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

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Message-ID: <87r1wdzlm5.fsf@yhuang-dev.intel.com>
Date:   Fri, 24 Apr 2020 10:02:58 +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 Thu, Apr 23, 2020 at 01:57:34PM +0800, Huang, Ying wrote:
>>Wei Yang <richard.weiyang@...il.com> 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@...il.com>
>>> CC: Hugh Dickins <hughd@...gle.com>
>>> CC: "Huang, Ying" <ying.huang@...el.com>
>>>
>>> ---
>>> 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?

In the original code, the loop can still stop.

Best Regards,
Huang, Ying

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