[PATCH v21 9/9] rust: page: add `ExclusivePage` for race-free page access

[Matthew Wilcox]

On Thu, Sep 10, 2026 at 11:00:13AM +0200, Andreas Hindborg wrote:
> `ExclusivePage` wraps a regular page but adds an invariant that the
> page data area does not incur data races. This means `ExclusivePage`
> cannot be mapped to user space or shared with devices, and it
> becomes simpler to directly reference the contents of the page.
> 
> Since `Page` implements `AlwaysRefCounted`, handing out a `&Page`
> from an `ExclusivePage` would allow safe code to obtain an
> `ARef<Page>` to the page and break the aliasing invariant of
> `ExclusivePage`. Thus, do not implement `Deref<Target = Page>` for
> `ExclusivePage`.

Given this description, I wonder if this shouldn't be implemented by
alloc_frozen_pages() / free_frozen_pages().  We'ree certainly trying
to move the C parts of the kernel away from thinking that pages have a
refcount (folios have a refcount; otheer memdescs may or may not have
a refcount).  Do your intended users of this actually have a need for a
refcount?

> +++ b/rust/kernel/page.rs
> @@ -17,7 +17,11 @@
>          AlwaysRefCounted,
>          RefCounted, //
>      },
> -    types::Opaque,
> +    types::{
> +        Opaque,
> +        Ownable,
> +        Owned, //
> +    },

While I'm asking stupid questions, what's the significance of the //
after Owned?  I see it after RefCounted too.