and resetting the charge_{target,addr}_from.
> 
>     Result: R2 is permanently skipped even though it has never been
>     processed.
> 
> However, it is important to note that this is a very minor issue.  This is
> because it is triggered only when the previous window saved/kept
> charge_{target,addr}_from, and in the next window, all regions except the
> last region were skipped by damos_skip_charged_region().
> 
> Fix this by only resetting the charge_{target,addr}_from when last region
> is reached, only skipping when it is applied or cannot split.
> 
> Link: https://lore.kernel.org/20260908134739.96919-1-sj@kernel.org
> Fixes: 50585192bc2e ("mm/damon/schemes: skip already charged targets and regions")
> Signed-off-by: Liew Rui Yan <aethernet65535@gmail.com>
> Reviewed-by: SJ Park <sj@kernel.org>
> Signed-off-by: SJ Park <sj@kernel.org>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> Cc: <stable@vger.kernel.org> # v5.16.x
> Signed-off-by: Sasha Levin <sashal@kernel.org>
> ---
>  mm/damon/core.c | 25 ++++++++++++++-----------
>  1 file changed, 14 insertions(+), 11 deletions(-)
> 
> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index 53d479da2007e..2a24e817e5360 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -1325,36 +1325,39 @@ static bool damos_skip_charged_region(struct damon_target *t,
>  {
>  	struct damos_quota *quota = &s->quota;
>  	unsigned long sz_to_skip;
> +	bool skip = false;
>  
>  	/* Skip previously charged regions */
>  	if (quota->charge_target_from) {
>  		if (t != quota->charge_target_from)
>  			return true;
> -		if (r == damon_last_region(t)) {
> -			quota->charge_target_from = NULL;
> -			quota->charge_addr_from = 0;
> -			return true;
> -		}
>  		if (quota->charge_addr_from &&
> -				r->ar.end <= quota->charge_addr_from)
> -			return true;
> +				r->ar.end <= quota->charge_addr_from) {
> +			skip = true;
> +			goto out;
> +		}
>  
>  		if (quota->charge_addr_from && r->ar.start <
>  				quota->charge_addr_from) {
>  			sz_to_skip = ALIGN_DOWN(quota->charge_addr_from -
>  					r->ar.start, min_region_sz);
>  			if (!sz_to_skip) {
> -				if (damon_sz_region(r) <= min_region_sz)
> -					return true;
> +				if (damon_sz_region(r) <= min_region_sz) {
> +					skip = true;
> +					goto out;
> +				}
>  				sz_to_skip = min_region_sz;
>  			}
>  			damon_split_region_at(t, r, sz_to_skip);
> -			return true;
> +			skip = true;
>  		}
> +	}
> +out:
> +	if (r == damon_last_region(t)) {
>  		quota->charge_target_from = NULL;
>  		quota->charge_addr_from = 0;
>  	}
> -	return false;
> +	return skip;
>  }
>  
>  static void damos_update_stat(struct damos *s,
> -- 
> 2.53.0
> 
> 

Did not apply :(