linux-kernel - Re: [PATCH v2] mm/swapfile.c: simplify the scan loop in scan_swap_map

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Message-ID: <20200426211958.m7aheswirqaj2nte@master>
Date:   Sun, 26 Apr 2020 21:19:58 +0000
From:   Wei Yang <richard.weiyang@...il.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Wei Yang <richard.weiyang@...il.com>, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH v2] mm/swapfile.c: simplify the scan loop in
 scan_swap_map_slots()

On Sun, Apr 26, 2020 at 09:07:11AM +0800, Huang, Ying wrote:
>Wei Yang <richard.weiyang@...il.com> writes:
>
>> On Fri, Apr 24, 2020 at 10:02:58AM +0800, Huang, Ying wrote:
>>>Wei Yang <richard.weiyang@...il.com> 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.

When (offset > scan_base), (offset >  si->highest_bit) means offset will be
set to si->lowest_bit.

When (offset < scan_base), next_offset() would always increase offset till
offset is scan_base.

Sorry, I didn't catch your case. Would you minding giving more detail?

>
>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

-- 
Wei Yang
Help you, Help me