linux-kernel - Re: [PATCH RFC 31/35] crypto: remove nth

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Message-ID: <0dc9936f-c977-4ff4-98f3-7941b2eba9d3@redhat.com>
Date: Thu, 21 Aug 2025 22:37:26 +0200
From: David Hildenbrand <david@...hat.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: linux-kernel@...r.kernel.org, Herbert Xu <herbert@...dor.apana.org.au>,
 "David S. Miller" <davem@...emloft.net>,
 Alexander Potapenko <glider@...gle.com>,
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 Zi Yan <ziy@...dia.com>
Subject: Re: [PATCH RFC 31/35] crypto: remove nth_page() usage within SG entry

On 21.08.25 22:29, David Hildenbrand wrote:
> On 21.08.25 22:24, Linus Torvalds wrote:
>> On Thu, 21 Aug 2025 at 16:08, David Hildenbrand <david@...hat.com> wrote:
>>>
>>> -       page = nth_page(page, offset >> PAGE_SHIFT);
>>> +       page += offset / PAGE_SIZE;
>>
>> Please keep the " >> PAGE_SHIFT" form.
> 
> No strong opinion.
> 
> I was primarily doing it to get rid of (in other cases) the parentheses.
> 
> Like in patch #29
> 
> -	/* Assumption: contiguous pages can be accessed as "page + i" */
> -	page = nth_page(sg_page(sg), (*offset >> PAGE_SHIFT));
> +	page = sg_page(sg) + *offset / PAGE_SIZE;
> 
>>
>> Is "offset" unsigned? Yes it is, But I had to look at the source code
>> to make sure, because it wasn't locally obvious from the patch. And
>> I'd rather we keep a pattern that is "safe", in that it doesn't
>> generate strange code if the value might be a 's64' (eg loff_t) on
>> 32-bit architectures.
>>
>> Because doing a 64-bit shift on x86-32 is like three cycles. Doing a
>> 64-bit signed division by a simple constant is something like ten
>> strange instructions even if the end result is only 32-bit.
> 
> I would have thought that the compiler is smart enough to optimize that?
> PAGE_SIZE is a constant.

It's late, I get your point: if the compiler can't optimize if it's a 
signed value ...

-- 
Cheers

David / dhildenb