[PATCH v3 0/3] rust: add a few helpers

[PATCH v3 0/3] 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. It also moves the `NumaNode`
struct from `alloc` to `numa`.

Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
---
Changes in v3:
- Drop Alice tag on patch 2 due to restructuring.
- Use a relaxed load to read `nr_online_nodes` (Gary).
- Move `NumaNode` from the `alloc` module to the `numa` module in a
  new patch (Alice).
- Rebase on v7.3-rc5.
- Link to v2: https://msgid.link/20260605-cpu-helpers-v2-0-0ad227f0d91b@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

---
Andreas Hindborg (3):
      rust: add a wrapper for the `num_possible_cpus` C function
      rust: add a wrapper for the `nr_online_nodes` C function
      rust: alloc: move `NumaNode` to the `numa` module

 rust/helpers/helpers.c         |  1 +
 rust/helpers/num_cpus.c        |  8 ++++++
 rust/kernel/alloc.rs           | 30 +++-----------------
 rust/kernel/alloc/allocator.rs |  6 ++--
 rust/kernel/alloc/kbox.rs      |  4 +--
 rust/kernel/alloc/kvec.rs      |  4 +--
 rust/kernel/cpu.rs             |  7 +++++
 rust/kernel/lib.rs             |  1 +
 rust/kernel/numa.rs            | 62 ++++++++++++++++++++++++++++++++++++++++++
 9 files changed, 90 insertions(+), 33 deletions(-)
---
base-commit: 72d3fcf802c45d00b300f25b848a93c3a2bd7c7e
change-id: 20260215-cpu-helpers-08efb2572487

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

[PATCH v3 1/3] 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.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
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 440fb7638e3c..89e38b08fd17 100644
--- a/rust/helpers/helpers.c
+++ b/rust/helpers/helpers.c
@@ -76,6 +76,7 @@
 #include "mm.c"
 #include "mutex.c"
 #include "net/genetlink.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 {

-- 
2.54.0

[PATCH v3 2/3] 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 | 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)
+    }
+    #[cfg(not(CONFIG_NUMA))]
+    {
+        bindings::nr_online_nodes
+    }
+}

-- 
2.54.0

[PATCH v3 3/3] rust: alloc: move `NumaNode` to the `numa` module

[Andreas Hindborg]

`NumaNode` identifies a NUMA node and is not specific to the allocator API.
Move it from the `alloc` module to the `numa` module so that NUMA related
items live together.

No functional change.

Suggested-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/all/aiZvKPFCkqJUsP0a@google.com/
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
---
 rust/kernel/alloc.rs           | 30 ++++--------------------------
 rust/kernel/alloc/allocator.rs |  6 +++---
 rust/kernel/alloc/kbox.rs      |  4 ++--
 rust/kernel/alloc/kvec.rs      |  4 ++--
 rust/kernel/numa.rs            | 35 ++++++++++++++++++++++++++++++++++-
 5 files changed, 45 insertions(+), 34 deletions(-)

diff --git a/rust/kernel/alloc.rs b/rust/kernel/alloc.rs
index 21067bde6860..9daddf540e7d 100644
--- a/rust/kernel/alloc.rs
+++ b/rust/kernel/alloc.rs
@@ -22,7 +22,10 @@
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 pub struct AllocError;
 
-use crate::prelude::*;
+use crate::{
+    numa::NumaNode,
+    prelude::*, //
+};
 
 use core::{
     alloc::Layout,
@@ -114,31 +117,6 @@ pub mod flags {
     pub const __GFP_NOWARN: Flags = Flags(bindings::__GFP_NOWARN);
 }
 
-/// Non Uniform Memory Access (NUMA) node identifier.
-#[derive(Clone, Copy, PartialEq)]
-pub struct NumaNode(i32);
-
-impl NumaNode {
-    /// Create a new NUMA node identifier (non-negative integer).
-    ///
-    /// Returns [`EINVAL`] if a negative id or an id exceeding [`bindings::MAX_NUMNODES`] is
-    /// specified.
-    pub fn new(node: i32) -> Result<Self> {
-        // MAX_NUMNODES never exceeds 2**10 because NODES_SHIFT is 0..10.
-        if node < 0 || node >= bindings::MAX_NUMNODES as i32 {
-            return Err(EINVAL);
-        }
-        Ok(Self(node))
-    }
-}
-
-/// Specify necessary constant to pass the information to Allocator that the caller doesn't care
-/// about the NUMA node to allocate memory from.
-impl NumaNode {
-    /// No node preference.
-    pub const NO_NODE: NumaNode = NumaNode(bindings::NUMA_NO_NODE);
-}
-
 /// The kernel's [`Allocator`] trait.
 ///
 /// An implementation of [`Allocator`] can allocate, re-allocate and free memory buffers described
diff --git a/rust/kernel/alloc/allocator.rs b/rust/kernel/alloc/allocator.rs
index cd4203f27aed..4af50e20933b 100644
--- a/rust/kernel/alloc/allocator.rs
+++ b/rust/kernel/alloc/allocator.rs
@@ -11,12 +11,12 @@
 use super::{
     AllocError,
     Allocator,
-    Flags,
-    NumaNode, //
+    Flags, //
 };
 
 use crate::{
     bindings,
+    numa::NumaNode,
     page, //
 };
 
