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

From: Wei Yang <richard.weiyang@gmail.com>
To: "Huang, Ying" <ying.huang@intel.com>
Cc: Wei Yang <richard.weiyang@gmail.com>,
	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()
Date: Tue, 28 Apr 2020 21:22:30 +0000	[thread overview]
Message-ID: <20200428212230.3aobygpy62bto4gz@master> (raw)
In-Reply-To: <87d07tycfu.fsf@yhuang-dev.intel.com>

On Mon, Apr 27, 2020 at 08:55:33AM +0800, Huang, Ying wrote:
>Wei Yang <richard.weiyang@gmail.com> writes:
>
>> On Sun, Apr 26, 2020 at 09:07:11AM +0800, Huang, Ying wrote:
>>>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.
>>
>> 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.
>

Yes, I am trying to think about it in parallel mode.

Here are the cases, it might happen in parallel when task B change highest_bit
to be less than scan_base.

(1)
                                                     offset
                                                       v
          +-------------------+------------------+
	  ^                   ^                  ^
          lowest_bit       highest_bit    scan_base

(2)
                                       offset
                                         v
          +-------------------+------------------+
	  ^                   ^                  ^
          lowest_bit       highest_bit    scan_base

(3)
                       offset
                         v
          +-------------------+------------------+
	  ^                   ^                  ^
          lowest_bit       highest_bit    scan_base

Case (1), (offset > highest) && (offset > scan_base),  offset would be set to
lowest_bit. This  looks good.

Case (2), (offset > highest) && (offset < scan_base),  since offset is less
than scan_base, it wouldn't be set to lowest. Instead it will continue to
scan_base.

Case (3), almost the same as Case (2).

In Case (2) and (3), one thing interesting is the loop won't stop at
highest_bit, while the behavior is the same as original code.

Maybe your concern is this one? I still not figure out your point about the
infinite loop. Hope you would share some light on it.

>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

-- 
Wei Yang
Help you, Help me