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

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Message-ID: <aPtDmn7fSn5LcuC7@google.com>
Date: Fri, 24 Oct 2025 09:15:06 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Yury Norov <yury.norov@...il.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 Thu, Oct 23, 2025 at 01:33:03PM -0400, Yury Norov wrote:
> 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>

Makes sense. Will reword to 'inline', thanks!

> > +        // 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;
>         }

Personally I think I would prefer to keep the union only on the `repr`
field like it is now.

Alice