Re: [PATCH v2 2/6] rust: page: define PageOwner trait

["Alexandre Courbot" <acourbot@nvidia.com>]

On Thu Aug 7, 2025 at 5:50 AM JST, Danilo Krummrich wrote:
> Introduce the PageOwner trait, which can be implemented by any entity
> that potentially owns one or multiple pages and allow users to borrow
> them.
>
> For instance, this is useful to access and borrow the backing pages of
> allocation primitives, such as Box and Vec, backing a scatterlist.
>
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
> ---
>  rust/kernel/page.rs | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
>
> diff --git a/rust/kernel/page.rs b/rust/kernel/page.rs
> index 631718a6ad7d..93ce4956f782 100644
> --- a/rust/kernel/page.rs
> +++ b/rust/kernel/page.rs
> @@ -103,6 +103,18 @@ fn deref(&self) -> &Self::Target {
>      }
>  }
>  
> +/// Represents a potential owner of one or multiple [`Page`]s.
> +///
> +/// This trait may be implemented by types that potentially hold ownership of memory pages. It

Why "potentially"? I'd expect types implement this to be certain about
their ownership of the memory they share. :)

> +/// allows users to iterate over those pages and borrow them as [`BorrowedPage`].
> +pub trait PageOwner {
> +    /// Returns an [`Iterator`] of [`BorrowedPage`] items over all pages owned by `self`.
> +    fn page_iter<'a>(&'a mut self) -> impl Iterator<Item = BorrowedPage<'a>>;
> +
> +    /// Returns the number of pages currently owned by `self`.
> +    fn page_count(&self) -> usize;

Should we add a mention that `page_iter` is guaranteed to yield
`page_count` pages? If this is part of the contract, should the trait be
made `unsafe` as this guarantee cannot be held by a safe trait?

If so, I am wondering whether we should also take the next step and make
`page_iter` return an `ExactSizeIterator` while requiring that its `len`
method is accurate. This would allow us to remove `page_count`
altogether.