linux-kernel - Re: [PATCH v8 5/5] rust: add dynamic ID pool abstraction for bitmap

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Message-ID: <CAH5fLghNJYjxPFUc2E4-2pJpGT5umUr1EJstZvs88ox3MsXDGQ@mail.gmail.com>
Date: Mon, 19 May 2025 20:46:37 -0700
From: Alice Ryhl <aliceryhl@...gle.com>
To: Boqun Feng <boqun.feng@...il.com>
Cc: Jann Horn <jannh@...gle.com>, Burak Emir <bqe@...gle.com>, Yury Norov <yury.norov@...il.com>, 
	Kees Cook <kees@...nel.org>, Rasmus Villemoes <linux@...musvillemoes.dk>, 
	Viresh Kumar <viresh.kumar@...aro.org>, Miguel Ojeda <ojeda@...nel.org>, 
	Alex Gaynor <alex.gaynor@...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>, "Gustavo A . R . Silva" <gustavoars@...nel.org>, 
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH v8 5/5] rust: add dynamic ID pool abstraction for bitmap

On Mon, May 19, 2025 at 4:56 PM Boqun Feng <boqun.feng@...il.com> wrote:
>
> On Tue, May 20, 2025 at 12:51:07AM +0200, Jann Horn wrote:
> > On Mon, May 19, 2025 at 6:20 PM Burak Emir <bqe@...gle.com> wrote:
> > > This is a port of the Binder data structure introduced in commit
> > > 15d9da3f818c ("binder: use bitmap for faster descriptor lookup") to
> > > Rust.
> >
> > Stupid high-level side comment:
> >
> > That commit looks like it changed a simple linear rbtree scan (which
> > is O(n) with slow steps) into a bitmap thing. A more elegant option
> > might have been to use an augmented rbtree, reducing the O(n) rbtree
> > scan to an O(log n) rbtree lookup, just like how finding a free area
>
> I think RBTree::cursor_lower_bound() [1] does exactly what you said

We need the smallest ID without a value, not the smallest ID in use.

Alice