linux-kernel - Re: [PATCH v2 06/10] rust: alloc: introduce the `BoxExt` trait

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Message-ID: <d9c96a73-bde7-4bd3-bb05-4422fa85c743@proton.me>
Date: Wed, 27 Mar 2024 17:09:47 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Wedson Almeida Filho <wedsonaf@...il.com>, rust-for-linux@...r.kernel.org
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, Andreas Hindborg <a.hindborg@...sung.com>, Alice Ryhl <aliceryhl@...gle.com>, linux-kernel@...r.kernel.org, Wedson Almeida Filho <walmeida@...rosoft.com>
Subject: Re: [PATCH v2 06/10] rust: alloc: introduce the `BoxExt` trait

On 27.03.24 03:35, Wedson Almeida Filho wrote:
> diff --git a/rust/kernel/alloc/allocator.rs b/rust/kernel/alloc/allocatorrs
> index 01ad139e19bc..fc0439455faa 100644
> --- a/rust/kernel/alloc/allocator.rs
> +++ b/rust/kernel/alloc/allocator.rs
> @@ -15,7 +15,11 @@
>  ///
>  /// - `ptr` can be either null or a pointer which has been allocated by this allocator.
>  /// - `new_layout` must have a non-zero size.
> -unsafe fn krealloc_aligned(ptr: *mut u8, new_layout: Layout, flags: bindings::gfp_t) -> *mut u8 {
> +pub(crate) unsafe fn krealloc_aligned(
> +    ptr: *mut u8,
> +    new_layout: Layout,
> +    flags: bindings::gfp_t,

I think it is a good idea to use `Flags` here as well.

> +) -> *mut u8 {
>      // Customized layouts from `Layout::from_size_align()` can have size < align, so pad first.
>      let layout = new_layout.pad_to_align();
> 
> diff --git a/rust/kernel/alloc/box_ext.rs b/rust/kernel/alloc/box_ext.rs
> new file mode 100644
> index 000000000000..aaa6a20d12b6
> --- /dev/null
> +++ b/rust/kernel/alloc/box_ext.rs
> @@ -0,0 +1,62 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Extensions to [`Box`] for fallible allocations.
> +
> +use super::Flags;
> +use alloc::boxed::Box;
> +use core::alloc::AllocError;
> +use core::mem::MaybeUninit;
> +use core::result::Result;
> +
> +/// Extensions to [`Box`].
> +pub trait BoxExt<T>: Sized {
> +    /// Allocates a new box.
> +    ///
> +    /// The allocation may fail, in which case an error is returned.
> +    fn new(x: T, flags: Flags) -> Result<Self, AllocError>;
> +
> +    /// Allocates a new uninitialised box.
> +    ///
> +    /// The allocation may fail, in which case an error is returned.
> +    fn new_uninit(flags: Flags) -> Result<Box<MaybeUninit<T>>, AllocError>;
> +}
> +
> +impl<T> BoxExt<T> for Box<T> {
> +    #[cfg(any(test, testlib))]
> +    fn new(x: T, _flags: Flags) -> Result<Self, AllocError> {
> +        Ok(Box::new(x))
> +    }
> +
> +    #[cfg(not(any(test, testlib)))]
> +    fn new(x: T, flags: Flags) -> Result<Self, AllocError> {
> +        let ptr = if core::mem::size_of::<T>() == 0 {
> +            core::ptr::NonNull::<T>::dangling().as_ptr()
> +        } else {
> +            let layout = core::alloc::Layout::new::<T>();
> +
> +            // SAFETY: Memory is being allocated (first arg is null). The only other source of
> +            // safety issues is sleeping on atomic context, which is addressed by klint. Lastly,
> +            // the type is not a SZT (checked above).
> +            let ptr = unsafe {
> +                super::allocator::krealloc_aligned(core::ptr::null_mut(), layout, flags.0)
> +            };
> +            if ptr.is_null() {
> +                return Err(AllocError);
> +            }
> +
> +            let ptr = ptr.cast::<T>();
> +
> +            // SAFETY: We just allocated the memory above, it is valid for write.
> +            unsafe { ptr.write(x) };
> +            ptr
> +        };
> +
> +        // SAFETY: For non-zero-sized types, we allocate above using the global allocator. For
> +        // zero-sized types, we use `NonNull::dangling`.
> +        Ok(unsafe { Box::from_raw(ptr) })
> +    }
> +
> +    fn new_uninit(flags: Flags) -> Result<Box<MaybeUninit<T>>, AllocError> {
> +        <Box<_> as BoxExt<_>>::new(MaybeUninit::<T>::uninit(), flags)

Note that the expression `MaybeUninit::<T>::uninit()` introduces a
temporary which will be allocated on the stack. If the type `T` is very
big, then this function will cause immediate stack overflows.

I would recommend to implement `new` in terms of `new_uninit`. The
`new_uninit` implementation can be almost the same as the current
implementation of `new`, just remove the `ptr.write(x)` statement.

-- 
Cheers,
Benno

> +    }
> +}