Re: [PATCH v4 1/2] rust: add a wrapper for the `nr_online_nodes` C function

[Boqun Feng <boqun@kernel.org>]

On Fri, Oct 30, 2026 at 05:08:03PM +0100, Andreas Hindborg wrote:
> This function returns the number of online NUMA nodes.
> 
> The wrapper is needed by the Rust null block driver.
> 
> Reviewed-by: Gary Guo <gary@garyguo.net>
> Reviewed-by: Yury Norov <ynorov@nvidia.com>
> Assisted-by: LLM
> Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
> ---
>  rust/kernel/lib.rs  |  1 +
>  rust/kernel/numa.rs | 29 +++++++++++++++++++++++++++++
>  2 files changed, 30 insertions(+)
> 
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index 4d5c96ddc49c..a7b1c9cf454f 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -105,6 +105,7 @@
>  #[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..164e622d325f
> --- /dev/null
> +++ b/rust/kernel/numa.rs
> @@ -0,0 +1,29 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! NUMA topology utilities.
> +//!
> +//! C header: [`include/linux/nodemask.h`](srctree/include/linux/nodemask.h)
> +
> +use crate::bindings;
> +#[cfg(CONFIG_NUMA)]
> +use crate::sync::atomic::{
> +    Atomic,
> +    Relaxed, //
> +};
> +
> +/// Returns the number of online NUMA nodes.
> +#[inline]
> +pub fn num_online_nodes() -> u32 {
> +    #[cfg(CONFIG_NUMA)]
> +    {
> +        // SAFETY: `nr_online_nodes` is a global variable that is valid for the lifetime of the
> +        // kernel. The C side updates it with plain writes during node hotplug and reads it with
> +        // plain reads. A relaxed load is the Rust equivalent of `READ_ONCE()`, which is the best
> +        // we can do on the reader side.
> +        unsafe { Atomic::from_ptr(&raw mut bindings::nr_online_nodes) }.load(Relaxed)

Just so you know you can use `atomic_load()`[1]:

	unsafe { atomic_load(&raw mut bindings::nr_online_nodes, Relaxed) }

With or without this:

Reviewed-by: Boqun Feng <boqun@kernel.org>

[1]: https://rust.docs.kernel.org/kernel/sync/atomic/fn.atomic_load.html

Regards,
Boqun

> +    }
> +    #[cfg(not(CONFIG_NUMA))]
> +    {
> +        bindings::nr_online_nodes
> +    }
> +}
> 
> -- 
> 2.54.0
> 
>