[PATCH v2 0/2] rust: add a few helpers

[PATCH v2 0/2] rust: add a few helpers

[Andreas Hindborg]

This series adds two helpers for obtaining information about the number
of numa domains and cpu count of the system.

Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
---
Changes in v2:
- Move helpers out of the crate root into the `cpu` and `numa`
  modules (Miguel).
- Mark Rust wrappers `#[inline]` (Alice).
- Mark the `num_possible_cpus` C helper `__rust_helper` (Dirk).
- Mention the Rust null block driver as the consumer in commit
  messages (Miguel).
- Link to v1: https://msgid.link/20260215-cpu-helpers-v1-0-fab13d817a5e@kernel.org

To: Miguel Ojeda <ojeda@kernel.org>
To: Boqun Feng <boqun@kernel.org>
To: Gary Guo <gary@garyguo.net>
To: Björn Roy Baron <bjorn3_gh@protonmail.com>
To: Benno Lossin <lossin@kernel.org>
To: Andreas Hindborg <a.hindborg@kernel.org>
To: Alice Ryhl <aliceryhl@google.com>
To: Trevor Gross <tmgross@umich.edu>
To: Danilo Krummrich <dakr@kernel.org>
To: Thomas Gleixner <tglx@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org
Cc: rust-for-linux@vger.kernel.org

---
Andreas Hindborg (2):
      rust: add a wrapper for the `num_possible_cpus` C function
      rust: add a wrapper for the `nr_online_nodes` C function

 rust/helpers/helpers.c  |  1 +
 rust/helpers/num_cpus.c |  8 ++++++++
 rust/kernel/cpu.rs      |  7 +++++++
 rust/kernel/lib.rs      |  1 +
 rust/kernel/numa.rs     | 19 +++++++++++++++++++
 5 files changed, 36 insertions(+)
---
base-commit: 7fd2df204f342fc17d1a0bfcd474b24232fb0f32
change-id: 20260215-cpu-helpers-08efb2572487

Best regards,
--  
Andreas Hindborg <a.hindborg@kernel.org>

[PATCH v2 1/2] rust: add a wrapper for the `num_possible_cpus` C function

[Andreas Hindborg]

This function returns the maximum number of CPUs that can be online.

The wrapper is needed by the Rust null block driver.

Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
---
 rust/helpers/helpers.c  | 1 +
 rust/helpers/num_cpus.c | 8 ++++++++
 rust/kernel/cpu.rs      | 7 +++++++
 3 files changed, 16 insertions(+)

diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
index 625921e27dfb..bb250648e0a9 100644
--- a/rust/helpers/helpers.c
+++ b/rust/helpers/helpers.c
@@ -70,6 +70,7 @@
 #include "maple_tree.c"
 #include "mm.c"
 #include "mutex.c"
+#include "num_cpus.c"
 #include "of.c"
 #include "page.c"
 #include "pci.c"
diff --git a/rust/helpers/num_cpus.c b/rust/helpers/num_cpus.c
new file mode 100644
index 000000000000..54e8bacb46c3
--- /dev/null
+++ b/rust/helpers/num_cpus.c
@@ -0,0 +1,8 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/cpumask.h>
+
+__rust_helper unsigned int rust_helper_num_possible_cpus(void)
+{
+	return num_possible_cpus();
+}
diff --git a/rust/kernel/cpu.rs b/rust/kernel/cpu.rs
index cb6c0338ef5a..b38b6914b12e 100644
--- a/rust/kernel/cpu.rs
+++ b/rust/kernel/cpu.rs
@@ -6,6 +6,13 @@
 
 use crate::{bindings, device::Device, error::Result, prelude::ENODEV};
 
+/// Returns the maximum number of CPUs that may be online on the system.
+#[inline]
+pub fn num_possible_cpus() -> u32 {
+    // SAFETY: FFI call with no additional requirements.
+    unsafe { bindings::num_possible_cpus() }
+}
+
 /// Returns the maximum number of possible CPUs in the current system configuration.
 #[inline]
 pub fn nr_cpu_ids() -> u32 {

-- 
2.51.2

Re: [PATCH v2 1/2] rust: add a wrapper for the `num_possible_cpus` C function

[Alice Ryhl]

On Fri, Jun 05, 2026 at 02:54:56PM +0200, Andreas Hindborg wrote:
> This function returns the maximum number of CPUs that can be online.
> 
> The wrapper is needed by the Rust null block driver.
> 
> Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>

Reviewed-by: Alice Ryhl <aliceryhl@google.com>

Re: [PATCH v2 1/2] rust: add a wrapper for the `num_possible_cpus` C function

[Gary Guo]

On Fri Jun 5, 2026 at 1:54 PM BST, Andreas Hindborg wrote:
> This function returns the maximum number of CPUs that can be online.
> 
> The wrapper is needed by the Rust null block driver.
> 
> Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>

Reviewed-by: Gary Guo <gary@garyguo.net>

> ---
>  rust/helpers/helpers.c  | 1 +
>  rust/helpers/num_cpus.c | 8 ++++++++
>  rust/kernel/cpu.rs      | 7 +++++++
>  3 files changed, 16 insertions(+)

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

[Andreas Hindborg]

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@kernel.org>
---
 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.
+    #[allow(unused_unsafe)]
+    unsafe {
+        bindings::nr_online_nodes
+    }
+}

-- 
2.51.2

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

[Alice Ryhl]

On Fri, Jun 05, 2026 at 02:54:57PM +0200, 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@kernel.org>

I guess the NumaNode struct should be moved to this new file.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>

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@kernel.org>
> ---
>  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
> +    }
> +}

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

[Andreas Hindborg]

"Gary Guo" <gary@garyguo.net> writes:

> 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@kernel.org>
>> ---
>>  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?

Yes, that sounds reasonable.

But what are the semantics of a relaxed load when other writers are not
using atomic primitives? Does it make a difference?


Best regards,
Andreas Hindborg

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

[Gary Guo]

On Mon Jun 8, 2026 at 12:52 PM BST, Andreas Hindborg wrote:
> "Gary Guo" <gary@garyguo.net> writes:
>
>> 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@kernel.org>
>>> ---
>>>  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?
>
> Yes, that sounds reasonable.
>
> But what are the semantics of a relaxed load when other writers are not
> using atomic primitives? Does it make a difference?

This is the whole CONFIG_KCSAN_ASSUME_PLAIN_WRITES_ATOMIC again.

Technically not okay, but this happens common in C code that people can choose
to ignore it :)

But let's not advocate for that in Rust code.

Best,
Gary