Re: [PATCH v5 4/4] rust: samples: Add debugfs sample

["Benno Lossin" <lossin@kernel.org>]

On Tue May 6, 2025 at 1:51 AM CEST, Matthew Maurer wrote:
> +impl kernel::Module for RustDebugFs {
> +    fn init(_this: &'static ThisModule) -> Result<Self> {
> +        // Create a debugfs directory in the root of the filesystem called "sample_debugfs".
> +        let debugfs = Dir::new(c_str!("sample_debugfs"));
> +
> +        {
> +            // Create a subdirectory, so "sample_debugfs/subdir" now exists.
> +            // We wrap it in `ManuallyDrop` so that the subdirectory is not automatically discarded
> +            // at the end of the scope - it will be cleaned up when `debugfs` is.
> +            let sub = ManuallyDrop::new(debugfs.subdir(c_str!("subdir")));

I dislike the direct usage of `ManuallyDrop`. To me the usage of
`ManuallyDrop` signifies that one has to opt out of `Drop` without the
support of the wrapped type. But in this case, `Dir` is sometimes
intended to not be dropped, so I'd rather have a `.keep()` function I
saw mentioned somewhere.

> +
> +            // Create a single file in the subdirectory called "example" that will read from the
> +            // `EXAMPLE` atomic variable.
> +            // We `forget` the result to avoid deleting the file at the end of the scope.
> +            forget(sub.fmt_file(c_str!("example"), &EXAMPLE, &|example, f| {
> +                writeln!(f, "Reading atomic: {}", example.load(Ordering::Relaxed))
> +            }));

Similarly here, I'd rather have a `.keep()` function than people start
using `forget` all over the place.

---
Cheers,
Benno

> +            // Now, "sample_debugfs/subdir/example" will print "Reading atomic: 8\n" when read.
> +        }
> +
> +        // Change the value in the variable displayed by the file. This is intended to demonstrate
> +        // that the module can continue to change the value used by the file.
> +        EXAMPLE.store(10, Ordering::Relaxed);
> +        // Now, "sample_debugfs/subdir/example" will print "Reading atomic: 10\n" when read.
> +
> +        // Save the root debugfs directory we created to our module object. It will be
> +        // automatically cleaned up when our module is unloaded because dropping the module object
> +        // will drop the `Dir` handle. The base directory, the subdirectory, and the file will all
> +        // continue to exist until the module is unloaded.
> +        Ok(Self { _debugfs: debugfs })
> +    }
> +}