Re: [PATCH v2 2/4] rust: debugfs: Bind file creation for long-lived Display

[Danilo Krummrich <dakr@kernel.org>]

On Thu, May 01, 2025 at 09:09:27AM -0700, Matthew Maurer wrote:
> On Thu, May 1, 2025 at 3:37 AM Danilo Krummrich <dakr@kernel.org> wrote:
> > On Wed, Apr 30, 2025 at 11:31:57PM +0000, Matthew Maurer wrote:
> > Besides that, don't we also need a separate type for a file to be able to attach
> > non-static data anyways? I.e. something like:
> >
> >         #[cfg(CONFIG_DEBUG_FS)]
> >         struct File<T> {
> >            dentry: *mut bindings::dentry,
> >            data: T,
> >         }
> >
> >         #[cfg(not(CONFIG_DEBUG_FS))]
> >         struct File<T> {
> >            _p: PhantomData<T>,
> >         }
> >
> > I'm not exactly sure how v1 did this; I haven't had time to look at v1 before v2
> > was posted. I seems like v1 relied on a separate structure storing the data,
> > which also held a reference to the corresponding dentry or something along those
> > lines?
> In v1, this was done via
> ```
> #[pin_data]
> struct Values<T> {
>   dir: /* ignore this type */,
>   #[pin]
>   backing: T
> }
> ```
> Then, there was an interface that let the user provide a building
> function which had to have a fully polymorphic lifetime, which would
> be passed a backing reference that it was allowed to attach to
> subdirectory files. Since the dir would be cleaned up before the
> backing went away, we could know that it successfully outlived it.
> It'll probably look a little different when I send the follow-up
> series on top of this one.
> 
> Attaching to the root directory rather than each individual file made
> sense to me because this meant that if you had
> ```
> struct Foo {
>   prop_a: u32,
>   prop_b: u32
> }
> ```
> it would not be as tricky to attach `prop_a` to one file and `prop_b`
> to another, because the directory would own `Foo`.

Thanks for the explanation! I need to think a bit more about this approach.

> This'll probably be
> clearer when I send up a dependent series on top of v3 later today.

At least from my side, no need to rush. :) I'll have quite limited bandwidth for
the next ~10 days anyways.