Re: [PATCH v3 3/7] rust: alloc: add Vec::push_within_capacity

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

On Tue, Apr 22, 2025 at 09:52:18AM +0000, Alice Ryhl wrote:
> This introduces a new method called `push_within_capacity` for appending
> to a vector without attempting to allocate if the capacity is full. Rust
> Binder will use this in various places to safely push to a vector while
> holding a spinlock.
> 
> The implementation is moved to a push_within_capacity_unchecked method.
> This is preferred over having push() call push_within_capacity()
> followed by an unwrap_unchecked() for simpler unsafe.
> 
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> ---
>  rust/kernel/alloc/kvec.rs | 41 ++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 38 insertions(+), 3 deletions(-)
> 
> diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
> index ebca0cfd31c67f3ce13c4825d7039e34bb54f4d4..a005a295262cb1e8b7c118125ffa07ae252e257c 100644
> --- a/rust/kernel/alloc/kvec.rs
> +++ b/rust/kernel/alloc/kvec.rs
> @@ -307,17 +307,52 @@ pub fn spare_capacity_mut(&mut self) -> &mut [MaybeUninit<T>] {
>      /// ```
>      pub fn push(&mut self, v: T, flags: Flags) -> Result<(), AllocError> {
>          self.reserve(1, flags)?;
> +        // SAFETY: The call to `reserve` was successful, so the capacity is at least one greater
> +        // than the length.
> +        unsafe { self.push_within_capacity_unchecked(v) };
> +        Ok(())
> +    }
> +
> +    /// Appends an element to the back of the [`Vec`] instance without reallocating.
> +    ///
> +    /// Fails if the vector does not have capacity for the new element.
> +    ///
> +    /// # Examples
> +    ///
> +    /// ```
> +    /// let mut v = KVec::with_capacity(10, GFP_KERNEL);

Should be:

    /// let mut v = KVec::with_capacity(10, GFP_KERNEL)?;

, right? I.e. a question mark is missing.

The rest looks good to me.

Regards,
Boqun

> +    /// for i in 0..10 {
> +    ///     v.push_within_capacity(i).unwrap();
> +    /// }
> +    ///
> +    /// assert!(v.push_within_capacity(10).is_err());
> +    /// # Ok::<(), Error>(())
> +    /// ```
> +    pub fn push_within_capacity(&mut self, v: T) -> Result<(), T> {
> +        if self.len() < self.capacity() {
> +            // SAFETY: The length is less than the capacity.
> +            unsafe { self.push_within_capacity_unchecked(v) };
> +            Ok(())
> +        } else {
> +            Err(v)
> +        }
> +    }
>  
> +    /// Appends an element to the back of the [`Vec`] instance without reallocating.
> +    ///
> +    /// # Safety
> +    ///
> +    /// The length must be less than the capacity.
> +    pub unsafe fn push_within_capacity_unchecked(&mut self, v: T) {
>          let spare = self.spare_capacity_mut();
>  
>          // SAFETY: The call to `reserve` was successful so the spare capacity is at least 1.
>          unsafe { spare.get_unchecked_mut(0) }.write(v);
>  
>          // SAFETY: We just initialised the first spare entry, so it is safe to increase the length
> -        // by 1. We also know that the new length is <= capacity because of the previous call to
> -        // `reserve` above.
> +        // by 1. We also know that the new length is <= capacity because the caller guarantees that
> +        // the length is less than the capacity at the beginning of this function.
>          unsafe { self.inc_len(1) };
> -        Ok(())
>      }
>  
>      /// Removes the last element from a vector and returns it, or `None` if it is empty.
> 
> -- 
> 2.49.0.805.g082f7c87e0-goog
> 
>