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rust: alloc: specify the minimum alignment of each allocator

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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.

Reviewed-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Acked-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Link: https://lore.kernel.org/r/20250811-align-min-allocator-v2-1-3386cc94f4fc@google.com
[ Add helper for ARCH_KMALLOC_MINALIGN; remove cast to usize. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
This commit is contained in:
Alice Ryhl
2025-08-11 12:31:50 +00:00
committed by Danilo Krummrich
parent f87de6919d
commit 1b1a946dc2
3 changed files with 17 additions and 0 deletions
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1
rust/bindings/bindings_helper.h
View File

@@ -84,6 +84,7 @@
/* `bindgen` gets confused at certain things. */
const size_t RUST_CONST_HELPER_ARCH_SLAB_MINALIGN = ARCH_SLAB_MINALIGN;
const size_t RUST_CONST_HELPER_ARCH_KMALLOC_MINALIGN = ARCH_KMALLOC_MINALIGN;
const size_t RUST_CONST_HELPER_PAGE_SIZE = PAGE_SIZE;
const gfp_t RUST_CONST_HELPER_GFP_ATOMIC = GFP_ATOMIC;
const gfp_t RUST_CONST_HELPER_GFP_KERNEL = GFP_KERNEL;

8
rust/kernel/alloc.rs
View File

@@ -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 is guaranteed to 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

8
rust/kernel/alloc/allocator.rs
View File

@@ -17,6 +17,8 @@ use crate::alloc::{AllocError, Allocator};
use crate::bindings;
use crate::pr_warn;
const ARCH_KMALLOC_MINALIGN: usize = bindings::ARCH_KMALLOC_MINALIGN;
/// The contiguous kernel allocator.
///
/// `Kmalloc` is typically used for physically contiguous allocations up to page size, but also
@@ -128,6 +130,8 @@ impl ReallocFunc {
// - 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 impl Allocator for Kmalloc {
// - 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 impl Allocator for Vmalloc {
// - 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>>,