Re: [PATCH v8 09/29] rust: alloc: implement `KVmalloc` allocator

[Gary Guo <gary@garyguo.net>]

On Tue,  1 Oct 2024 16:59:44 +0200
Danilo Krummrich <dakr@kernel.org> wrote:

> Implement `Allocator` for `KVmalloc`, an `Allocator` that tries to
> allocate memory wth `kmalloc` first and, on failure, falls back to
> `vmalloc`.
> 
> All memory allocations made with `KVmalloc` end up in
> `kvrealloc_noprof()`; all frees in `kvfree()`.
> 
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> Reviewed-by: Benno Lossin <benno.lossin@proton.me>
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>

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

> ---
>  rust/helpers/slab.c                 |  6 +++++
>  rust/kernel/alloc/allocator.rs      | 36 +++++++++++++++++++++++++++++
>  rust/kernel/alloc/allocator_test.rs |  1 +
>  3 files changed, 43 insertions(+)
> 
> diff --git a/rust/helpers/slab.c b/rust/helpers/slab.c
> index f043e087f9d6..a842bfbddcba 100644
> --- a/rust/helpers/slab.c
> +++ b/rust/helpers/slab.c
> @@ -7,3 +7,9 @@ rust_helper_krealloc(const void *objp, size_t new_size, gfp_t flags)
>  {
>  	return krealloc(objp, new_size, flags);
>  }
> +
> +void * __must_check __realloc_size(2)
> +rust_helper_kvrealloc(const void *p, size_t size, gfp_t flags)
> +{
> +	return kvrealloc(p, size, flags);
> +}
> diff --git a/rust/kernel/alloc/allocator.rs b/rust/kernel/alloc/allocator.rs
> index f341e9ed5b7c..d0a8819c6741 100644
> --- a/rust/kernel/alloc/allocator.rs
> +++ b/rust/kernel/alloc/allocator.rs
> @@ -34,6 +34,15 @@
>  /// For more details see [self].
>  pub struct Vmalloc;
>  
> +/// The kvmalloc kernel allocator.
> +///
> +/// `KVmalloc` attempts to allocate memory with `Kmalloc` first, but falls back to `Vmalloc` upon
> +/// failure. This allocator is typically used when the size for the requested allocation is not
> +/// known and may exceed the capabilities of `Kmalloc`.
> +///
> +/// 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.
> @@ -76,6 +85,9 @@ impl ReallocFunc {
>      // INVARIANT: `vrealloc` satisfies the type invariants.
>      const VREALLOC: Self = Self(bindings::vrealloc);
>  
> +    // INVARIANT: `kvrealloc` satisfies the type invariants.
> +    const KVREALLOC: Self = Self(bindings::kvrealloc);
> +
>      /// # Safety
>      ///
>      /// This method has the same safety requirements as [`Allocator::realloc`].
> @@ -203,6 +215,30 @@ unsafe fn realloc(
>      }
>  }
>  
> +// 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 KVmalloc {
> +    #[inline]
> +    unsafe fn realloc(
> +        ptr: Option<NonNull<u8>>,
> +        layout: Layout,
> +        old_layout: Layout,
> +        flags: Flags,
> +    ) -> Result<NonNull<[u8]>, AllocError> {
> +        // 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");
> +            return Err(AllocError);
> +        }
> +
> +        // SAFETY: If not `None`, `ptr` is guaranteed to point to valid memory, which was previously
> +        // allocated with this `Allocator`.
> +        unsafe { ReallocFunc::KVREALLOC.call(ptr, layout, old_layout, flags) }
> +    }
> +}
> +
>  #[global_allocator]
>  static ALLOCATOR: Kmalloc = Kmalloc;
>  
> diff --git a/rust/kernel/alloc/allocator_test.rs b/rust/kernel/alloc/allocator_test.rs
> index e7bf2982f68f..1b2642c547ec 100644
> --- a/rust/kernel/alloc/allocator_test.rs
> +++ b/rust/kernel/alloc/allocator_test.rs
> @@ -8,6 +8,7 @@
>  
>  pub struct Kmalloc;
>  pub type Vmalloc = Kmalloc;
> +pub type KVmalloc = Kmalloc;
>  
>  unsafe impl Allocator for Kmalloc {
>      unsafe fn realloc(