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

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Message-ID: <20250915102918.77adc04a@nimda.home>
Date: Mon, 15 Sep 2025 10:29:18 +0300
From: Onur <work@...rozkan.dev>
To: Alice Ryhl <aliceryhl@...gle.com>
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, 15 Sep 2025 09:26:00 +0200
Alice Ryhl <aliceryhl@...gle.com> wrote:

> 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

Good to know, thanks!

Onur