On Wed, Jul 8, 2026 at 11:08 PM Alice Ryhl <[email protected]> wrote: > > On Wed, Jul 08, 2026 at 04:37:38PM -0400, Yury Norov wrote: > > On Wed, Jul 08, 2026 at 07:33:16AM +0200, Greg KH wrote: > > > > ... > > > > > > I asked exactly the same question when Alice and Burak added wrappers > > > > for bitmaps to implement their ID pool. This is the answer: > > > > > > > > An alternative route of vendoring an existing Rust bitmap package was > > > > considered but suboptimal overall. Reusing the C implementation is > > > > preferable for a basic data structure like bitmaps. It enables Rust > > > > code to be a lot more similar and predictable with respect to C code > > > > that uses the same data structures and enables the use of code that > > > > has been tried-and-tested in the kernel, with the same performance > > > > characteristics whenever possible. > > > > > > > > And now it's in a commit message: 11eca92a2caeb > > > > > > > > They measured the affect of their wrapper on performance, and it appears > > > > to be ~5%. See lib/find_bit_benchmark_rust.rs. > > > > > > You are comparing the C vs. Rust data structures here, which is not what > > > I am proposing. > > > > > > Also, is this code being used on a "hot path" like the binder stuff is? > > > > > > > I didn't see any side-to-side comparison between any native Rust API vs > > > > imported C bitmaps. I'm sure, I asked for that, and I still believe > > > > it's the important piece of data to avoid this back-and-forth type of > > > > discussions. So, Alice, Burak or anybody... > > > > > > Again, I'm not talking about Rust API vs. imported C bitmaps, I'm asking > > > to use the C structures like maple-tree and idr instead of open-coding > > > logic around the bitmap code. > > > > I understand your point. I asked both questions: are they sure that bitmap > > is the most optimal data structure for the ID pool, and if so, why not use > > the built-in Rust bitmaps? The answer was: yes, it's the most optimal, and > > using built-in bitmaps is suboptimal overall. > > The Rust standard library doesn't really have a bitmap abstraction to > begin with, so if we want to use bitmap then it's either handrolled > bit-manipulation or the C bitmap api. > > cc'ing Burak's new email >
(Thanks, I will update my email in the MAINTAINERS file.) Binder needs to acquire bits *while holding a spinlock*. This is the biggest constraint. My understanding is that the IDA API call might need to allocate memory (and sleep) when getting a new ID, as a side-effect. So for the binder use case, it was not an option. @Yury, about the alternative of vendoring Rust bitmap crates: that would have duplicated the kernel's C bitmap functionality. There does not seem much point in duplicating that, and opening the door to have Rust and C code differently named API functions and such. So beside performance, maintenance and coherence are also a factor. The Rust ID pool code is merely a port of some existing bitmap-specific binder C code (that can hopefully be deleted one day) that deals with spinlock allocation fun. If the constraints match, I think it is good to see it generalized and used elsewhere. That is a conditional statement, I am not familiar with the nova use case's performance/allocation/sleep constraints. -- Burak
