Re: [PATCH v7 15/26] rust: alloc: implement `collect` for `IntoIter`

Gary Guo
2024-09-28T19:27:39Z

On Thu, 12 Sep 2024 00:52:51 +0200
Danilo Krummrich <dakr@kernel.org> wrote:

> Currently, we can't implement `FromIterator`. There are a couple of
> issues with this trait in the kernel, namely:
>
> - Rust's specialization feature is unstable. This prevents us to
> optimze for the special case where `I::IntoIter` equals `Vec`'s
> `IntoIter` type.
> - We also can't use `I::IntoIter`'s type ID either to work around this,
> since `FromIterator` doesn't require this type to be `'static`.
> - `FromIterator::from_iter` does return `Self` instead of
> `Result<Self, AllocError>`, hence we can't properly handle allocation
> failures.
> - Neither `Iterator::collect` nor `FromIterator::from_iter` can handle
> additional allocation flags.
>
> Instead, provide `IntoIter::collect`, such that we can at least convert
> `IntoIter` into a `Vec` again.
>
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>

A question is how useful is this? The way this can be used seems
fairly limited: you `into_iter`, consume a few elements, and the
`collect()`?

It feels whatever user this serves, it would make more sense for them
to use a `VecDeque` and just `pop_front()`.

Also, inline comments below.

> ---
> rust/kernel/alloc/kvec.rs | 86 +++++++++++++++++++++++++++++++++++++++
> 1 file changed, 86 insertions(+)
>
> diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
> index e91761c5c52d..686e969463f8 100644
> --- a/rust/kernel/alloc/kvec.rs
> +++ b/rust/kernel/alloc/kvec.rs
> @@ -690,6 +690,92 @@ impl<T, A> IntoIter<T, A>
> fn as_raw_mut_slice(&mut self) -> *mut [T] {
> ptr::slice_from_raw_parts_mut(self.ptr, self.len)
> }
> +
> + fn into_raw_parts(self) -> (*mut T, NonNull<T>, usize, usize) {
> + let me = ManuallyDrop::new(self);
> + let ptr = me.ptr;
> + let buf = me.buf;
> + let len = me.len;
> + let cap = me.cap;
> + (ptr, buf, len, cap)
> + }
> +
> + /// Same as `Iterator::collect` but specialized for `Vec`'s `IntoIter`.
> + ///
> + /// # Examples
> + ///
> + /// ```
> + /// let v = kernel::kvec![1, 2, 3]?;
> + /// let mut it = v.into_iter();
> + ///
> + /// assert_eq!(it.next(), Some(1));
> + ///
> + /// let v = it.collect(GFP_KERNEL);
> + /// assert_eq!(v, [2, 3]);
> + ///
> + /// # Ok::<(), Error>(())
> + /// ```
> + /// # Implementation Details
> + ///
> + /// Currently, we can't implement `FromIterator`. There are a couple of issues with this trait
> + /// in the kernel, namely:
> + ///
> + /// - Rust's specialization feature is unstable. This prevents us to optimze for the special
> + /// case where `I::IntoIter` equals `Vec`'s `IntoIter` type.
> + /// - We also can't use `I::IntoIter`'s type ID either to work around this, since `FromIterator`
> + /// doesn't require this type to be `'static`.
> + /// - `FromIterator::from_iter` does return `Self` instead of `Result<Self, AllocError>`, hence
> + /// we can't properly handle allocation failures.
> + /// - Neither `Iterator::collect` nor `FromIterator::from_iter` can handle additional allocation
> + /// flags.
> + ///
> + /// Instead, provide `IntoIter::collect`, such that we can at least convert a `IntoIter` into a
> + /// `Vec` again.
> + ///
> + /// Note that `IntoIter::collect` doesn't require `Flags`, since it re-uses the existing backing
> + /// buffer. However, this backing buffer may be shrunk to the actual count of elements.
> + pub fn collect(self, flags: Flags) -> Vec<T, A> {
> + let (mut ptr, buf, len, mut cap) = self.into_raw_parts();
> + let has_advanced = ptr != buf.as_ptr();
> +
> + if has_advanced {
> + // Copy the contents we have advanced to at the beginning of the buffer.
> + //
> + // SAFETY:
> + // - `ptr` is valid for reads of `len * size_of::<T>()` bytes,
> + // - `buf.as_ptr()` is valid for writes of `len * size_of::<T>()` bytes,
> + // - `ptr` and `buf.as_ptr()` are not be subject to aliasing restrictions relative to
> + // each other,
> + // - both `ptr` and `buf.ptr()` are properly aligned.
> + unsafe { ptr::copy(ptr, buf.as_ptr(), len) };
> + ptr = buf.as_ptr();
> + }
> +
> + // This can never fail, `len` is guaranteed to be smaller than `cap`.
> + let layout = core::alloc::Layout::array::<T>(len).unwrap();

nit: could be `unwrap_unchecked()`. Although feel free to leave it to
avoid an additional unsafe.

> +
> + // SAFETY: `buf` points to the start of the backing buffer and `len` is guaranteed to be
> + // smaller than `cap`. Depending on `alloc` this operation may shrink the buffer or leaves
> + // it as it is.
> + ptr = match unsafe { A::realloc(Some(buf.cast()), layout, flags) } {
> + // If we fail to shrink, which likely can't even happen, continue with the existing
> + // buffer.
> + Err(_) => ptr,
> + Ok(ptr) => {
> + cap = len;
> + ptr.as_ptr().cast()
> + }
> + };

This should be moved to `Vec::shrink_to_fit`. And then this function
can just `Vec::from_raw_parts(...)` and then `vec.shrink_to_fit`.

But my question would be why this function needs to shrink in the first
place.

> +
> + // SAFETY: If the iterator has been advanced, the advanced elements have been copied to
> + // the beginning of the buffer and `len` has been adjusted accordingly.
> + //
> + // - `ptr` is guaranteed to point to the start of the backing buffer.
> + // - `cap` is either the original capacity or, after shrinking the buffer, equal to `len`.
> + // - `alloc` is guaranteed to be unchanged since `into_iter` has been called on the original
> + // `Vec`.
> + unsafe { Vec::from_raw_parts(ptr, len, cap) }
> + }
> }
>
> impl<T, A> Iterator for IntoIter<T, A>