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

[Wei Yang]

On Thu, Apr 23, 2020 at 01:57:34PM +0800, Huang, Ying wrote:
>Wei Yang <richard.weiyang@xxxxxxxxx> writes:
>
>> After commit c60aa176c6de8 ("swapfile: swap allocation cycle if
>> nonrot"), swap allocation is cyclic. Current approach is done with two
>> separate loop on the upper and lower half. This looks a little
>> redundant.
>
>I can understand that the redundant code doesn't smell good.  But I
>don't think the new code is easier to be understood than the original
>one.
>
>> From another point of view, the loop iterates [lowest_bit, highest_bit]
>> range starting with (offset + 1) but except scan_base. So we can
>> simplify the loop with condition (next_offset() != scan_base) by
>> introducing next_offset() which makes sure offset fit in that range
>> with correct order.
>>
>> Signed-off-by: Wei Yang <richard.weiyang@xxxxxxxxx>
>> CC: Hugh Dickins <hughd@xxxxxxxxxx>
>> CC: "Huang, Ying" <ying.huang@xxxxxxxxx>
>>
>> ---
>> v2:
>>   * return scan_base if the lower part is eaten
>>   * only start over when iterating on the upper part
>> ---
>>  mm/swapfile.c | 31 ++++++++++++++-----------------
>>  1 file changed, 14 insertions(+), 17 deletions(-)
>>
>> diff --git a/mm/swapfile.c b/mm/swapfile.c
>> index f903e5a165d5..0005a4a1c1b4 100644
>> --- a/mm/swapfile.c
>> +++ b/mm/swapfile.c
>> @@ -729,6 +729,19 @@ static void swap_range_free(struct swap_info_struct *si, unsigned long offset,
>>  	}
>>  }
>>  
>> +static unsigned long next_offset(struct swap_info_struct *si,
>> +				unsigned long *offset, unsigned long scan_base)
>> +{
>> +	/* only start over when iterating on the upper part */
>> +	if (++(*offset) > si->highest_bit && *offset > scan_base) {
>> +		*offset = si->lowest_bit;
>> +		/* someone has eaten the lower part */
>> +		if (si->lowest_bit >= scan_base)
>> +			return scan_base;
>> +	}
>
>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?

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

-- 
Wei Yang
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