Re: [PATCH v2 1/2] rust: kernel: add `drop_contents` to `BoxExt`

[Benno Lossin]

On 08.07.24 23:52, Boqun Feng wrote:
> On Mon, Jul 08, 2024 at 02:14:50PM -0700, Boqun Feng wrote:
>> On Mon, Jul 08, 2024 at 08:53:38PM +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@xxxxxxxxx>
>>> Link: https://lore.kernel.org/rust-for-linux/20240418-b4-rbtree-v3-5-323e134390ce@xxxxxxxxxx/ [1]
>>> ---
>>>  rust/kernel/alloc/box_ext.rs | 21 +++++++++++++++++++++
>>>  1 file changed, 21 insertions(+)
>>>
>>> diff --git a/rust/kernel/alloc/box_ext.rs b/rust/kernel/alloc/box_ext.rs
>>> index cdbb5ad166d9..6cf79f96d6c7 100644
>>> --- a/rust/kernel/alloc/box_ext.rs
>>> +++ b/rust/kernel/alloc/box_ext.rs
>>> @@ -5,6 +5,7 @@
>>>  use super::{AllocError, Flags};
>>>  use alloc::boxed::Box;
>>>  use core::mem::MaybeUninit;
>>> +use core::ptr;
>>>  use core::result::Result;
>>>
>>>  /// Extensions to [`Box`].
>>> @@ -18,6 +19,18 @@ 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.
>>> +    ///
> 
> (I spoke too soon ;-))
> 
>>> +    /// # Examples
>>> +    ///
>>> +    /// ```
> 
> need a `use` here:
> 
> 	use kernel::alloc::{flags, box_ext::BoxExt};

Oops, should have checked that.

>>> +    /// let value = Box::new([0; 32], flags::GFP_KERNEL)
> 
> missing a '?' and a ';' at the end of this line.
> 
>>> +    /// let value = value.drop_contents();
>>> +    /// // Now we can re-use `value`:
>>> +    /// Box::write(value, [1; 32]);
> 
> Need a line:
> 
> 	# Ok::<(), Error>(())
> 
> here.
>>> +    /// ```
>>> +    fn drop_contents(self) -> Box<MaybeUninit<T>>;
>>>  }
>>>
> [...]
> 
> I queued this patch in rust-dev with these fixes applied, FYI.

Thanks!

@Miguel, when you take this, then apply the fixes that Boqun suggested.

---
Cheers,
Benno