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

[Alice Ryhl <aliceryhl@google.com>]

On Tue, Jul 01, 2025 at 05:10:47PM +0200, Danilo Krummrich wrote:
> On Tue, Jul 01, 2025 at 04:21:56PM +0200, Greg Kroah-Hartman wrote:
> > On Tue, Jul 01, 2025 at 04:13:28PM +0200, Danilo Krummrich wrote:
> > > Instead this should just be:
> > > 
> > > 	struct GPU {
> > > 	   fw: debugfs::File<Firmware>,
> > > 	}
> > > 
> > > and then I would initialize it the following way:
> > > 
> > > 	let fw = KBox::new(Firmware::new(), GFP_KERNEL)?;
> > > 	let file = dir.create_file("firmware", fw);
> > > 
> > > 	// debugfs::File<Firmware> dereferences to Firmware
> > > 	file.do_something();
> > > 
> > > 	// Access to fw is prevented by the compiler, since it has been moved
> > > 	// into file.
> > > 
> > > This is much better, since now I have the guarantee that my Firmare instance
> > > can't out-live the GPU instance.
> > 
> > That's better, yes, but how would multiple files for the same
> > "structure" work here?  Like a debugfs-file-per-field of a structure
> > that we often have?
> 
> That is a very good question and I thought about this as well, because with only
> the current API this would require us to have more and more dynamic allocations
> if we want to have a more fine grained filesystem representations of structures.
> 
> The idea I have for this is to use pin-init, which we do in quite some other
> places as well.
> 
> I think we can add an additional API like this:
> 
> 	impl Dir {
> 	   pub fn create_file<T>(&self, data: impl PinInit<T>) -> impl PinInit<Self> {
> 	      pin_init!(Self {
> 	         data <- data,
> 	         ...
> 	      })
> 	   }
> 	}
> 
> This allows us to do things like:
> 
> 	#[pin_data]
> 	struct Firmware {
> 	   #[pin]
> 	   minor: debugfs::File<u32>,
> 	   #[pin]
> 	   major: debugfs::File<u32>,
> 	   #[pin]
> 	   buffer: debugfs::File<[u8]>,
> 	}
> 
> 	impl Firmware {
> 	   pub fn new(&dir: debugfs::Dir, buffer: [u8]) -> impl PinInit<Self> {
> 	      pin_init!(Self {
> 	         minor <- dir.create_file("minor", 1),
> 	         major <- dir.create_file("major", 2),
> 	         buffer <- dir.create_file("buffer", buffer),
> 	      })
> 	   }
> 	}
> 
> 	// This is the only allocation we need.
> 	let fw = KBox::pin_init(Firmware::new(...), GFP_KERNEL)?;
> 
> With this everything is now in a single allocation and since we're using
> pin-init, Dir::create_file() can safely store pointers of the corresponding data
> in debugfs_create_file(), since this structure is guaranteed to be pinned in
> memory.
> 
> Actually, we can also implement *only this*, since with this my previous example
> would just become this:
> 
> 	struct GPU {
> 	   fw: debugfs::File<Firmware>,
> 	}
> 
> 	let file = dir.create_file("firmware", Firmware::new());
> 	let file = KBox::pin_init(file, GFP_KERNEL)?;
> 
> 	// debugfs::File<Firmware> dereferences to Firmware
> 	file.do_something();
> 
> Given that, I think we should change things to use pin-init right away for the
> debugfs::File API.

Does this actually work in practice for anything except immutable data?
I mean, let's take Rust Binder as an example and lets say that I want to
expose a directory for each Process object with some of the fields
exposed. Let's just simplify Rust Binder a bit and only include some of
the fields:

#[pin_data]
struct Process {
    task: ARef<Task>,
    #[pin]
    inner: SpinLock<ProcessInner>,
}

pub(crate) struct ProcessInner {
    threads: RBTree<i32, Arc<Thread>>,
    nodes: RBTree<u64, DArc<Node>>,
    requested_thread_count: u32,
    max_threads: u32,
    started_thread_count: u32,
}

Rust Binder already does expose some debugging data through a file
system, though it doesn't do so using debugfs. It exposes a lot of data,
but among them are the pid, the number of threads and nodes, as well as
the values of requested_thread_count, started_thread_count, and
max_threads.

Now, we run into problem number one: pinning is not supported inside
mutexes. But let's say we solved that and we could do this:

#[pin_data]
struct Process {
    task: File<ARef<Task>>, // prints the pid
    #[pin]
    inner: SpinLock<ProcessInner>,
}

pub(crate) struct ProcessInner {
    threads: File<RBTree<i32, Arc<Thread>>>, // prints the count
    nodes: File<RBTree<u64, DArc<Node>>>, // prints the count
    requested_thread_count: File<u32>,
    max_threads: File<u32>,
    started_thread_count: File<u32>,
}

However, this still doesn't work! Debugfs may get triggered at any time
and need to read these fields, and there's no way for it to take the
spinlock with the above design - it doesn't know where the spinlock is.
For the integers I guess we could make them atomic to allow reading them
in parallel with mutation, but that option is not available for the
red/black trees.

What is the intended solution in this case? If the argument is that this
is a rare case, then keep in mind that this is a real-world example of
debugging information that we actually expose today in a real driver.
With Matt's current approach, it's relatively easy - just store a bunch
of File<Arc<Process>> instances somewhere and define each one to take
the mutex and print the relevant value.

Alice