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From:   "Huang\, Ying" <ying.huang@intel.com>
To:     Wei Yang <richard.weiyang@gmail.com>
Cc:     <akpm@linux-foundation.org>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>, Hugh Dickins <hughd@google.com>
Subject: Re: [PATCH v2] mm/swapfile.c: simplify the scan loop in scan_swap_map_slots()
References: <20200422214111.19370-1-richard.weiyang@gmail.com>
        <87d07y2181.fsf@yhuang-dev.intel.com>
        <20200423131507.2rgrk3okh42oo6gh@master>
        <87r1wdzlm5.fsf@yhuang-dev.intel.com>
        <20200425003012.uuqh547feq3kz4y5@master>
Date:   Sun, 26 Apr 2020 09:07:11 +0800
In-Reply-To: <20200425003012.uuqh547feq3kz4y5@master> (Wei Yang's message of
        "Sat, 25 Apr 2020 00:30:12 +0000")
Message-ID: <87tv17xdfk.fsf@yhuang-dev.intel.com>
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Wei Yang <richard.weiyang@gmail.com> writes:

> On Fri, Apr 24, 2020 at 10:02:58AM +0800, Huang, Ying wrote:
>>Wei Yang <richard.weiyang@gmail.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.

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