linux-kernel - Re: [PATCH v6 13/26] rust: alloc: implement kernel `Vec` type

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Message-ID: <367ca85d-3ed2-422a-bd90-2fc53fdf25b5@proton.me>
Date: Tue, 10 Sep 2024 19:33:43 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Danilo Krummrich <dakr@...nel.org>, Boqun Feng <boqun.feng@...il.com>
Cc: ojeda@...nel.org, alex.gaynor@...il.com, wedsonaf@...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 v6 13/26] rust: alloc: implement kernel `Vec` type

On 10.09.24 20:26, Danilo Krummrich wrote:
> On Tue, Sep 03, 2024 at 12:08:08PM -0700, Boqun Feng wrote:
>> On Fri, Aug 16, 2024 at 02:10:55AM +0200, Danilo Krummrich wrote:
>>> `Vec` provides a contiguous growable array type (such as `Vec`) with
>>> contents allocated with the kernel's allocators (e.g. `Kmalloc`,
>>> `Vmalloc` or `KVmalloc`).
>>>
>>> In contrast to Rust's `Vec` type, the kernel `Vec` type considers the
>>> kernel's GFP flags for all appropriate functions, always reports
>>> allocation failures through `Result<_, AllocError>` and remains
>>> independent from unstable features.
>>>
>>> Signed-off-by: Danilo Krummrich <dakr@...nel.org>
>>> ---
>> [...]
>>> +
>>> +impl<T, A> Vec<T, A>
>>> +where
>>> +    A: Allocator,
>>> +{
>> [...]
>>> +    /// Forcefully sets `self.len` to `new_len`.
>>> +    ///
>>> +    /// # Safety
>>> +    ///
>>> +    /// - `new_len` must be less than or equal to [`Self::capacity`].
>>> +    /// - If `new_len` is greater than `self.len`, all elements within the interval
>>> +    ///   [`self.len`,`new_len`] must be initialized.
>>
>> Maybe use "[`self.len`, `new_len`)" to indicate `new_len` side is open?
> 
> Agreed.

Alternatively just use the rust range operator `self.len..new_len`, it
is exclusive by default (for an inclusive range you can use `..=`).

>> Also `self.len` may confuse people whether it's the old length or new
>> length, could you use `old_len` and add note saying "`old_len` is the
>> length before `set_len()`?

I personally think this is not a big issue, since safety requirements
are preconditions to calling a function (so the function couldn't have
been called yet).

> What about:
> 
>    /// - If `new_len` is greater than `self.len` (from before calling this function), all elements
>    ///   within the interval [`self.len`,`new_len`] must be initialized.

You will still apply the range fix, right?

---
Cheers,
Benno