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

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Message-ID: <875xeicllr.fsf@t14s.mail-host-address-is-not-set>
Date: Wed, 20 Aug 2025 12:51:12 +0200
From: Andreas Hindborg <a.hindborg@...nel.org>
To: Oliver Mangold <oliver.mangold@...me>, Benno Lossin <lossin@...nel.org>
Cc: 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

"Oliver Mangold" <oliver.mangold@...me> writes:

> On 250820 0941, Benno Lossin wrote:
>> 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>`.
>
> Ah, okay. Makes sense, I guess.

I applied Linas patch with this change to my tree [1].

Best regards,
Andreas Hindborg


[1] https://lore.kernel.org/all/20250202-rust-page-v1-2-e3170d7fe55e@asahilina.net/