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

Tamir Duberstein
2025-04-23T15:39:08Z

On Tue, Apr 22, 2025 at 5:53 AM Alice Ryhl <aliceryhl@google.com> 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);
> + /// 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) {

Did you intend for this to be pub? The commit message doesn't
obviously indicate it.

> let spare = self.spare_capacity_mut();
>
> // SAFETY: The call to `reserve` was successful so the spare capacity is at least 1.

What call to reserve?

> 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
>
>