Re: [PATCH v3 1/2] rust: kernel: add `drop_contents` to `BoxExt`

[Danilo Krummrich <dakr@kernel.org>]

On 8/3/24 4:16 PM, 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.

Is this (and the stuff in patch 2) used somewhere? Otherwise, I think it
would probably make sense to base this work on top of my allocator work.

> 
> 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]
> ---
>   rust/kernel/alloc/box_ext.rs | 26 +++++++++++++++++++++++++-
>   1 file changed, 25 insertions(+), 1 deletion(-)
> 
> diff --git a/rust/kernel/alloc/box_ext.rs b/rust/kernel/alloc/box_ext.rs
> index 829cb1c1cf9e..076d5de5f47d 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 +69,12 @@ 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) };
> +        // SAFETY: `ptr` is valid, because it came from `Box::into_raw`.
> +        unsafe { Box::from_raw(ptr.cast()) }
> +    }
>   }