Re: [PATCH v4] rust: Return Option from page_align and ensure no usize overflow

[Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>]

Hi Brendan,

This looks much better now with the examples!

On Sat, Nov 29, 2025 at 1:54 AM Brendan Shephard <bshephar@bne-home.net> wrote:
>
> +/// Returns a page aligned [`usize`] in cases where the value can be aligned. Otherwise, returns `None`
> +/// if the aligned size will overflow a [`usize`].

[`None`]

> +/// # Examples

Newline before `# Examples` header.

> +/// Assuming a `PAGE_SIZE` of 4096 (0x1000):

Can we assume that? i.e. these tests run as KUnit tests and we support
architectures that allow for other sizes, so we may need to guard
these with a `cfg` or `if` (which may be hidden), or perhaps better,
you could compute the values based on `PAGE_SIZE`, e.g. something
like:

    assert_eq!(page_align(PAGE_SIZE + 1), Some(2 * PAGE_SIZE));

It would be nice if you can confirm the tests fail (and then work
again) running them in a kernel config with a different page size:

    https://docs.kernel.org/rust/testing.html#the-kunit-tests

> +/// // The check asserts that None is returned when a value is requested within one PAGE_SIZE of
> +/// // usize::MAX.

`None`
`PAGE_SIZE`
`usize::MAX`

> +/// let overflow_addr = usize::MAX - (PAGE_SIZE / 2);
> +/// assert_eq!(page_align(overflow_addr), None);

Is there a reason for that particular address? If not, perhaps just
using `usize::MAX` (the maximum value) would be simpler. Or perhaps
the edge case (the first value that returns `None`) would be better.
Or even better, both.

Thanks!

Cheers,
Miguel