linux-kernel - Re: [PATCH 1/2] rust: Add initial cpumask abstractions

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Message-ID: <CAH5fLghUDz8tpkTuvWZgwT2_VKgxuS+iZKnoT4prKwS9tbp8wA@mail.gmail.com>
Date: Tue, 25 Feb 2025 12:53:40 +0100
From: Alice Ryhl <aliceryhl@...gle.com>
To: Viresh Kumar <viresh.kumar@...aro.org>
Cc: Yury Norov <yury.norov@...il.com>, Rasmus Villemoes <linux@...musvillemoes.dk>, 
	Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, 
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Trevor Gross <tmgross@...ch.edu>, linux-kernel@...r.kernel.org, 
	Danilo Krummrich <dakr@...hat.com>, rust-for-linux@...r.kernel.org, 
	Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH 1/2] rust: Add initial cpumask abstractions

On Tue, Feb 25, 2025 at 12:48 PM Viresh Kumar <viresh.kumar@...aro.org> wrote:
>
> On 25-02-25, 10:55, Alice Ryhl wrote:
> > On Tue, Feb 25, 2025 at 10:47 AM Viresh Kumar <viresh.kumar@...aro.org> wrote:
> > > +    /// Creates cpumask.
> > > +    #[cfg(not(CONFIG_CPUMASK_OFFSTACK))]
> > > +    fn new_inner(empty: bool) -> Result<Self> {
> > > +        let ptr = KBox::into_raw(KBox::new([bindings::cpumask::default(); 1], GFP_KERNEL)?);
> >
> > I don't really understand this CPUMASK_OFFSTACK logic. You seem to
> > always allocate memory, but if OFFSTACK=n, then shouldn't it be on the
> > stack ...?
>
> IIUC, the idea of the config option is to prevent stack overflow on
> systems with high number of CPUs (> 256), in which case the memory for
> the masks is allocated dynamically. Otherwise a local variable, in a
> function or a struct (which may itself get allocated dynamically) is
> fine.
>
> In the CONFIG_CPUMASK_OFFSTACK=y case, the cpumask C core does the
> allocation and the Rust code doesn't need to take care of the same.
>
> In the CONFIG_CPUMASK_OFFSTACK=n case, the allocation must be done by
> the caller (on stack or heap) and the cpumask C core will only clear
> the mask.
>
> I tried with an on-stack variable earlier but ran into problems as the
> memory is shared with the C FFI and Rust moves it unless it is pinned.

Is it a problem if a value of type struct cpumask is moved? It looks
like it is just an array of longs?

Alice