Re: [PATCH v2] mm/swapfile.c: simplify the scan loop in scan_swap_map_slots()

[Huang\, Ying]

Wei Yang <richard.weiyang@xxxxxxxxx> writes:

> On Fri, Apr 24, 2020 at 10:02:58AM +0800, Huang, Ying wrote:
>>Wei Yang <richard.weiyang@xxxxxxxxx> writes:
>>
> [...]
>>>>
>>>>if "offset > si->highest_bit" is true and "offset < scan_base" is true,
>>>>scan_base need to be returned.
>>>>
>>>
>>> When this case would happen in the original code?
>>
>>In the original code, the loop can still stop.
>>
>
> Sorry, I don't get your point yet.
>
> In original code, there are two separate loops
>
>     while (++offset <= si->highest_bit) {
>     }
>
>     while (offset < scan_base) {
>     }
>
> And for your condition, (offset > highest_bit) && (offset < scan_base), which
> terminates the first loop and fits the second loop well.
>
> Not sure how this condition would stop the loop in original code?

Per my understanding, in your code, if some other task changes
si->highest_bit to be less than scan_base in parallel.  The loop may
cannot stop.

Best Regards,
Huang, Ying

>>Best Regards,
>>Huang, Ying
>>
>>>>Again, the new code doesn't make it easier to find this kind of issues.
>>>>
>>>>Best Regards,
>>>>Huang, Ying