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

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Message-ID: <ZqzK2LeUgX1AQP4E@pollux>
Date: Fri, 2 Aug 2024 14:02:32 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: ojeda@...nel.org, alex.gaynor@...il.com, wedsonaf@...il.com,
	boqun.feng@...il.com, gary@...yguo.net, bjorn3_gh@...tonmail.com,
	benno.lossin@...ton.me, a.hindborg@...sung.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,
	acurrid@...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 v3 17/25] rust: alloc: implement `collect` for `IntoIter`

On Fri, Aug 02, 2024 at 09:08:48AM +0200, Alice Ryhl wrote:
> On Thu, Aug 1, 2024 at 5:37 PM Danilo Krummrich <dakr@...nel.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@...nel.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@...nel.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.

Even once we have our own `FromIterator` trait, we probably want to keep this
specific one besides the generic `collect` for optimization. With the generic
one we'd otherwise copy into a new `Vec`.

> 
> > > > +
> > > > +        // 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.

It actually does, see [1]. But Rust's `Vec` does it less efficiently. It either
keeps the `Vec` as it is, or creates a whole new one.

[1] https://github.com/rust-lang/rust/blob/master/library/alloc/src/vec/spec_from_iter.rs#L39

> 
> Alice
>