linux-kernel - Re: [PATCH v8 4/6] rust: debugfs: Support arbitrary owned backing for File

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Message-ID: <ce8f428b-fcb0-48dc-b13e-6717c9a851b4@kernel.org>
Date: Mon, 30 Jun 2025 19:39:44 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: Matthew Maurer <mmaurer@...gle.com>
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>,
 Andreas Hindborg <a.hindborg@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>,
 Trevor Gross <tmgross@...ch.edu>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 "Rafael J. Wysocki" <rafael@...nel.org>,
 Sami Tolvanen <samitolvanen@...gle.com>, Timur Tabi <ttabi@...dia.com>,
 Benno Lossin <lossin@...nel.org>, linux-kernel@...r.kernel.org,
 rust-for-linux@...r.kernel.org, Dirk Behme <dirk.behme@...bosch.com>
Subject: Re: [PATCH v8 4/6] rust: debugfs: Support arbitrary owned backing for
 File

On 6/30/25 7:34 PM, Matthew Maurer wrote:
> On Mon, Jun 30, 2025 at 10:30 AM Danilo Krummrich <dakr@...nel.org> wrote:
>>
>> On 6/28/25 1:18 AM, Matthew Maurer wrote:
>>> +    fn create_file<D: ForeignOwnable>(&self, _name: &CStr, data: D) -> File
>>> +    where
>>> +        for<'a> D::Borrowed<'a>: Display,
>>> +    {
>>> +        File {
>>> +            _foreign: ForeignHolder::new(data),
>>> +        }
>>>        }
>>
>> What's the motivation for the ForeignHolder abstraction? Why not just make it
>> File<D> and store data directly?
> 
> 1. A `File<D>` can't be held in collection data structures as easily
> unless all your files contain the *same* backing type.

That sounds reasonable.

> 2. None of the APIs or potential APIs for `File` care about which type
> it's wrapping, nor are they supposed to. If nothing you can do with a
> `File` is different depending on the backing type, making it
> polymorphic is just needlessly confusing.

What if I want to access file.data() and do something with the data? Then I'd
necessarily need to put my data in an Arc and reference count it to still be
able to access it.

That doesn't seem like a reasonable requirement to be able to access data
exposed via debugfs.