linux-kernel - Re: [PATCH v7 13/26] rust: alloc: implement kernel `Vec` type

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Message-ID: <Zvf58aEqGEamm7s8@pollux>
Date: Sat, 28 Sep 2024 14:43:29 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: Benno Lossin <benno.lossin@...ton.me>
Cc: ojeda@...nel.org, alex.gaynor@...il.com, wedsonaf@...il.com,
	boqun.feng@...il.com, gary@...yguo.net, bjorn3_gh@...tonmail.com,
	a.hindborg@...sung.com, aliceryhl@...gle.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,
	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 v7 13/26] rust: alloc: implement kernel `Vec` type

On Thu, Sep 26, 2024 at 01:47:04PM +0000, Benno Lossin wrote:
> On 12.09.24 00:52, Danilo Krummrich wrote:
> > +    /// Appends an element to the back of the [`Vec`] instance.
> > +    ///
> > +    /// # Examples
> > +    ///
> > +    /// ```
> > +    /// let mut v = KVec::new();
> > +    /// v.push(1, GFP_KERNEL)?;
> > +    /// assert_eq!(&v, &[1]);
> > +    ///
> > +    /// v.push(2, GFP_KERNEL)?;
> > +    /// assert_eq!(&v, &[1, 2]);
> > +    /// # Ok::<(), Error>(())
> > +    /// ```
> > +    pub fn push(&mut self, v: T, flags: Flags) -> Result<(), AllocError> {
> > +        Vec::reserve(self, 1, flags)?;
> > +
> > +        // SAFETY:
> > +        // - `self.len` is smaller than `self.capacity` and hence, the resulting pointer is
> > +        //   guaranteed to be part of the same allocated object.
> > +        // - `self.len` can not overflow `isize`.
> > +        let ptr = unsafe { self.as_mut_ptr().add(self.len) };
> > +
> > +        // SAFETY:
> > +        // - `ptr` is properly aligned and valid for writes.
> > +        unsafe { core::ptr::write(ptr, v) };
> 
> Why not use `self.spare_capacity_mut()[0].write(v);`?

Before v7 I did exactly that, but in v6 you suggested to use the raw pointer
instead to avoid the bounds check.

> 
> If you want to avoid the bounds check, you can do
> 
>     let first = self.spare_capacity_mut().first();
>     // SAFETY: the call to `Vec::reserve` above ensures that `spare_capacity_mut()` is non-empty.
>     unsafe { first.unwrap_unchecked() }.write(v);

`first` does a similar check to create the `Option<&T>`, right?. I'd rather keep
the raw pointer access as suggested in v6.