linux-kernel - [PATCH 2/3] rust: xarray: abstract `xa_alloc

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Message-ID: <20251006163024.18473-3-work@onurozkan.dev>
Date: Mon,  6 Oct 2025 19:30:23 +0300
From: Onur Özkan <work@...rozkan.dev>
To: rust-for-linux@...r.kernel.org
Cc: ojeda@...nel.org,
	alex.gaynor@...il.com,
	boqun.feng@...il.com,
	gary@...yguo.net,
	bjorn3_gh@...tonmail.com,
	lossin@...nel.org,
	aliceryhl@...gle.com,
	tmgross@...ch.edu,
	dakr@...nel.org,
	linux-kernel@...r.kernel.org,
	acourbot@...dia.com,
	airlied@...il.com,
	simona@...ll.ch,
	maarten.lankhorst@...ux.intel.com,
	mripard@...nel.org,
	tzimmermann@...e.de,
	corbet@....net,
	lyude@...hat.com,
	linux-doc@...r.kernel.org,
	Onur Özkan <work@...rozkan.dev>
Subject: [PATCH 2/3] rust: xarray: abstract `xa_alloc_cyclic`

Implements `alloc_cyclic` function to `XArray<T>` that
wraps `xa_alloc_cyclic` safely.

Resolves a task from the nova/core task list under the "XArray
bindings [XARR]" section in "Documentation/gpu/nova/core/todo.rst"
file.

Signed-off-by: Onur Özkan <work@...rozkan.dev>
---
 rust/kernel/xarray.rs | 43 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 43 insertions(+)

diff --git a/rust/kernel/xarray.rs b/rust/kernel/xarray.rs
index 1b882cd2f58b..4c2fdf53c7af 100644
--- a/rust/kernel/xarray.rs
+++ b/rust/kernel/xarray.rs
@@ -305,6 +305,49 @@ pub fn alloc(

         Ok(id)
     }
+
+    /// Allocates an empty slot within the given `limit`, storing `value` and cycling from `*next`.
+    ///
+    /// May drop the lock if needed to allocate memory, and then reacquire it afterwards.
+    ///
+    /// On success, returns the allocated id.
+    ///
+    /// On failure, returns the element which was attempted to be stored.
+    pub fn alloc_cyclic(
+        &mut self,
+        limit: bindings::xa_limit,
+        next: &mut u32,
+        value: T,
+        gfp: alloc::Flags,
+    ) -> Result<u32, StoreError<T>> {
+        build_assert!(
+            T::FOREIGN_ALIGN >= 4,
+            "pointers stored in XArray must be 4-byte aligned"
+        );
+
+        let new = value.into_foreign();
+
+        // `__xa_alloc_cyclic` overwrites this.
+        let mut id: u32 = 0;
+
+        // SAFETY:
+        // - `self.xa.xa` is valid by the type invariant.
+        // - `new` came from `T::into_foreign`.
+        let ret = unsafe {
+            bindings::__xa_alloc_cyclic(self.xa.xa.get(), &mut id, new, limit, next, gfp.as_raw())
+        };
+
+        if ret < 0 {
+            // SAFETY: `__xa_alloc_cyclic` doesn't take ownership on error.
+            let value = unsafe { T::from_foreign(new) };
+            return Err(StoreError {
+                value,
+                error: Error::from_errno(ret),
+            });
+        }
+
+        Ok(id)
+    }
 }

 // SAFETY: `XArray<T>` has no shared mutable state so it is `Send` iff `T` is `Send`.
--
2.51.0