Re: [PATCH] mm/damon: make region calculations more precise

[Enze Li <lienze@kylinos.cn>]

Hi SeongJae,

On Wed, May 21 2025 at 10:18:09 AM -0700, SeongJae Park wrote:

> Hi Enze,
>
> On Wed, 21 May 2025 15:07:47 +0800 Enze Li <lienze@kylinos.cn> wrote:
>
>> The damon_sz_region() function misses counting one element when
>> calculating region size, which leads to inaccurate results.  This patch
>> corrects the size calculation by properly accounting for all elements.
>
> Thank you for this patch, but I don't think the current calculation is wrong.
> Please refer to the below comment.
>
>> 
>> Signed-off-by: Enze Li <lienze@kylinos.cn>
>> ---
>>  include/linux/damon.h | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/include/linux/damon.h b/include/linux/damon.h
>> index 47e36e6ea203..70473863f7fe 100644
>> --- a/include/linux/damon.h
>> +++ b/include/linux/damon.h
>> @@ -808,7 +808,7 @@ static inline struct damon_region *damon_first_region(struct damon_target *t)
>>  
>>  static inline unsigned long damon_sz_region(struct damon_region *r)
>>  {
>> -	return r->ar.end - r->ar.start;
>> +	return r->ar.end - r->ar.start + 1;
>>  }
>
> 'ar' here is 'struct damon_addr_range' which is for a half-open range.  Refer
> to the comment on 'struct damon_addr_range' definition on include/linux/damon.h
> for detail.  So I don't think the current calculation is wrong.

Thanks for pointing out this - I overlooked it :(

>
> If you think this function also deserves a short comment for clarifying this,
> your patch for that wil be welcomed :)

Since this is already documented in the definition, repeating it here
would be redundant.  If someone submits a similar patch for this in the
future, we can consider adding comments then.

Thanks,
Enze

[...]

>
> Please let me know if I'm missing something.
>
>
> Thanks,
> SJ
>
> [...]