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

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Date:   Mon, 27 Apr 2020 08:55:33 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Wei Yang <richard.weiyang@...il.com>
Cc:     <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()

Wei Yang <richard.weiyang@...il.com> writes:

> 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?

Don't think in single thread model.  There's no lock to prevent other
tasks to change si->highest_bit simultaneously.  For example, task B may
change si->highest_bit to be less than scan_base in task A.

Best Regards,
Huang, Ying

>>
>>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