linux-kernel - Re: [PATCH v2 2/3] rust: alloc: add Vec::resize method

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Message-ID: <CAJ-ks9=23qEqxT5rivsbfNYC6iUP4RXsKbcDU9XDx32ERnKbYg@mail.gmail.com>
Date: Wed, 19 Mar 2025 12:13:14 -0400
From: Tamir Duberstein <tamird@...il.com>
To: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Cc: Benno Lossin <benno.lossin@...ton.me>, Andrew Ballance <andrewjballance@...il.com>, dakr@...nel.org, 
	airlied@...il.com, simona@...ll.ch, maarten.lankhorst@...ux.intel.com, 
	mripard@...nel.org, tzimmermann@...e.de, corbet@....net, ojeda@...nel.org, 
	alex.gaynor@...il.com, boqun.feng@...il.com, gary@...yguo.net, 
	bjorn3_gh@...tonmail.com, a.hindborg@...nel.org, aliceryhl@...gle.com, 
	tmgross@...ch.edu, acourbot@...dia.com, nouveau@...ts.freedesktop.org, 
	dri-devel@...ts.freedesktop.org, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v2 2/3] rust: alloc: add Vec::resize method

On Wed, Mar 19, 2025 at 12:06 PM Miguel Ojeda
<miguel.ojeda.sandonis@...il.com> wrote:
>
> On Wed, Mar 19, 2025 at 4:59 PM Tamir Duberstein <tamird@...il.com> wrote:
> >
> > If we're talking about the same thing then I think we're both wrong
> > and the correct phrasing would have been: "you can avoid underflow
> > checking when CONFIG_RUST_OVERFLOW_CHECKS=y by using `checked_sub`". I
> > was referring to the underflow check implicit in `new_len -
> > self.len()`.
>
> `checked_sub` always checks (if not optimized away). The config option
> is about the implicit one.
>
> Do you mean avoiding panics?

No, I meant avoiding the check. The existing code already explicitly
checks `new_len > self.len()` before evaluating `new_len -
self.len()`. This means the check occurs twice. `checked_sub` reduces
the number of checks by 1. Perhaps my wording could have been clearer
("avoid *an* underflow check").

Tamir