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

[Alice Ryhl <aliceryhl@google.com>]

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.

Alice