Re: [PATCH v3 06/10] rust: alloc: introduce the `BoxExt` trait

[Boqun Feng <boqun.feng@gmail.com>]

On Wed, Mar 27, 2024 at 10:35:59PM -0300, Wedson Almeida Filho wrote:
> From: Wedson Almeida Filho <walmeida@microsoft.com>
> 
> Make fallible versions of `new` and `new_uninit` methods available in
> `Box` even though it doesn't implement them because we build `alloc`
> with the `no_global_oom_handling` config.
> 
> They also have an extra `flags` parameter that allows callers to pass
> flags to the allocator.
> 
> Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>

Reviewed-by: Boqun Feng <boqun.feng@gmail.com>

A small nit below..

> ---
>  rust/kernel/alloc.rs           |  1 +
>  rust/kernel/alloc/allocator.rs |  2 +-
>  rust/kernel/alloc/box_ext.rs   | 60 ++++++++++++++++++++++++++++++++++
>  rust/kernel/init.rs            | 13 ++++----
>  rust/kernel/prelude.rs         |  2 +-
>  rust/kernel/sync/arc.rs        |  3 +-
>  6 files changed, 72 insertions(+), 9 deletions(-)
>  create mode 100644 rust/kernel/alloc/box_ext.rs
> 
> diff --git a/rust/kernel/alloc.rs b/rust/kernel/alloc.rs
> index fd5541991a22..9bc1b48b5641 100644
> --- a/rust/kernel/alloc.rs
> +++ b/rust/kernel/alloc.rs
> @@ -5,6 +5,7 @@
>  #[cfg(not(test))]
>  #[cfg(not(testlib))]
>  mod allocator;
> +pub mod box_ext;
>  pub mod vec_ext;
>  
>  /// Flags to be used when allocating memory.
> diff --git a/rust/kernel/alloc/allocator.rs b/rust/kernel/alloc/allocator.rs
> index 8cc7099d6ae1..ff88bce04fd4 100644
> --- a/rust/kernel/alloc/allocator.rs
> +++ b/rust/kernel/alloc/allocator.rs
> @@ -16,7 +16,7 @@
>  ///
>  /// - `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: Flags) -> *mut u8 {
> +pub(crate) unsafe fn krealloc_aligned(ptr: *mut u8, new_layout: Layout, flags: Flags) -> *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..c62db0aa4dd1
> --- /dev/null
> +++ b/rust/kernel/alloc/box_ext.rs
> @@ -0,0 +1,60 @@
> +// 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> {
> +    fn new(x: T, flags: Flags) -> Result<Self, AllocError> {
> +        let mut b = <Self as BoxExt<_>>::new_uninit(flags)?;
> +        b.write(x);
> +        // SAFETY: The contents were just initialised in the line above.
> +        Ok(unsafe { b.assume_init() })

, since we have the new_uninit feature selected, we can actually do

	fn new(x: T, flags: Flags) -> Result<Self, AllocError> {
	    <Self as BoxExt<_>>::new_uninit(flags).map(|b| Box::write(b, x))
	}

here.

Regards,
Boqun

> +    }
> +
[...]