linux-kernel - Re: [PATCH v2 2/3] rust: alloc: add Vec::resize method

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Message-ID: <CAJ-ks9kAROOfyPtxMe6LE4-UPsvXca2sQ2VDjhRchZp3HLddGg@mail.gmail.com>
Date: Tue, 18 Mar 2025 16:12:53 -0400
From: Tamir Duberstein <tamird@...il.com>
To: Andrew Ballance <andrewjballance@...il.com>
Cc: dakr@...nel.org, airlied@...il.com, simona@...ll.ch, 
	maarten.lankhorst@...ux.intel.com, mripard@...nel.org, tzimmermann@...e.de, 
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	gary@...yguo.net, bjorn3_gh@...tonmail.com, benno.lossin@...ton.me, 
	a.hindborg@...nel.org, aliceryhl@...gle.com, tmgross@...ch.edu, 
	acourbot@...dia.com, nouveau@...ts.freedesktop.org, 
	dri-devel@...ts.freedesktop.org, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v2 2/3] rust: alloc: add Vec::resize method

On Sun, Mar 16, 2025 at 7:17 AM Andrew Ballance
<andrewjballance@...il.com> wrote:
>
> implement the equivalent of the rust std's Vec::resize
> on the kernel's Vec type.
>
> Signed-off-by: Andrew Ballance <andrewjballance@...il.com>
> ---
>  rust/kernel/alloc/kvec.rs | 26 ++++++++++++++++++++++++++
>  1 file changed, 26 insertions(+)
>
> diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
> index 18bcc59f0b38..eb6d40a1bf8b 100644
> --- a/rust/kernel/alloc/kvec.rs
> +++ b/rust/kernel/alloc/kvec.rs
> @@ -554,6 +554,32 @@ pub fn from_elem(value: T, n: usize, flags: Flags) -> Result<Self, AllocError> {
>
>          Ok(v)
>      }
> +
> +    /// Resizes the [`Vec`] so that `len` is equal to `new_len`.
> +    ///
> +    /// If `new_len` is smaller than `len`, the `Vec` is [`Vec::truncate`]d.
> +    /// If `new_len` is larger, each new slot is filled with clones of `value`.
> +    ///
> +    /// # Examples
> +    ///
> +    /// ```
> +    /// let mut v = kernel::kvec![1, 2, 3]?;
> +    /// v.resize(1, 42, GFP_KERNEL)?;
> +    /// assert_eq!(&v, &[1]);
> +    ///
> +    /// v.resize(3, 42, GFP_KERNEL)?;
> +    /// assert_eq!(&v, &[1, 42, 42]);
> +    ///
> +    /// # Ok::<(), Error>(())
> +    /// ```
> +    pub fn resize(&mut self, new_len: usize, value: T, flags: Flags) -> Result<(), AllocError> {
> +        if new_len > self.len() {
> +            self.extend_with(new_len - self.len(), value, flags)
> +        } else {
> +            self.truncate(new_len);
> +            Ok(())
> +        }
> +    }

You can avoid underflow checking in debug builds by using `checked_sub`:

        match new_len.checked_sub(self.len()) {
            Some(n) => self.extend_with(n, value, flags),
            None => {
                self.truncate(new_len);
                Ok(())
            }
        }

>  }
>
>  impl<T, A> Drop for Vec<T, A>
> --
> 2.48.1
>
>

Either way:

Reviewed-by: Tamir Duberstein <tamird@...il.com>