linux-kernel - Re: [PATCH v4 12/15] rust: block: add `GenDisk` private data support

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Message-ID: <CAH5fLgjezWnFLaEKZkfvb9ko0RHG-c5g6yO0KvOJ8nyennEXOQ@mail.gmail.com>
Date: Wed, 13 Aug 2025 15:50:25 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Andreas Hindborg <a.hindborg@...nel.org>
Cc: Boqun Feng <boqun.feng@...il.com>, Miguel Ojeda <ojeda@...nel.org>, 
	Alex Gaynor <alex.gaynor@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <lossin@...nel.org>, Trevor Gross <tmgross@...ch.edu>, 
	Danilo Krummrich <dakr@...nel.org>, Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org, 
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 12/15] rust: block: add `GenDisk` private data support

On Wed, Aug 13, 2025 at 3:47 PM Andreas Hindborg <a.hindborg@...nel.org> wrote:
>
> "Alice Ryhl" <aliceryhl@...gle.com> writes:
>
> > On Tue, Aug 12, 2025 at 10:44:30AM +0200, Andreas Hindborg wrote:
> >> Allow users of the rust block device driver API to install private data in
> >> the `GenDisk` structure.
> >>
> >> Signed-off-by: Andreas Hindborg <a.hindborg@...nel.org>
> >
> > Overall LGTM.
> > Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>
> >
> >>          self,
> >>          name: fmt::Arguments<'_>,
> >>          tagset: Arc<TagSet<T>>,
> >> +        queue_data: T::QueueData,
> >>      ) -> Result<GenDisk<T>> {
> >> +        let data = queue_data.into_foreign();
> >> +        let recover_data = ScopeGuard::new(|| {
> >> +            drop(
> >> +                // SAFETY: T::QueueData was created by the call to `into_foreign()` above
> >> +                unsafe { T::QueueData::from_foreign(data) },
> >> +            );
> >
> > This is usually formatted as:
> >
> > // SAFETY: T::QueueData was created by the call to `into_foreign()` above
> > drop(unsafe { T::QueueData::from_foreign(data) });
>
> I don't really have a preference, my optimization function was to
> minimize distance to the unsafe block. Are there any rust guidelines on this?

I would say that the unsafe keyword just has to be on the next line
from the safety comment. Optimizing further than that leads to wonky
formatting. A similar example that I also think is going too far:

let var =
    // SAFETY: bla bla
    unsafe { value };

Alice