linux-kernel - Re: [RFC v11 08/14] mm: page_frag: some minor refactoring before adding new API

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Message-ID: <86368be7-3344-4fc0-bd03-d79e84f48c35@huawei.com>
Date: Thu, 1 Aug 2024 20:53:59 +0800
From: Yunsheng Lin <linyunsheng@...wei.com>
To: Alexander H Duyck <alexander.duyck@...il.com>
CC: <davem@...emloft.net>, <kuba@...nel.org>, <pabeni@...hat.com>,
	<netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>, Andrew Morton
	<akpm@...ux-foundation.org>, <linux-mm@...ck.org>
Subject: Re: [RFC v11 08/14] mm: page_frag: some minor refactoring before
 adding new API

On 2024/8/1 1:02, Alexander H Duyck wrote:
> On Wed, 2024-07-31 at 20:35 +0800, Yunsheng Lin wrote:
>> On 2024/7/30 23:12, Alexander H Duyck wrote:
>>
>> ...
>>
>>>>         }
>>>>
>>>>         nc->pagecnt_bias--;
>>>>         nc->remaining = remaining - fragsz;
>>>>
>>>>         return encoded_page_address(encoded_va) +
>>>>                 (page_frag_cache_page_size(encoded_va) - remaining);
>>>
>>> Parenthesis here shouldn't be needed, addition and subtractions
>>> operations can happen in any order with the result coming out the same.
>>
>> I am playing safe to avoid overflow here, as I am not sure if the allocator
>> will give us the last page. For example, '0xfffffffffffff000 + 0x1000' will
>> have a overflow.
> 
> So what if it does though? When you subtract remaining it will
> underflow and go back to the correct value shouldn't it?

I guess that it is true that underflow will bring back the correct value.
But I am not sure what does it hurt to have a parenthesis here, doesn't having
a parenthesis make it more obvious that 'size - remaining' indicate the offset
of allocated fragment and not having to scratch my head and wondering if there
is overflow/underflow problem? Or is there any performance trick behind the above
comment?