Re: [PATCH] mm, sparse: remove check with __highest_present_section_nr in for_each_present_section_nr()

[Wei Yang <richard.weiyang@gmail.com>]

On Tue, Dec 11, 2018 at 10:44:41AM +0100, Michal Hocko wrote:
>On Tue 11-12-18 11:51:28, Wei Yang wrote:
>> A valid present section number is in [0, __highest_present_section_nr].
>> And the return value of next_present_section_nr() meets this
>> requirement. This means it is not necessary to check it with
>> __highest_present_section_nr again in for_each_present_section_nr().
>> 
>> Since we pass an unsigned long *section_nr* to
>> for_each_present_section_nr(), we need to cast it to int before
>> comparing.
>
>Why do we want this patch? Is it an improvement? If yes, it is
>performance visible change or does it make the code easier to maintain?
>

Michal

I know you concern, maintainance is a very critical part of review.

>To me at least the later seems dubious to be honest because it adds a
>non-obvious dependency of the terminal condition to the
>next_present_section_nr implementation and that might turn out error
>prone.
>

While I think the original code is not that clear about the syntax.

When we look at the next_present_section_nr(section_nr), the return
value falls into two categories:

  -1   : no more present section after section_nr
  other: the next present section number after section_nr

Based on this syntax, the iteration could be simpler to terminate
when the return value is less than 0. This is what the patch tries to
do.

Maybe I could do more to help the maintainance:

  * add some comment about the return value of next_present_section_nr
  * terminate the loop when section_nr == -1

Hope this would help a little.

-- 
Wei Yang
Help you, Help me