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@proton.me>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20241004154149.93856-7-dakr@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: Danilo Krummrich

rust/kernel/alloc/allocator.rs

@@ -13,10 +13,16 @@ use core::alloc::{GlobalAlloc, Layout};
 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 @@ impl ReallocFunc {
     }
 }
 
+// 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 {