linux-kernel - Re: [PATCH v11 1/4] rust: types: Add Ownable/Owned types

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Message-Id: <DC732XTLKE1U.244I3Q2DR8JNK@kernel.org>
Date: Wed, 20 Aug 2025 09:41:04 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Oliver Mangold" <oliver.mangold@...me>
Cc: "Andreas Hindborg" <a.hindborg@...nel.org>, "Miguel Ojeda"
 <ojeda@...nel.org>, "Alex Gaynor" <alex.gaynor@...il.com>, "Boqun Feng"
 <boqun.feng@...il.com>, "Gary Guo" <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>, "Alice Ryhl"
 <aliceryhl@...gle.com>, "Trevor Gross" <tmgross@...ch.edu>, "Asahi Lina"
 <lina+kernel@...hilina.net>, <rust-for-linux@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v11 1/4] rust: types: Add Ownable/Owned types

On Wed Aug 20, 2025 at 8:02 AM CEST, Oliver Mangold wrote:
> On 250819 1913, Benno Lossin wrote:
>> On Tue Aug 19, 2025 at 10:53 AM CEST, Andreas Hindborg wrote:
>> > "Benno Lossin" <lossin@...nel.org> writes:
>> >> On Tue Aug 19, 2025 at 8:04 AM CEST, Oliver Mangold wrote:
>> >>> On 250819 0027, Benno Lossin wrote:
>> >>>> On Mon Aug 18, 2025 at 3:04 PM CEST, Oliver Mangold wrote:
>> >>>> > On 250818 1446, Andreas Hindborg wrote:
>> >>>> >> "Oliver Mangold" <oliver.mangold@...me> writes:
>> >>>> >> > +impl<T: OwnableMut> DerefMut for Owned<T> {
>> >>>> >> > +    fn deref_mut(&mut self) -> &mut Self::Target {
>> >>>> >> > +        // SAFETY: The type invariants guarantee that the object is valid, and that we can safely
>> >>>> >> > +        // return a mutable reference to it.
>> >>>> >> > +        unsafe { self.ptr.as_mut() }
>> >>>> >> > +    }
>> >>>> >> > +}
>> >>>> >>
>> >>>> >> I think someone mentioned this before, but handing out mutable
>> >>>> >> references can be a problem if `T: !Unpin`. For instance, we don't want
>> >>>> >> to hand out `&mut Page` in case of `Owned<Page>`.
>> >>>> >>
>> >>>> >
>> >>>> > That was the reason, why `OwnableMut` was introduced in the first place.
>> >>>> > It's clear, I guess, that as-is it cannot be implemented on many classes.
>> >>>>
>> >>>> Yeah the safety requirements ensure that you can't implement it on
>> >>>> `!Unpin` types.
>> >>>>
>> >>>> But I'm not sure it's useful then? As you said there aren't many types
>> >>>> that will implement the type then, so how about we change the meaning
>> >>>> and make it give out a pinned mutable reference instead?
>> >>>
>> >>> Making `deref_mut()` give out a pinned type won't work. The return types of
>> >>> deref() are required to match.
>> >>
>> >> I meant the changes that Andreas suggested.
>> >
>> > Not sure what you are asking, but I need to assert exclusive access to
>> > an `Page`. I could either get this by taking a `&mut Owned<Page>` or a
>> > `Pin<&mut Page>`. I think the latter is more agnostic.
>> 
>> The former isn't really correct? It's like having a `&mut Box<Page>`
>> which is weird. I was saying we can have a `DerefMut` impl gated on `T:
>> Unpin` and a `fn get_pin_mut(&mut self) -> Pin<&mut T>`.
>
> Yes. I think `Page` is the wrong example, as it already has owned semantics
> and does its own cleanup. Wrapping it in an Owned would be redundant.

After we have these owned patches, we are going to change `Page` to
`Opaque<bindings::page>`.

---
Cheers,
Benno