Re: [PATCH v6 15/26] rust: alloc: implement `collect` for `IntoIter`

[Danilo Krummrich <dakr@kernel.org>]

On Tue, Sep 10, 2024 at 08:12:24PM +0000, Benno Lossin wrote:
> On 16.08.24 02:10, Danilo Krummrich 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>
> > ---
> >  rust/kernel/alloc/kvec.rs | 78 +++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 78 insertions(+)
> > 
> > diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
> > index 3b79f977b65e..ad96f4c3af9e 100644
> > --- a/rust/kernel/alloc/kvec.rs
> > +++ b/rust/kernel/alloc/kvec.rs
> > @@ -681,6 +681,84 @@ 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`.
> > +    ///
> > +    /// 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.
> 
> I think it's great that you include this in the code, but I don't think
> that it should be visible in the documentation,

Why not? I think this information is valuable for users of this API.

> can you move it under
> the `Examples` section and turn it into normal comments?
> 
> > +    ///
> > +    /// 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.
> > +    ///
> > +    /// # 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>(())
> > +    /// ```
> > +    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 {
> > +            // SAFETY: Copy the contents we have advanced to at the beginning of the buffer.
> 
> This first sentence should not be part of the SAFETY comment.
> 
> > +            // `ptr` is guaranteed to be between `buf` and `buf.add(cap)` and `ptr.add(len)` is
> > +            // guaranteed to be smaller than `buf.add(cap)`.
> 
> This doesn't justify all the requirements documented in [1].
> 
> [1]: https://doc.rust-lang.org/core/ptr/fn.copy.html#safety
> 
> > +            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();
> > +
> > +        // 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()
> > +            }
> > +        };
> > +
> > +        // 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`.
> 
> Turn this into bullet points please.
> 
> ---
> Cheers,
> Benno
> 
> > +        unsafe { Vec::from_raw_parts(ptr, len, cap) }
> > +    }
> >  }
> > 
> >  impl<T, A> Iterator for IntoIter<T, A>
> > --
> > 2.46.0
> > 
>