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 > > > > > > Why isn't the built-in idr library being used here instead of > > > > > > rolling > > > > > > your own data structure? > > > > > > Now having more context, the ID pool's primary goal is to allocate > > > individual IDs, which naturally lays on find_bit() API in C. The > > > native Rust alternative is considered and found 'suboptimal overall'. > > > > Allocating IDs is a probe() thing, which can be as slow as it wants, > > right? Or is this some other hot-path where performance matters? The > > patch was not very specific as to the tradeoffs needed. > > I am a bitmaps maintainer, and I want them in Rust to perform equally well. > The ID pool case was just the first user. Even if bitmap performance is > not a critical path for ID pool, my role is to make sure that bitmap in > Rust is implemented well, including the performance part. > > Said that, I recall that bitmaps performance was important for the ID > pool, and Alice and Burak spent some time optimizing the ID allocation > path. Particularly, added a logic to avoid allocation of small bitmaps. Yeah the Binder use-case is not the same as probe(). It's a per-fd data structure used to give names to kernel objects that userspace is manipulating, and the mapping is per-process. Alice
