linux-kernel - Re: [PATCH v2 2/5] rust: bitmap: add BitmapVec::new

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Message-ID: <aPpmxy_oYoJeeqfr@yury>
Date: Thu, 23 Oct 2025 13:33:03 -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 2/5] rust: bitmap: add BitmapVec::new_small()

On Tue, Oct 21, 2025 at 01:32:44PM +0000, Alice Ryhl wrote:
> This constructor is useful when you just want to create a BitmapVec
> without allocating but don't care how large it is.
> 
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
> ---
>  rust/kernel/bitmap.rs | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/rust/kernel/bitmap.rs b/rust/kernel/bitmap.rs
> index 15fa23b45054b9272415fcc000e3e3b52c74d7c1..4ffe9eb0f208a3d62016e00297f5a0800aa33336 100644
> --- a/rust/kernel/bitmap.rs
> +++ b/rust/kernel/bitmap.rs
> @@ -232,6 +232,16 @@ impl BitmapVec {
>      /// The maximum length that avoids allocating.
>      pub const NO_ALLOC_MAX_LEN: usize = BITS_PER_LONG;
>  
> +    /// Constructs a new [`BitmapVec`] without allocating.
> +    #[inline]
> +    pub fn new_small() -> Self {

Nit: maybe:

        /// Construct a longest possible inline [`BitmapVec`].
        #[inline]
        pub fn new_inline() ...

This 'small vs large' lingo is internal to bitmaps. I don't think it
is worth to expose it in the interfaces. 'Inline' or 'inplace' sounds
better to me.

With that,

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

> +        // INVARIANT: `nbits <= NO_ALLOC_MAX_LEN`, so an inline bitmap is the right repr.
> +        BitmapVec {
> +            repr: BitmapRepr { bitmap: 0 },
> +            nbits: BitmapVec::NO_ALLOC_MAX_LEN,

A side note: after merging bitfields, we may switch inline bitmaps to to

        bitfield!() {
                0:31    nbits;
                32:64   bitmap;
        }

Thanks,
Yury

> +        }
> +    }
> +
>      /// Constructs a new [`BitmapVec`].
>      ///
>      /// Fails with [`AllocError`] when the [`BitmapVec`] could not be allocated. This
> 
> -- 
> 2.51.0.869.ge66316f041-goog