Re: [PATCH v4 1/2] rust: kernel: add `drop_contents` to `BoxExt`

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

On Mon, Aug 19, 2024 at 11:24:35AM +0000, Benno Lossin wrote:
> Sometimes (see [1]) it is necessary to drop the value inside of a
> `Box<T>`, but retain the allocation. For example to reuse the allocation
> in the future.
> Introduce a new function `drop_contents` that turns a `Box<T>` into
> `Box<MaybeUninit<T>>` by dropping the value.
> 
> Signed-off-by: Benno Lossin <benno.lossin@proton.me>
> Link: https://lore.kernel.org/rust-for-linux/20240418-b4-rbtree-v3-5-323e134390ce@google.com/ [1]


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

Regards,
Boqun

> ---
>  rust/kernel/alloc/box_ext.rs | 31 ++++++++++++++++++++++++++++++-
>  1 file changed, 30 insertions(+), 1 deletion(-)
> 
> diff --git a/rust/kernel/alloc/box_ext.rs b/rust/kernel/alloc/box_ext.rs
> index 829cb1c1cf9e..b68ade26a42d 100644
> --- a/rust/kernel/alloc/box_ext.rs
> +++ b/rust/kernel/alloc/box_ext.rs
> @@ -4,7 +4,7 @@
>  
>  use super::{AllocError, Flags};
>  use alloc::boxed::Box;
> -use core::mem::MaybeUninit;
> +use core::{mem::MaybeUninit, ptr, result::Result};
>  
>  /// Extensions to [`Box`].
>  pub trait BoxExt<T>: Sized {
> @@ -17,6 +17,22 @@ pub trait BoxExt<T>: Sized {
>      ///
>      /// The allocation may fail, in which case an error is returned.
>      fn new_uninit(flags: Flags) -> Result<Box<MaybeUninit<T>>, AllocError>;
> +
> +    /// Drops the contents, but keeps the allocation.
> +    ///
> +    /// # Examples
> +    ///
> +    /// ```
> +    /// use kernel::alloc::{flags, box_ext::BoxExt};
> +    /// let value = Box::new([0; 32], flags::GFP_KERNEL)?;
> +    /// assert_eq!(*value, [0; 32]);
> +    /// let value = Box::drop_contents(value);
> +    /// // Now we can re-use `value`:
> +    /// let value = Box::write(value, [1; 32]);
> +    /// assert_eq!(*value, [1; 32]);
> +    /// # Ok::<(), Error>(())
> +    /// ```
> +    fn drop_contents(this: Self) -> Box<MaybeUninit<T>>;
>  }
>  
>  impl<T> BoxExt<T> for Box<T> {
> @@ -53,4 +69,17 @@ fn new_uninit(flags: Flags) -> Result<Box<MaybeUninit<T>>, AllocError> {
>          // zero-sized types, we use `NonNull::dangling`.
>          Ok(unsafe { Box::from_raw(ptr) })
>      }
> +
> +    fn drop_contents(this: Self) -> Box<MaybeUninit<T>> {
> +        let ptr = Box::into_raw(this);
> +        // SAFETY: `ptr` is valid, because it came from `Box::into_raw`.
> +        unsafe { ptr::drop_in_place(ptr) };
> +
> +        // CAST: `MaybeUninit<T>` is a transparent wrapper of `T`.
> +        let ptr = ptr.cast::<MaybeUninit<T>>();
> +
> +        // SAFETY: `ptr` is valid for writes, because it came from `Box::into_raw` and it is valid for
> +        // reads, since the pointer came from `Box::into_raw` and the type is `MaybeUninit<T>`.
> +        unsafe { Box::from_raw(ptr) }
> +    }
>  }
> 
> base-commit: c4d7f546dd9aa9780716cdb07416ca97264dce43
> -- 
> 2.45.2
> 
> 
>