linux-kernel - Re: [PATCH v4 1/6] rust: bitmap: add MAX_LEN and NO_ALLOC_MAX

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Message-ID: <CAJ-ks9kvMQ9tUMZyM07jRr8O+pJ6RRvCZodenB==tzDChhHT=A@mail.gmail.com>
Date: Mon, 10 Nov 2025 08:59:36 -0500
From: Tamir Duberstein <tamird@...il.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Yury Norov <yury.norov@...il.com>, 
	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 v4 1/6] rust: bitmap: add MAX_LEN and NO_ALLOC_MAX_LEN constants

On Mon, Nov 10, 2025 at 8:06 AM Alice Ryhl <aliceryhl@...gle.com> wrote:
>
> To avoid hard-coding these values in drivers, define constants for them
> that drivers can reference.
>
> Acked-by: Danilo Krummrich <dakr@...nel.org>
> Reviewed-by: Burak Emir <bqe@...gle.com>
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
> ---
>  rust/kernel/bitmap.rs | 16 +++++++++++-----
>  1 file changed, 11 insertions(+), 5 deletions(-)
>
> diff --git a/rust/kernel/bitmap.rs b/rust/kernel/bitmap.rs
> index aa8fc7bf06fc99865ae755d8694e4bec3dc8e7f0..15fa23b45054b9272415fcc000e3e3b52c74d7c1 100644
> --- a/rust/kernel/bitmap.rs
> +++ b/rust/kernel/bitmap.rs
> @@ -149,14 +149,14 @@ macro_rules! bitmap_assert_return {
>  ///
>  /// # Invariants
>  ///
> -/// * `nbits` is `<= i32::MAX` and never changes.
> +/// * `nbits` is `<= MAX_LEN`.
>  /// * if `nbits <= bindings::BITS_PER_LONG`, then `repr` is a `usize`.

Should this and other references to bindings::BITS_PER_LONG be
`NO_ALLOC_MAX_LEN` instead?

>  /// * otherwise, `repr` holds a non-null pointer to an initialized
>  ///   array of `unsigned long` that is large enough to hold `nbits` bits.
>  pub struct BitmapVec {
>      /// Representation of bitmap.
>      repr: BitmapRepr,
> -    /// Length of this bitmap. Must be `<= i32::MAX`.
> +    /// Length of this bitmap. Must be `<= MAX_LEN`.
>      nbits: usize,
>  }
>
> @@ -226,10 +226,16 @@ fn drop(&mut self) {
>  }
>
>  impl BitmapVec {
> +    /// The maximum possible length of a `BitmapVec`.
> +    pub const MAX_LEN: usize = i32::MAX as usize;
> +
> +    /// The maximum length that avoids allocating.
> +    pub const NO_ALLOC_MAX_LEN: usize = BITS_PER_LONG;
> +
>      /// Constructs a new [`BitmapVec`].
>      ///
>      /// Fails with [`AllocError`] when the [`BitmapVec`] could not be allocated. This
> -    /// includes the case when `nbits` is greater than `i32::MAX`.
> +    /// includes the case when `nbits` is greater than `MAX_LEN`.
>      #[inline]
>      pub fn new(nbits: usize, flags: Flags) -> Result<Self, AllocError> {
>          if nbits <= BITS_PER_LONG {
> @@ -238,11 +244,11 @@ pub fn new(nbits: usize, flags: Flags) -> Result<Self, AllocError> {
>                  nbits,
>              });
>          }
> -        if nbits > i32::MAX.try_into().unwrap() {
> +        if nbits > Self::MAX_LEN {
>              return Err(AllocError);
>          }
>          let nbits_u32 = u32::try_from(nbits).unwrap();
> -        // SAFETY: `BITS_PER_LONG < nbits` and `nbits <= i32::MAX`.
> +        // SAFETY: `BITS_PER_LONG < nbits` and `nbits <= MAX_LEN`.
>          let ptr = unsafe { bindings::bitmap_zalloc(nbits_u32, flags.as_raw()) };
>          let ptr = NonNull::new(ptr).ok_or(AllocError)?;
>          // INVARIANT: `ptr` returned by C `bitmap_zalloc` and `nbits` checked.
>
> --
> 2.51.2.1041.gc1ab5b90ca-goog
>
>