Re: [PATCH v2 2/2] rust: add a wrapper for the `nr_online_nodes` C function

[Gary Guo]

On Fri Jun 5, 2026 at 1:54 PM BST, Andreas Hindborg wrote:
> This function returns the number of online NUMA nodes.
>
> The wrapper is needed by the Rust null block driver.
>
> Signed-off-by: Andreas Hindborg <a.hindborg@xxxxxxxxxx>
> ---
>  rust/kernel/lib.rs  |  1 +
>  rust/kernel/numa.rs | 19 +++++++++++++++++++
>  2 files changed, 20 insertions(+)
>
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index b72b2fbe046d..00713b2adcac 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -97,6 +97,7 @@
>  #[cfg(CONFIG_NET)]
>  pub mod net;
>  pub mod num;
> +pub mod numa;
>  pub mod of;
>  #[cfg(CONFIG_PM_OPP)]
>  pub mod opp;
> diff --git a/rust/kernel/numa.rs b/rust/kernel/numa.rs
> new file mode 100644
> index 000000000000..760db072a6c1
> --- /dev/null
> +++ b/rust/kernel/numa.rs
> @@ -0,0 +1,19 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! NUMA topology utilities.
> +//!
> +//! C header: [`include/linux/nodemask.h`](srctree/include/linux/nodemask.h)
> +
> +use crate::bindings;
> +
> +/// Returns the number of online NUMA nodes.
> +#[inline]
> +pub fn num_online_nodes() -> u32 {
> +    // NOTE: In some configurations, we can read this variable without an unsafe block.
> +    // SAFETY: When NUMA is enabled, this is a global mutable static. We do as C and just read it,
> +    // even though it might race.

Can we do better and do a relaxed load instead?

Best,
Gary

> +    #[allow(unused_unsafe)]
> +    unsafe {
> +        bindings::nr_online_nodes
> +    }
> +}