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

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Message-ID: <aCwRZlkBWekRmDg7@Mac.home>
Date: Mon, 19 May 2025 22:21:42 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Alice Ryhl <aliceryhl@...gle.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 08:46:37PM -0700, Alice Ryhl wrote:
> 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.
> 

Ok, but it shouldn't be hard to write a Rust function that search that,
right? My point was mostly the Rust rbtree binding can do O(log n)
search. I have no idea about "even so, should we try something like Jann
suggested". And I think your other reply basically says no.

Regards,
Boqun

> Alice