linux-kernel - Re: [PATCH 2/2] rust: alloc: add `Vec::dec

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Message-ID: <CAJ-ks9mJYyJK7iQz+qS2TnwNgTqp3rRWGTv6hiPhJ3v6aWsLng@mail.gmail.com>
Date: Sun, 16 Mar 2025 19:27:22 -0400
From: Tamir Duberstein <tamird@...il.com>
To: Danilo Krummrich <dakr@...nel.org>
Cc: Andrew Ballance <andrewjballance@...il.com>, Alice Ryhl <aliceryhl@...gle.com>, 
	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>, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] rust: alloc: add `Vec::dec_len`

On Sun, Mar 16, 2025 at 7:02 PM Danilo Krummrich <dakr@...nel.org> wrote:
>
> On Sun, Mar 16, 2025 at 06:47:42PM -0400, Tamir Duberstein wrote:
> > On Sun, Mar 16, 2025 at 6:42 PM Danilo Krummrich <dakr@...nel.org> wrote:
> > >
> > > On Sun, Mar 16, 2025 at 06:32:01PM -0400, Tamir Duberstein wrote:
> > > > Add `Vec::dec_len` that reduces the length of the receiver. This method
> > > > is intended to be used from methods that remove elements from `Vec` such
> > > > as `truncate`, `pop`, `remove`, and others. This method is intentionally
> > > > not `pub`.
> > > >
> > > > Signed-off-by: Tamir Duberstein <tamird@...il.com>
> > > > ---
> > > >  rust/kernel/alloc/kvec.rs | 15 +++++++++++++++
> > > >  1 file changed, 15 insertions(+)
> > > >
> > > > diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
> > > > index d43a1d609434..5d604e04b9a5 100644
> > > > --- a/rust/kernel/alloc/kvec.rs
> > > > +++ b/rust/kernel/alloc/kvec.rs
> > > > @@ -195,6 +195,21 @@ pub unsafe fn inc_len(&mut self, additional: usize) {
> > > >          self.len += additional;
> > > >      }
> > > >
> > > > +    /// Decreases `self.len` by `count`.
> > > > +    ///
> > > > +    /// Returns a mutable reference to the removed elements.
> > > > +    ///
> > > > +    /// # Safety
> > > > +    ///
> > > > +    /// - `count` must be less than or equal to `self.len`.
> > >
> > > Why? We can catch this, no?
> > >
> > > We can keep the debug_assert!(), but use self.len.saturating_sub(count) instead.
> >
> > This is why I didn't want to write this until we had an actual caller :)
>
> That just defers this question, the methods you mention in your commit message
> will be added, hence I think it's better to do it right away.
>
> > We can, but it's not clear why that's better. What does it mean if the
> > caller asked to decrement by more than self.len?
>
> It tells us that the caller is buggy, but that's what the debug_assert!() is
> for.
>
> But to me both is fine, it's also good when the caller has to justify.

Ok! I've left this as-is.

> Forgot to mention, for dec_len(), please add the corresponding invariant comment
> when adjusting self.len.

Does this suit?

>         // INVARIANT: By the safety requirements of this method `self.len - count` represents the
>         // exact number of elements stored within `self`.