Re: [PATCH RFC 31/35] crypto: remove nth_page() usage within SG entry

Linus Torvalds
2025-08-21T23:17:03Z

On Thu, 21 Aug 2025 at 16:08, David Hildenbrand <david@redhat.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