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

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Message-ID: <2926d7d9-b44e-40c0-b05d-8c42e99c511d@redhat.com>
Date: Thu, 21 Aug 2025 22:29:41 +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>,
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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 22:24, 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.

> 
> And again - not the case *here*, but just a general "let's keep to one
> pattern", and the shift pattern is simply the better choice.

It's a wild mixture, but I can keep doing what we already do in these cases.

-- 
Cheers

David / dhildenb