linux-kernel - Re: [PATCH] rust: less allocation in CString::try

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Message-ID: <CAH5fLgi-VrhDSEjJA4Bg+5Wmr2JDAmmOc1FkfZAZ29SMqkX++w@mail.gmail.com>
Date: Mon, 15 Sep 2025 09:26:00 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Onur Özkan <work@...rozkan.dev>
Cc: rust-for-linux@...r.kernel.org, ojeda@...nel.org, alex.gaynor@...il.com, 
	boqun.feng@...il.com, gary@...yguo.net, bjorn3_gh@...tonmail.com, 
	lossin@...nel.org, a.hindborg@...nel.org, tmgross@...ch.edu, dakr@...nel.org, 
	tamird@...il.com, daniel@...lak.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rust: less allocation in CString::try_from

On Mon, Sep 15, 2025 at 8:54 AM Onur Özkan <work@...rozkan.dev> wrote:
>
> Allocates buffer with the correct capacity upfront by
> using the length of the `CStr` to avoid extra and unnecessary
> re-allocation when converting from `CStr` to `CString`.
>
> Signed-off-by: Onur Özkan <work@...rozkan.dev>
> ---
>  rust/kernel/str.rs | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/rust/kernel/str.rs b/rust/kernel/str.rs
> index 6c892550c0ba..98d41d995e45 100644
> --- a/rust/kernel/str.rs
> +++ b/rust/kernel/str.rs
> @@ -946,9 +946,10 @@ impl<'a> TryFrom<&'a CStr> for CString {
>      type Error = AllocError;
>
>      fn try_from(cstr: &'a CStr) -> Result<CString, AllocError> {
> -        let mut buf = KVec::new();
> +        let bytes = cstr.to_bytes_with_nul();
>
> -        buf.extend_from_slice(cstr.to_bytes_with_nul(), GFP_KERNEL)?;
> +        let mut buf = KVec::with_capacity(bytes.len(), GFP_KERNEL)?;
> +        buf.extend_from_slice(bytes, GFP_KERNEL)?;

I don't think this changes the number of allocations. KVec::new() does
not allocate.

Alice