Re: [PATCH v4 1/6] rust: bitmap: add MAX_LEN and NO_ALLOC_MAX_LEN constants

Tamir Duberstein
2025-11-10T14:00:14Z

On Mon, Nov 10, 2025 at 8:06 AM Alice Ryhl <aliceryhl@google.com> wrote:
>
> To avoid hard-coding these values in drivers, define constants for them
> that drivers can reference.
>
> Acked-by: Danilo Krummrich <dakr@kernel.org>
> Reviewed-by: Burak Emir <bqe@google.com>
> Signed-off-by: Alice Ryhl <aliceryhl@google.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
>
>