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rust: alloc: replace aligned_size() with Kmalloc::aligned_layout()

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aligned_size() dates back to when Rust did support kmalloc() only, but
is now used in ReallocFunc::call() and hence for all allocators.

However, the additional padding applied by aligned_size() is only
required by the kmalloc() allocator backend.

Hence, replace aligned_size() with Kmalloc::aligned_layout() and use it
for the affected allocators, i.e. kmalloc() and kvmalloc(), only.

While at it, make Kmalloc::aligned_layout() public, such that Rust
abstractions, which have to call subsystem specific kmalloc() based
allocation primitives directly, can make use of it.

Fixes: 8a799831fc ("rust: alloc: implement `ReallocFunc`")
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250731154919.4132-2-dakr@kernel.org
[ Remove `const` from Kmalloc::aligned_layout(). - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
This commit is contained in:
Danilo Krummrich 2025-07-31 17:48:06 +02:00
parent f87d597e83
commit fde578c862
1 changed files with 18 additions and 12 deletions
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30
rust/kernel/alloc/allocator.rs
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@ -43,17 +43,6 @@ pub struct Vmalloc;
/// For more details see [self].
pub struct KVmalloc;
/// Returns a proper size to alloc a new object aligned to `new_layout`'s alignment.
fn aligned_size(new_layout: Layout) -> usize {
// Customized layouts from `Layout::from_size_align()` can have size < align, so pad first.
let layout = new_layout.pad_to_align();
// Note that `layout.size()` (after padding) is guaranteed to be a multiple of `layout.align()`
// which together with the slab guarantees means the `krealloc` will return a properly aligned
// object (see comments in `kmalloc()` for more information).
layout.size()
}
/// # Invariants
///
/// One of the following: `krealloc`, `vrealloc`, `kvrealloc`.
@ -88,7 +77,7 @@ impl ReallocFunc {
old_layout: Layout,
flags: Flags,
) -> Result<NonNull<[u8]>, AllocError> {
let size = aligned_size(layout);
let size = layout.size();
let ptr = match ptr {
Some(ptr) => {
if old_layout.size() == 0 {
@ -123,6 +112,17 @@ impl ReallocFunc {
}
}
impl Kmalloc {
/// Returns a [`Layout`] that makes [`Kmalloc`] fulfill the requested size and alignment of
/// `layout`.
pub fn aligned_layout(layout: Layout) -> Layout {
// Note that `layout.size()` (after padding) is guaranteed to be a multiple of
// `layout.align()` which together with the slab guarantees means that `Kmalloc` will return
// a properly aligned object (see comments in `kmalloc()` for more information).
layout.pad_to_align()
}
}
// SAFETY: `realloc` delegates to `ReallocFunc::call`, which guarantees that
// - memory remains valid until it is explicitly freed,
// - passing a pointer to a valid memory allocation is OK,
@ -135,6 +135,8 @@ unsafe impl Allocator for Kmalloc {
old_layout: Layout,
flags: Flags,
) -> Result<NonNull<[u8]>, AllocError> {
let layout = Kmalloc::aligned_layout(layout);
// SAFETY: `ReallocFunc::call` has the same safety requirements as `Allocator::realloc`.
unsafe { ReallocFunc::KREALLOC.call(ptr, layout, old_layout, flags) }
}
@ -176,6 +178,10 @@ unsafe impl Allocator for KVmalloc {
old_layout: Layout,
flags: Flags,
) -> Result<NonNull<[u8]>, AllocError> {
// `KVmalloc` may use the `Kmalloc` backend, hence we have to enforce a `Kmalloc`
// compatible layout.
let layout = Kmalloc::aligned_layout(layout);
// TODO: Support alignments larger than PAGE_SIZE.
if layout.align() > bindings::PAGE_SIZE {
pr_warn!("KVmalloc does not support alignments larger than PAGE_SIZE yet.\n");