linux-kernel - [PATCH v3] rust: alloc: implement Box::pin

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Message-Id: <20250811101456.2901694-1-vitaly.wool@konsulko.se>
Date: Mon, 11 Aug 2025 12:14:56 +0200
From: Vitaly Wool <vitaly.wool@...sulko.se>
To: rust-for-linux@...r.kernel.org
Cc: linux-kernel@...r.kernel.org,
	Uladzislau Rezki <urezki@...il.com>,
	Danilo Krummrich <dakr@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	"Liam R . Howlett" <Liam.Howlett@...cle.com>,
	Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Boqun Feng <boqun.feng@...il.com>,
	Gary Guo <gary@...yguo.net>,
	Bjorn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <lossin@...nel.org>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Trevor Gross <tmgross@...ch.edu>,
	Vitaly Wool <vitaly.wool@...sulko.se>
Subject: [PATCH v3] rust: alloc: implement Box::pin_slice()

From: Alice Ryhl <aliceryhl@...gle.com>

Add a new constructor to Box to facilitate Box creation from a pinned
slice of elements. This allows to efficiently allocate memory for e.g.
slices of structrures containing spinlocks or mutexes. Such slices may
be used in kmemcache like or zpool API implementations.

Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
Signed-off-by: Vitaly Wool <vitaly.wool@...sulko.se>
---
 rust/kernel/alloc/kbox.rs | 72 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 72 insertions(+)

diff --git a/rust/kernel/alloc/kbox.rs b/rust/kernel/alloc/kbox.rs
index 1fef9beb57c8..ea9b08e3f8ea 100644
--- a/rust/kernel/alloc/kbox.rs
+++ b/rust/kernel/alloc/kbox.rs
@@ -290,6 +290,78 @@ pub fn pin(x: T, flags: Flags) -> Result<Pin<Box<T, A>>, AllocError>
         Ok(Self::new(x, flags)?.into())
     }
 
+    /// Construct a pinned slice of elements `Pin<Box<[T], A>>`.
+    ///
+    /// This is a convenient means for creation of e.g. slices of structrures containing spinlocks
+    /// or mutexes.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use kernel::sync::{new_spinlock, SpinLock};
+    ///
+    /// struct Inner {
+    ///     a: u32,
+    ///     b: u32,
+    /// }
+    /// #[pin_data]
+    /// struct Example {
+    ///     c: u32,
+    ///     #[pin]
+    ///     d: SpinLock<Inner>,
+    /// }
+    ///
+    /// impl Example {
+    ///     fn new() -> impl PinInit<Self, Error> {
+    ///         try_pin_init!(Self {
+    ///             c: 10,
+    ///             d <- new_spinlock!(Inner { a: 20, b: 30 }),
+    ///         })
+    ///     }
+    /// }
+    /// // Allocate a boxed slice of 10 `Example`s.
+    /// let s = KBox::pin_slice(
+    ///     | _i | Example::new(),
+    ///     10,
+    ///     GFP_KERNEL
+    /// )?;
+    /// assert_eq!(s[5].c, 10);
+    /// assert_eq!(s[3].d.lock().a, 20);
+    /// # Ok::<(), Error>(())
+    /// ```
+    pub fn pin_slice<Func, Item, E>(
+        mut init: Func,
+        len: usize,
+        flags: Flags,
+    ) -> Result<Pin<Box<[T], A>>, E>
+    where
+        Func: FnMut(usize) -> Item,
+        Item: PinInit<T, E>,
+        E: From<AllocError>,
+    {
+        let mut buffer = super::Vec::<T, A>::with_capacity(len, flags)?;
+        for i in 0..len {
+            let ptr = buffer.spare_capacity_mut().as_mut_ptr().cast();
+            // SAFETY:
+            // - `ptr` is a valid pointer to uninitialized memory.
+            // - `ptr` is not used if an error is returned.
+            // - `ptr` won't be moved until it is dropped, i.e. it is pinned.
+            unsafe { init(i).__pinned_init(ptr)? };
+
+            // SAFETY:
+            // - `i + 1 <= len`, hence we don't exceed the capacity, due to the call to
+            // `with_capacity()` above
+            // - the new value at index buffer.len() + 1 is the only element being added here, and
+            // it has been initialized above by `init(i).__pinned_init(ptr)`
+            unsafe { buffer.inc_len(1) };
+        }
+        let (ptr, _, _) = buffer.into_raw_parts();
+        let slice = core::ptr::slice_from_raw_parts_mut(ptr, len);
+        // SAFETY: `slice` points to an allocation allocated with `A` (`buffer`) and holds a valid
+        // `[T]`
+        Ok(Pin::from(unsafe { Box::from_raw(slice) }))
+    }
+
     /// Convert a [`Box<T,A>`] to a [`Pin<Box<T,A>>`]. If `T` does not implement
     /// [`Unpin`], then `x` will be pinned in memory and can't be moved.
     pub fn into_pin(this: Self) -> Pin<Self> {
-- 
2.39.2