@@ -122,7 +122,7 @@ unsafe fn call(
         // - Those functions provide the guarantees of this function.
         let raw_ptr = unsafe {
             // If `size == 0` and `ptr != NULL` the memory behind the pointer is freed.
-            self.0(ptr.cast(), size, layout.align(), flags.0, nid.0).cast()
+            self.0(ptr.cast(), size, layout.align(), flags.0, nid.as_raw()).cast()
         };
 
         let ptr = if size == 0 {
diff --git a/rust/kernel/alloc/kbox.rs b/rust/kernel/alloc/kbox.rs
index c63d6acdbb6f..0fa736c81287 100644
--- a/rust/kernel/alloc/kbox.rs
+++ b/rust/kernel/alloc/kbox.rs
@@ -13,12 +13,12 @@
 use super::{
     AllocError,
     Allocator,
-    Flags,
-    NumaNode, //
+    Flags, //
 };
 
 use crate::{
     fmt,
+    numa::NumaNode,
     page::AsPageIter,
     prelude::*,
     types::ForeignOwnable, //
diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
index c7546b9da4fa..b669ca10b3f7 100644
--- a/rust/kernel/alloc/kvec.rs
+++ b/rust/kernel/alloc/kvec.rs
@@ -14,12 +14,12 @@
     AllocError,
     Allocator,
     Box,
-    Flags,
-    NumaNode, //
+    Flags, //
 };
 
 use crate::{
     fmt,
+    numa::NumaNode,
     page::{
         AsPageIter,
         PAGE_SIZE, //
diff --git a/rust/kernel/numa.rs b/rust/kernel/numa.rs
index 164e622d325f..4a3a880308f3 100644
--- a/rust/kernel/numa.rs
+++ b/rust/kernel/numa.rs
@@ -4,12 +4,15 @@
 //!
 //! C header: [`include/linux/nodemask.h`](srctree/include/linux/nodemask.h)
 
-use crate::bindings;
 #[cfg(CONFIG_NUMA)]
 use crate::sync::atomic::{
     Atomic,
     Relaxed, //
 };
+use crate::{
+    bindings,
+    prelude::*, //
+};
 
 /// Returns the number of online NUMA nodes.
 #[inline]
@@ -27,3 +30,33 @@ pub fn num_online_nodes() -> u32 {
         bindings::nr_online_nodes
     }
 }
+
+/// Non Uniform Memory Access (NUMA) node identifier.
+#[derive(Clone, Copy, PartialEq)]
+pub struct NumaNode(i32);
+
+impl NumaNode {
+    /// Create a new NUMA node identifier (non-negative integer).
+    ///
+    /// Returns [`EINVAL`] if a negative id or an id exceeding [`bindings::MAX_NUMNODES`] is
+    /// specified.
+    pub fn new(node: i32) -> Result<Self> {
+        // MAX_NUMNODES never exceeds 2**10 because NODES_SHIFT is 0..10.
+        if node < 0 || node >= bindings::MAX_NUMNODES as i32 {
+            return Err(EINVAL);
+        }
+        Ok(Self(node))
+    }
+
+    /// Get the raw representation of this node identifier.
+    pub(crate) fn as_raw(self) -> i32 {
+        self.0
+    }
+}
+
+/// Specify necessary constant to pass the information to Allocator that the caller doesn't care
+/// about the NUMA node to allocate memory from.
+impl NumaNode {
+    /// No node preference.
+    pub const NO_NODE: NumaNode = NumaNode(bindings::NUMA_NO_NODE);
+}

-- 
2.54.0

Re: [PATCH v3 1/3] rust: add a wrapper for the `num_possible_cpus` C function

[Miguel Ojeda]

On Tue, Sep 29, 2026 at 7:58 PM Andreas Hindborg <a.hindborg@kernel.org> wrote:
>
> This function returns the maximum number of CPUs that can be online.
>
> The wrapper is needed by the Rust null block driver.
>
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> Reviewed-by: Gary Guo <gary@garyguo.net>
> Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>

I can pick this one -- it could have been an independent patch anyway, no?

Thanks!

Cheers,
Miguel