linux-kernel - [PATCH v9 06/29] rust: alloc: implement `Allocator` for `Kmalloc`

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Message-ID: <20241004154149.93856-7-dakr@kernel.org>
Date: Fri,  4 Oct 2024 17:41:10 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: ojeda@...nel.org,
	alex.gaynor@...il.com,
	wedsonaf@...il.com,
	boqun.feng@...il.com,
	gary@...yguo.net,
	bjorn3_gh@...tonmail.com,
	benno.lossin@...ton.me,
	a.hindborg@...sung.com,
	aliceryhl@...gle.com,
	akpm@...ux-foundation.org
Cc: daniel.almeida@...labora.com,
	faith.ekstrand@...labora.com,
	boris.brezillon@...labora.com,
	lina@...hilina.net,
	mcanal@...lia.com,
	zhiw@...dia.com,
	cjia@...dia.com,
	jhubbard@...dia.com,
	airlied@...hat.com,
	ajanulgu@...hat.com,
	lyude@...hat.com,
	linux-kernel@...r.kernel.org,
	rust-for-linux@...r.kernel.org,
	linux-mm@...ck.org,
	Danilo Krummrich <dakr@...nel.org>
Subject: [PATCH v9 06/29] rust: alloc: implement `Allocator` for `Kmalloc`

Implement `Allocator` for `Kmalloc`, the kernel's default allocator,
typically used for objects smaller than page size.

All memory allocations made with `Kmalloc` end up in `krealloc()`.

It serves as allocator for the subsequently introduced types `KBox` and
`KVec`.

Reviewed-by: Benno Lossin <benno.lossin@...ton.me>
Reviewed-by: Gary Guo <gary@...yguo.net>
Signed-off-by: Danilo Krummrich <dakr@...nel.org>
---
 rust/kernel/alloc/allocator.rs | 31 ++++++++++++++++++++++++++++---
 1 file changed, 28 insertions(+), 3 deletions(-)

diff --git a/rust/kernel/alloc/allocator.rs b/rust/kernel/alloc/allocator.rs
index e5bac90046a4..b1894c800448 100644
--- a/rust/kernel/alloc/allocator.rs
+++ b/rust/kernel/alloc/allocator.rs
@@ -13,10 +13,16 @@
 use core::ptr;
 use core::ptr::NonNull;
 
-use crate::alloc::AllocError;
+use crate::alloc::{AllocError, Allocator};
 use crate::bindings;
 
-struct Kmalloc;
+/// The contiguous kernel allocator.
+///
+/// `Kmalloc` is typically used for physically contiguous allocations up to page size, but also
+/// supports larger allocations up to `bindings::KMALLOC_MAX_SIZE`, which is hardware specific.
+///
+/// For more details see [self].
+pub struct Kmalloc;
 
 /// Returns a proper size to alloc a new object aligned to `new_layout`'s alignment.
 fn aligned_size(new_layout: Layout) -> usize {
@@ -53,8 +59,10 @@ struct ReallocFunc(
     unsafe extern "C" fn(*const core::ffi::c_void, usize, u32) -> *mut core::ffi::c_void,
 );
 
-#[expect(dead_code)]
 impl ReallocFunc {
+    // INVARIANT: `krealloc` satisfies the type invariants.
+    const KREALLOC: Self = Self(bindings::krealloc);
+
     /// # Safety
     ///
     /// This method has the same safety requirements as [`Allocator::realloc`].
@@ -106,6 +114,23 @@ unsafe fn call(
     }
 }
 
+// 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,
+// - `realloc` satisfies the guarantees, since `ReallocFunc::call` has the same.
+unsafe impl Allocator for Kmalloc {
+    #[inline]
+    unsafe fn realloc(
+        ptr: Option<NonNull<u8>>,
+        layout: Layout,
+        old_layout: Layout,
+        flags: Flags,
+    ) -> Result<NonNull<[u8]>, AllocError> {
+        // SAFETY: `ReallocFunc::call` has the same safety requirements as `Allocator::realloc`.
+        unsafe { ReallocFunc::KREALLOC.call(ptr, layout, old_layout, flags) }
+    }
+}
+
 // SAFETY: TODO.
 unsafe impl GlobalAlloc for Kmalloc {
     unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
-- 
2.46.1