Re: [PATCH v3 17/25] rust: alloc: implement `collect` for `IntoIter`

Alice Ryhl
2024-08-02T07:09:02Z

On Thu, Aug 1, 2024 at 5:37 PM Danilo Krummrich <dakr@kernel.org> wrote:
>
> On Thu, Aug 01, 2024 at 05:10:22PM +0200, Alice Ryhl wrote:
> > On Thu, Aug 1, 2024 at 2:08 AM 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.
> > >
> > > Signed-off-by: Danilo Krummrich <dakr@kernel.org>
> >
> > I'm not convinced a collect implementation specific to IntoIter is necessary?
>
> For the reasons above, we can't implement `FromIterator`. At some point we may
> want to implement our own kernel `FromIterator` trait, but that's out of scope
> for this series.
>
> For now, I just want to provide a way to get a `Vec` from `IntoIter` again,
> which without `Vec::collect` would be impossible.

If you have a need for a collect on this particular kind of iterator, then okay.

> > > +
> > > + // 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) } {
> >
> > Why would you shrink it? You can just keep the capacity.
>
> What if the vector was pretty huge and meanwhile as advanced by a lot? I think
> we don't want to waste those resources.
>
> Ideally the corresponding `Allocator` implements a proper heuristic for when to
> actually shrink. For instance, krealloc() never shrinks, and it's probably not
> worth it. For vrealloc() though we clearly want to shrink properly (i.e. unmap
> and free spare pages) at some point.

The Rust Vec never shrinks unless explicitly requested. But I guess
it's okay either way.

Alice