Re: [PATCH 2/4] rust: bitmap: add contiguous area operations

[Alice Ryhl <aliceryhl@google.com>]

On Mon, Jul 06, 2026 at 12:29:00PM -0400, Yury Norov wrote:
> On Fri, Jul 03, 2026 at 07:16:05PM +0900, Eliot Courtney wrote:
> > Add bindings and helpers for area operations on bitmaps. Each one is
> > made safe by adding some extra checks compared to the underlying C code
> > (for example, checking bounds) and with additional checks to catch
> > likely erroneous usage if `CONFIG_RUST_BITMAP_HARDENED` is on.
> > 
> > The C code uses signed integers for some parameters, for example the
> > length for `__bitmap_set`, so bounds check against i32::MAX. We can't
> > rely on `BitmapVec::MAX_LEN` because `Bitmap` may not necessarily be
> > backed by `BitmapVec`. There's also a few cases where a non power of two
> > minus one `align_mask` can cause an infinite loop in the C code (can
> > happen on overflow), so check for that.
> > 
> > Add tests demonstrating the edge cases.
> > 
> > Signed-off-by: Eliot Courtney <ecourtney@nvidia.com>
> > ---
> >  rust/helpers/bitmap.c |  22 +++++
> >  rust/kernel/bitmap.rs | 219 ++++++++++++++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 241 insertions(+)
> > 
> > diff --git a/rust/helpers/bitmap.c b/rust/helpers/bitmap.c
> > index e4e9f4361270..dac5c03f2448 100644
> > --- a/rust/helpers/bitmap.c
> > +++ b/rust/helpers/bitmap.c
> > @@ -8,3 +8,25 @@ void rust_helper_bitmap_copy_and_extend(unsigned long *to, const unsigned long *
> >  {
> >  	bitmap_copy_and_extend(to, from, count, size);
> >  }
> > +
> > +__rust_helper
> > +unsigned long rust_helper_bitmap_find_next_zero_area(unsigned long *map,
> > +						     unsigned long size,
> > +						     unsigned long start,
> > +						     unsigned int nr,
> > +						     unsigned long align_mask)
> > +{
> > +	return bitmap_find_next_zero_area(map, size, start, nr, align_mask);
> > +}
> > +
> > +__rust_helper
> > +void rust_helper_bitmap_set(unsigned long *map, unsigned int start, unsigned int nbits)
> > +{
> > +	bitmap_set(map, start, nbits);
> > +}
> > +
> > +__rust_helper
> > +void rust_helper_bitmap_clear(unsigned long *map, unsigned int start, unsigned int nbits)
> > +{
> > +	bitmap_clear(map, start, nbits);
> > +}
> 
> All three are the wrappers around the regular outline functions:
> bitmap_find_next_zero_area_off(), __bitmap_set() and __bitmap_clear().
> 
> The inlined version in headers is optimized for small bitmaps. But the
> optimization is all based on inlining in the C code. It doesn't work
> if you wrap it with a rust helper.
> 
> There was a discussion about the similar find_next_bit(). The function
> itself is an inliner, but it's a wrapper around the true outlined
> _find_next_bit(). So we decided to minimize the binder size for that
> type of functions.
> 
> Please, keep the binder minimal unless necessary.

If we can just call __bitmap_clear() without any downsides, then I agree
we should do that, but we actually do support inlining the helpers now.
See CONFIG_RUST_INLINE_HELPERS.

Alice