Re: [PATCH 3/4] rust: id_pool: add contiguous area allocation

[Yury Norov <ynorov@nvidia.com>]

On Tue, Jul 07, 2026 at 04:13:30PM +0200, Greg KH wrote:
> On Tue, Jul 07, 2026 at 10:25:27PM +0900, Eliot Courtney wrote:
> > On Fri Jul 3, 2026 at 7:31 PM JST, Greg KH wrote:
> > > On Fri, Jul 03, 2026 at 07:16:06PM +0900, Eliot Courtney wrote:
> > >> Add support for contiguous area allocation. Add a new type,
> > >> `UnusedArea`, following the same pattern as `UnusedId`.
> > >> 
> > >> Signed-off-by: Eliot Courtney <ecourtney@nvidia.com>
> > >
> > > Why isn't the built-in idr library being used here instead of rolling
> > > your own data structure?
> > >
> > > thanks,
> > >
> > > greg k-h
> > 
> > For nova-core in this series, we need allocation of a contiguous
> > sequence of IDs with a specific length and sometimes a specific
> > alignment. IIUC, IDA/xarray do not support that (I checked
> > ida_alloc_range and it only allocates a single ID in a range, not a
> > contiguous sequence).
> > 
> > For IdPool before this series, I think it could have used IDA/xarray.
> > See [1] where Alice has posted some more context.
> > 
> > w.r.t. the structure choice, the IDs we need to allocate are channel
> > IDs, and the total range is limited to 2048 of them, so IMO bitmaps are
> > a better fit than e.g. maple tree.
> 
> But again, you are having to "roll your own" logic here, please reuse
> the data structures we already have in the kernel for this type of
> thing.  If a maple tree works, please use it.

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.

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...

> > > 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'.

This series extends the existing data structure in Rust for Linux, not
adding a new one. So, the series itself looks justified to me because
it's a logical extension of the existing functionality.

Regarding C bindings vs native API... Yes we lack thorough performance
analysis. Do we have such analysis for every data structure in kernel?

No. So to me it's as simple as: they are developers, they know better.

Thanks,
Yury