Re: [PATCH v6 1/6] rust: kernel: add `drop_contents` to `BoxExt`

[Benno Lossin <benno.lossin@proton.me>]

On 31.07.24 22:02, Gary Guo wrote:
> On Thu, 11 Jul 2024 16:20:57 +0000
> Matt Gilbride <mattgilbride@google.com> wrote:
> 
>> From: Benno Lossin <benno.lossin@proton.me>
>>
>> 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]
>> ---
>>  rust/kernel/alloc/box_ext.rs | 24 +++++++++++++++++++++++-
>>  1 file changed, 23 insertions(+), 1 deletion(-)
>>
>> diff --git a/rust/kernel/alloc/box_ext.rs b/rust/kernel/alloc/box_ext.rs
>> index 829cb1c1cf9e..557895db4f48 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};
>>
>>  /// Extensions to [`Box`].
>>  pub trait BoxExt<T>: Sized {
>> @@ -17,6 +17,20 @@ 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;
>> +    ///
>> +    /// let value = Box::new([0; 32], flags::GFP_KERNEL);
>> +    /// let value = value.unwrap().drop_contents();
>> +    /// // Now we can re-use `value`:
>> +    /// Box::write(value, [1; 32]);
>> +    /// ```
>> +    fn drop_contents(self) -> Box<MaybeUninit<T>>;
> 
> Should this have signature `fn drop_contents(this: Self) -> ...`? Box
> types normally don't have methods to avoid shadowing functions that the
> pointee may have.

Note that the newest version of this patch can be found at [1]. I agree,
it should probably take `this: Self` instead of a `self` receiver.

@Miguel, do you think I should send a new version with that change?

[1]: https://lore.kernel.org/rust-for-linux/20240708205325.1275473-1-benno.lossin@proton.me/

---
Cheers,
Benno

>>  }
>>
>>  impl<T> BoxExt<T> for Box<T> {
>> @@ -53,4 +67,12 @@ 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(self) -> Box<MaybeUninit<T>> {
>> +        let ptr = Box::into_raw(self);
>> +        // 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()) }
>> +    }
>>  }
>>
> 
> Best,
> Gary