linux-kernel - Re: [PATCH v2 3/5] rust: id_pool: do not supply starting capacity

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Message-ID: <aPpnwXGo4P4-WI6v@yury>
Date: Thu, 23 Oct 2025 13:37:05 -0400
From: Yury Norov <yury.norov@...il.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Arve Hjønnevåg <arve@...roid.com>,
	Todd Kjos <tkjos@...roid.com>, Martijn Coenen <maco@...roid.com>,
	Joel Fernandes <joelagnelf@...dia.com>,
	Christian Brauner <brauner@...nel.org>,
	Carlos Llamas <cmllamas@...gle.com>,
	Suren Baghdasaryan <surenb@...gle.com>, Burak Emir <bqe@...gle.com>,
	Miguel Ojeda <ojeda@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
	Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <lossin@...nel.org>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Trevor Gross <tmgross@...ch.edu>,
	Danilo Krummrich <dakr@...nel.org>, rust-for-linux@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/5] rust: id_pool: do not supply starting capacity

On Tue, Oct 21, 2025 at 01:32:45PM +0000, Alice Ryhl wrote:
> When creating the initial IdPool, Rust Binder simply wants the largest
> value that does not allocate. Having to handle allocating error failures

That "value that does not allocate" wording is pretty confusing.
Maybe:
        Rust binder is initially created with an arbitrary capacity
        such that the underlying bitmap is held inplace.

Regardless:

Acked-by: Yury Norov (NVIDIA) <yury.norov@...il.com>

> that cannot happen is inconvenient, so make the constructor infallible
> by removing the size argument.
> 
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
> ---
>  rust/kernel/id_pool.rs | 20 +++++++++++---------
>  1 file changed, 11 insertions(+), 9 deletions(-)
> 
> diff --git a/rust/kernel/id_pool.rs b/rust/kernel/id_pool.rs
> index a41a3404213ca92d53b14c80101afff6ac8c416e..126e57f34c3407cb1dab3169417f01917e172dee 100644
> --- a/rust/kernel/id_pool.rs
> +++ b/rust/kernel/id_pool.rs
> @@ -96,16 +96,11 @@ pub fn realloc(&self, flags: Flags) -> Result<PoolResizer, AllocError> {
>  
>  impl IdPool {
>      /// Constructs a new [`IdPool`].
> -    ///
> -    /// A capacity below [`BITS_PER_LONG`] is adjusted to
> -    /// [`BITS_PER_LONG`].
> -    ///
> -    /// [`BITS_PER_LONG`]: srctree/include/asm-generic/bitsperlong.h
>      #[inline]
> -    pub fn new(num_ids: usize, flags: Flags) -> Result<Self, AllocError> {
> -        let num_ids = core::cmp::max(num_ids, BITS_PER_LONG);
> -        let map = BitmapVec::new(num_ids, flags)?;
> -        Ok(Self { map })
> +    pub fn new() -> Self {
> +        Self {
> +            map: BitmapVec::new_small(),
> +        }
>      }
>  
>      /// Returns how many IDs this pool can currently have.
> @@ -224,3 +219,10 @@ pub fn release_id(&mut self, id: usize) {
>          self.map.clear_bit(id);
>      }
>  }
> +
> +impl Default for IdPool {
> +    #[inline]
> +    fn default() -> Self {
> +        Self::new()
> +    }
> +}
> 
> -- 
> 2.51.0.869.ge66316f041-goog