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Message-ID: <ZuDiurdQfvmewzDh@pollux>
Date: Wed, 11 Sep 2024 02:22:18 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: Benno Lossin <benno.lossin@...ton.me>
Cc: ojeda@...nel.org, alex.gaynor@...il.com, wedsonaf@...il.com,
boqun.feng@...il.com, gary@...yguo.net, bjorn3_gh@...tonmail.com,
a.hindborg@...sung.com, aliceryhl@...gle.com,
akpm@...ux-foundation.org, daniel.almeida@...labora.com,
faith.ekstrand@...labora.com, boris.brezillon@...labora.com,
lina@...hilina.net, mcanal@...lia.com, zhiw@...dia.com,
cjia@...dia.com, jhubbard@...dia.com, airlied@...hat.com,
ajanulgu@...hat.com, lyude@...hat.com, linux-kernel@...r.kernel.org,
rust-for-linux@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v6 15/26] rust: alloc: implement `collect` for `IntoIter`
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@...gle.com>
> > Signed-off-by: Danilo Krummrich <dakr@...nel.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
> >
>
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