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Message-ID: <20250715-align-min-allocator-v1-1-3e1b2a5516c0@google.com>
Date: Tue, 15 Jul 2025 13:46:34 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, "Liam R. Howlett" <Liam.Howlett@...cle.com>,
Andrew Morton <akpm@...ux-foundation.org>, Danilo Krummrich <dakr@...nel.org>,
Matthew Wilcox <willy@...radead.org>, Tamir Duberstein <tamird@...il.com>,
Andreas Hindborg <a.hindborg@...nel.org>, Miguel Ojeda <ojeda@...nel.org>
Cc: Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
"Björn Roy Baron" <bjorn3_gh@...tonmail.com>, Benno Lossin <lossin@...nel.org>,
Trevor Gross <tmgross@...ch.edu>, linux-mm@...ck.org, rust-for-linux@...r.kernel.org,
linux-kernel@...r.kernel.org, Alice Ryhl <aliceryhl@...gle.com>
Subject: [PATCH 1/2] rust: alloc: specify the minimum alignment of each allocator
The kernel's allocators sometimes provide a higher alignment than the
end-user requested, so add a new constant on the Allocator trait to let
the allocator specify what its minimum guaranteed alignment is.
This allows the ForeignOwnable trait to provide a more accurate value of
FOREIGN_ALIGN when using a pointer type such as Box, which will be
useful with certain collections such as XArray that store its own data
in the low bits of pointers.
Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
---
rust/kernel/alloc.rs | 8 ++++++++
rust/kernel/alloc/allocator.rs | 8 ++++++++
2 files changed, 16 insertions(+)
diff --git a/rust/kernel/alloc.rs b/rust/kernel/alloc.rs
index a2c49e5494d334bfde67328464dafcdb31052947..c12753a5fb1c7423a4063553674b537a775c860e 100644
--- a/rust/kernel/alloc.rs
+++ b/rust/kernel/alloc.rs
@@ -137,6 +137,14 @@ pub mod flags {
/// - Implementers must ensure that all trait functions abide by the guarantees documented in the
/// `# Guarantees` sections.
pub unsafe trait Allocator {
+ /// The minimum alignment satisfied by all allocations from this allocator.
+ ///
+ /// # Guarantees
+ ///
+ /// Any pointer allocated by this allocator must be aligned to `MIN_ALIGN` even if the
+ /// requested layout has a smaller alignment.
+ const MIN_ALIGN: usize;
+
/// Allocate memory based on `layout` and `flags`.
///
/// On success, returns a buffer represented as `NonNull<[u8]>` that satisfies the layout
diff --git a/rust/kernel/alloc/allocator.rs b/rust/kernel/alloc/allocator.rs
index aa2dfa9dca4c309e5a9eafc7da6a8a9bd7b54b11..25fc9f9ae3b4e471a08d77130b374bd1397f7384 100644
--- a/rust/kernel/alloc/allocator.rs
+++ b/rust/kernel/alloc/allocator.rs
@@ -17,6 +17,8 @@
use crate::bindings;
use crate::pr_warn;
+const ARCH_KMALLOC_MINALIGN: usize = bindings::ARCH_KMALLOC_MINALIGN as usize;
+
/// The contiguous kernel allocator.
///
/// `Kmalloc` is typically used for physically contiguous allocations up to page size, but also
@@ -128,6 +130,8 @@ unsafe fn call(
// - passing a pointer to a valid memory allocation is OK,
// - `realloc` satisfies the guarantees, since `ReallocFunc::call` has the same.
unsafe impl Allocator for Kmalloc {
+ const MIN_ALIGN: usize = ARCH_KMALLOC_MINALIGN;
+
#[inline]
unsafe fn realloc(
ptr: Option<NonNull<u8>>,
@@ -145,6 +149,8 @@ unsafe fn realloc(
// - passing a pointer to a valid memory allocation is OK,
// - `realloc` satisfies the guarantees, since `ReallocFunc::call` has the same.
unsafe impl Allocator for Vmalloc {
+ const MIN_ALIGN: usize = kernel::page::PAGE_SIZE;
+
#[inline]
unsafe fn realloc(
ptr: Option<NonNull<u8>>,
@@ -169,6 +175,8 @@ unsafe fn realloc(
// - passing a pointer to a valid memory allocation is OK,
// - `realloc` satisfies the guarantees, since `ReallocFunc::call` has the same.
unsafe impl Allocator for KVmalloc {
+ const MIN_ALIGN: usize = ARCH_KMALLOC_MINALIGN;
+
#[inline]
unsafe fn realloc(
ptr: Option<NonNull<u8>>,
--
2.50.0.727.gbf7dc18ff4-goog
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