linux-kernel - Re: [PATCH 04/20] rust: alloc: implement `Allocator` for `Kmalloc`

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Message-ID: <033c72ba-fb31-4eff-b98c-5c5d35057280@proton.me>
Date: Sat, 06 Jul 2024 10:37:00 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Danilo Krummrich <dakr@...hat.com>, ojeda@...nel.org, alex.gaynor@...il.com, wedsonaf@...il.com, boqun.feng@...il.com, gary@...yguo.net, bjorn3_gh@...tonmail.com, a.hindborg@...sung.com, aliceryhl@...gle.com
Cc: daniel.almeida@...labora.com, faith.ekstrand@...labora.com, boris.brezillon@...labora.com, lina@...hilina.net, mcanal@...lia.com, zhiw@...dia.com, acurrid@...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
Subject: Re: [PATCH 04/20] rust: alloc: implement `Allocator` for `Kmalloc`

On 04.07.24 19:06, Danilo Krummrich wrote:
> @@ -48,20 +57,54 @@ pub(crate) unsafe fn krealloc_aligned(ptr: *mut u8, new_layout: Layout, flags: F
>      }
>  }
> 
> +unsafe impl Allocator for Kmalloc {
> +    unsafe fn realloc(
> +        &self,
> +        old_ptr: *mut u8,
> +        _old_size: usize,
> +        layout: Layout,
> +        flags: Flags,
> +    ) -> Result<NonNull<[u8]>, AllocError> {
> +        let size = aligned_size(layout);
> +
> +        // SAFETY: `src` is guaranteed to point to valid memory with a size of at least
> +        // `old_size`, which was previously allocated with this `Allocator` or NULL.
> +        let raw_ptr = unsafe {
> +            // If `size == 0` and `old_ptr != NULL` `krealloc()` frees the memory behind the
> +            // pointer.
> +            bindings::krealloc(old_ptr.cast(), size, flags.0).cast()
> +        };
> +
> +        let ptr = if size == 0 {
> +            NonNull::dangling()
> +        } else {
> +            NonNull::new(raw_ptr).ok_or(AllocError)?
> +        };
> +
> +        Ok(NonNull::slice_from_raw_parts(ptr, size))
> +    }
> +}
> +
>  unsafe impl GlobalAlloc for Kmalloc {

Since you remove `alloc` entirely at the end of this series, do we even
need this?
If not, it would be best to just leave the implementation as-is until a
patch removes it entirely.

---
Cheers,
Benno

>      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
> -        // requirement.
> -        unsafe { krealloc_aligned(ptr::null_mut(), layout, GFP_KERNEL) }
> +        let this: &dyn Allocator = self;
> +
> +        match this.alloc(layout, GFP_KERNEL) {
> +            Ok(ptr) => ptr.as_ptr().cast(),
> +            Err(_) => ptr::null_mut(),
> +        }
>      }