linux-kernel - Re: [PATCH v3 03/12] rust: xarray: add `contains

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Message-ID: <q43rzzvpfh6ke74eyn3nypiek2ihaq6nat552asiobqge4vxmn@knwkb5dexnga>
Date: Tue, 10 Feb 2026 17:52:03 +0000
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Andreas Hindborg <a.hindborg@...nel.org>
Cc: Tamir Duberstein <tamird@...il.com>, Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>,
        Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Benno Lossin <lossin@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>,
        Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>,
        Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...two.org>,
        David Rientjes <rientjes@...gle.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Harry Yoo <harry.yoo@...cle.com>, Daniel Gomez <da.gomez@...nel.org>,
        rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH v3 03/12] rust: xarray: add `contains_index` method

* Andreas Hindborg <a.hindborg@...nel.org> [260209 14:38]:
> Add a convenience method `contains_index` to check whether an element
> exists at a given index in the XArray. This method provides a more
> ergonomic API compared to calling `get` and checking for `Some`.

I think this is going to result in less efficient code for most uses.

Most users use the results returned, not just checking if there is or is
not a value.  So if you find the value an xarray state and then just
throw it away and find it again, it'll be less efficient.

If there are users that do use the xarray to just check if something
exists or not (which there probably are?), then it should be in a
wrapper for that code and not the generic API.  Otherwise we will have
users pop up to use this method when they should not.

> 
> Signed-off-by: Andreas Hindborg <a.hindborg@...nel.org>
> ---
>  rust/kernel/xarray.rs | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
> 
> diff --git a/rust/kernel/xarray.rs b/rust/kernel/xarray.rs
> index d9762c6bef19c..ede48b5e1dba3 100644
> --- a/rust/kernel/xarray.rs
> +++ b/rust/kernel/xarray.rs
> @@ -218,6 +218,27 @@ fn load<F, U>(&self, index: usize, f: F) -> Option<U>
>          Some(f(ptr))
>      }
>  
> +    /// Checks if the XArray contains an element at the specified index.
> +    ///
> +    /// # Examples
> +    ///
> +    /// ```
> +    /// # use kernel::{alloc::{flags::GFP_KERNEL, kbox::KBox}, xarray::{AllocKind, XArray}};
> +    /// let xa = KBox::pin_init(XArray::new(AllocKind::Alloc), GFP_KERNEL)?;
> +    ///
> +    /// let mut guard = xa.lock();
> +    /// assert_eq!(guard.contains_index(42), false);
> +    ///
> +    /// guard.store(42, KBox::new(0u32, GFP_KERNEL)?, GFP_KERNEL)?;
> +    ///
> +    /// assert_eq!(guard.contains_index(42), true);
> +    ///
> +    /// # Ok::<(), kernel::error::Error>(())
> +    /// ```
> +    pub fn contains_index(&self, index: usize) -> bool {
> +        self.get(index).is_some()
> +    }
> +
>      /// Provides a reference to the element at the given index.
>      pub fn get(&self, index: usize) -> Option<T::Borrowed<'_>> {
>          self.load(index, |ptr| {
> 
> -- 
> 2.51.2
> 
>