Re: [PATCH v8 06/29] rust: alloc: implement `Allocator` for `Kmalloc`

[Gary Guo <gary@garyguo.net>]

On Tue,  1 Oct 2024 16:59:41 +0200
Danilo Krummrich <dakr@kernel.org> wrote:

> 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`.
> 
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>

Reviewed-by: Gary Guo <gary@garyguo.net>

> ---
>  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 1f28b004b447..4f3c3ab7c7a2 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) }
> +    }
> +}
> +
>  unsafe impl GlobalAlloc for Kmalloc {
>      unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
>          // SAFETY: `ptr::null_mut()` is null and `layout` has a non-zero size by the function safety