Re: [PATCH v20 4/8] rust: page: convert to `Ownable`'

[Andreas Hindborg <a.hindborg@kernel.org>]

Alice Ryhl <aliceryhl@google.com> writes:

> On Sun, Sep 6, 2026 at 3:02 PM Gary Guo <gary@garyguo.net> wrote:
>>
>> On Tue Aug 25, 2026 at 2:20 PM BST, Alice Ryhl wrote:
>> > On Mon, Aug 24, 2026 at 01:17:56PM +0200, Andreas Hindborg wrote:
>> >> +        // SAFETY: We just successfully allocated a page, so we now have ownership of the newly
>> >> +        // allocated page. We transfer that ownership to the new `Owned<Page>` object.
>> >> +        // Since `Page` is transparent, we can cast the pointer directly.
>> >> +        Ok(unsafe { Owned::from_raw(page.cast()) })
>> >
>> > This doesn't satisfy the safety requirements of Owned::from_raw()
>> > because the page may be used with vm_insert_page(), which increments its
>> > refcount and causes it to be shared the vma system, and this occurs
>> > before Page::release() is called.
>>
>> I suppose the existing vm_insert_page() abstraction we have is already
>> problematic, because it uses `&Page`?
>>
>> Maybe we want to change the API to use `ARef<Page>` so it already has to be
>> shared? Conceptually it takes a reference count from a `&Page`, which isn't
>> possible because `Page` is not `AlwaysRefCounted`, so it needs a `&ARef<Page>`
>> to be able to do that op.
>
> Honestly, the problem is the safety requirements of Owned::from_raw().
> Pages have a "special" main reference, and free_page() does more than
> put_page(). It even does something when the refcount does not hit
> zero.
>
> The correct behavior for Page is to allow the user to hold one
> Owned<Page> whose drop calls free_page(), *plus* any number of
> ARef<Page> references that invoke put_page() on drop. This way, the
> owned page controls the special drop codepath.

This does not mesh well with the model of `Owned` behaving like
`UniqueArc` to `ARef` behaving like an `Arc`.

Please help me understand; with page having a main ref and an auxiliary
refcount, if we model that with a single `Owned<Page>` and a number of
`ARef<Page>`, what would happen in the case where the main ref (`Owned`)
is dropped first? Is this legal?

My intuition here would be to follow Garry's suggestion and have
`vm_insert_page` take an `ARef<Page>`. Can you elaborate why this is not
an option?


Best regards,
Andreas Hindborg