linux-kernel - Re: [PATCH 02/10] rust: alloc: introduce the `VecExt` trait

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Date: Mon, 25 Mar 2024 22:05:33 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Wedson Almeida Filho <wedsonaf@...il.com>
Cc: rust-for-linux@...r.kernel.org, Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, Andreas Hindborg <a.hindborg@...sung.com>, Alice Ryhl <aliceryhl@...gle.com>, linux-kernel@...r.kernel.org, Wedson Almeida Filho <walmeida@...rosoft.com>
Subject: Re: [PATCH 02/10] rust: alloc: introduce the `VecExt` trait

On 25.03.24 20:54, Wedson Almeida Filho wrote:> From: Wedson Almeida Filho <walmeida@...rosoft.com>
> 
> Make `try_with_capacity`, `try_push`, and `try_extend_from_slice`
> methods available in `Vec` even though it doesn't implement them. It is
> implemented with `try_reserve` and `push_within_capacity`.
> 
> This is in preparation for switching to the upstream `alloc` crate.
> 
> Suggested-by: Gary Guo <gary@...yguo.net>
> Signed-off-by: Wedson Almeida Filho <walmeida@...rosoft.com>
> ---
>   rust/kernel/alloc.rs        |  1 +
>   rust/kernel/alloc/vecext.rs | 52 +++++++++++++++++++++++++++++++++++++
>   rust/kernel/lib.rs          |  1 +
>   rust/kernel/prelude.rs      |  2 ++
>   4 files changed, 56 insertions(+)
>   create mode 100644 rust/kernel/alloc/vecext.rs
> 
> diff --git a/rust/kernel/alloc.rs b/rust/kernel/alloc.rs
> index ccd4149932c3..8ad57a2e693e 100644
> --- a/rust/kernel/alloc.rs
> +++ b/rust/kernel/alloc.rs
> @@ -5,3 +5,4 @@
>   #[cfg(not(test))]
>   #[cfg(not(testlib))]
>   mod allocator;
> +pub mod vecext;
> diff --git a/rust/kernel/alloc/vecext.rs b/rust/kernel/alloc/vecext.rs
> new file mode 100644
> index 000000000000..59e92bab534e
> --- /dev/null
> +++ b/rust/kernel/alloc/vecext.rs
> @@ -0,0 +1,52 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Extensions to [`Vec`] for fallible allocations.
> +
> +use alloc::{collections::TryReserveError, vec::Vec};
> +use core::result::Result;
> +
> +/// Extensions to [`Vec`].
> +pub trait VecExt<T>: Sized {
> +    /// Creates a new [`Vec`] instance with at least the given capacity.
> +    fn try_with_capacity(capacity: usize) -> Result<Self, TryReserveError>;
> +
> +    /// Appends an element to the back of the [`Vec`] instance.
> +    fn try_push(&mut self, v: T) -> Result<(), TryReserveError>;
> +
> +    /// Pushes clones of the elements of slice into the [`Vec`] instance.
> +    fn try_extend_from_slice(&mut self, other: &[T]) -> Result<(), TryReserveError>
> +    where
> +        T: Clone;

All of the non `try_` prefix functions require 
`not(no_global_oom_handling)`, so we could also drop the `try_` prefix
here.
What do you think?

> +}
> +
> +impl<T> VecExt<T> for Vec<T> {
> +    fn try_with_capacity(capacity: usize) -> Result<Self, TryReserveError> {
> +        let mut v = Vec::new();
> +        v.try_reserve(capacity)?;
> +        Ok(v)
> +    }
> +
> +    fn try_push(&mut self, v: T) -> Result<(), TryReserveError> {
> +        if let Err(retry) = self.push_within_capacity(v) {
> +            self.try_reserve(1)?;
> +            let _ = self.push_within_capacity(retry);
> +        }
> +        Ok(())
> +    }
> +
> +    fn try_extend_from_slice(&mut self, other: &[T]) -> Result<(), TryReserveError>
> +    where
> +        T: Clone,
> +    {
> +        let extra_cap = self.capacity() - self.len();
> +        if extra_cap > 0 {
> +            self.try_reserve(extra_cap)?;
> +        }

I am confused, why are you doing this? Did you mean to do this?:
    let extra_cap = self.capacity() - self.len() - other.len();

-- 
Cheers,
Benno

> +
> +        for item in other {
> +            self.try_push(item.clone())?;
> +        }
> +
> +        Ok(())
> +    }
> +}
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index 51f30e55bd00..7f2841a18d05 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -19,6 +19,7 @@
>   #![feature(offset_of)]
>   #![feature(receiver_trait)]
>   #![feature(unsize)]
> +#![feature(vec_push_within_capacity)]
> 
>   // Ensure conditional compilation based on the kernel configuration works;
>   // otherwise we may silently break things like initcall handling.
> diff --git a/rust/kernel/prelude.rs b/rust/kernel/prelude.rs
> index ae21600970b3..a0177f195dec 100644
> --- a/rust/kernel/prelude.rs
> +++ b/rust/kernel/prelude.rs
> @@ -14,6 +14,8 @@
>   #[doc(no_inline)]
>   pub use core::pin::Pin;
> 
> +pub use crate::alloc::vecext::VecExt;
> +
>   #[doc(no_inline)]
>   pub use alloc::{boxed::Box, vec::Vec};
> 
> --
> 2.34.1
>