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

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Message-ID: <CAHk-=wjGzyGPgqKDNXM6_2Puf7OJ+DQAXMg5NgtSASN8De1roQ@mail.gmail.com>
Date: Thu, 21 Aug 2025 16:24:23 -0400
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, Herbert Xu <herbert@...dor.apana.org.au>, 
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	x86@...nel.org, Zi Yan <ziy@...dia.com>
Subject: Re: [PATCH RFC 31/35] crypto: remove nth_page() usage within SG entry

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.

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.

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.

             Linus