Re: [PATCH v3] rust: add bindings and API for bitmap.h and bitops.h.

From: Tamir Duberstein
Date: Mon Mar 10 2025 - 12:51:47 EST

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In reply to: Burak Emir: "[PATCH v3] rust: add bindings and API for bitmap.h and bitops.h."
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Hi Burak, some comments inline. Hopefully I haven't missed important
context from previous versions.

On Mon, Mar 10, 2025 at 12:21 PM Burak Emir <bqe@xxxxxxxxxx> wrote:
>
> Adds a Rust bitmap API and necessary bitmap and bitops bindings.
> These are for porting the approach from commit 15d9da3f818c ("binder:
> use bitmap for faster descriptor lookup") to Rust. The functionality
> in dbitmap.h makes use of bitmap and bitops.
>
> The Rust bitmap API provides an abstraction to underlying bitmap
> and bitops operations. For now, we only include methods that are
> necessary for reimplementing dbitmap.h. It is straightforward to add
> more methods later, as needed. We offer a safe API through
> bounds checks which panic if violated.
>
> We introduce bindings for the non-atomic variants __set_bit and
> __clear_bit, and use the _find_* variants instead of the find_*
> wrappers which enable small size optimization in C. These C
> small size optimizations do not carry over to Rust. The
> principle followed is that whenever there are plain variants, we use
> those.
>
> This series uses the usize type for sizes and indices into the bitmap,
> because Rust generally always uses that type for indices and lengths
> and it will be more convenient if the API accepts that type. This means
> that we need to perform some casts to/from u32 and usize, since the C
> headers use unsigned int instead of size_t/unsigned long for these
> numbers in some places.
>
> Suggested-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
> Signed-off-by: Burak Emir <bqe@xxxxxxxxxx>
> ---
> This is v3 of a patch introducing Rust bitmap API [v2]. Thanks
> for all the helpful comments!
>
> Changes v2 --> v3:
> - change `bitmap_copy` to `copy_from_bitmap_and_extend` which
> zeroes out extra bits. This enables dbitmap shrink and grow use
> cases while offering a consistent and understandable Rust API for
> other uses (Alice)
>
> Changes v1 --> v2:
> - Rebased on Yury's v2 patch [1] and Viresh's v2 patch [2]
> - Removed import of `bindings::*`, keeping only prefix (Miguel)
> - Renamed panic methods to make more explicit (Miguel)
> - use markdown in doc comments and added example/kunit test (Miguel)
> - Added maintainer section for BITOPS API BINDINGS [RUST] (Yury)
> - Added M: entry for bitmap.rs which goes to Alice (Viresh, Alice)
> - Changed calls from find_* to _find_*, removed helpers (Yury)
> - Use non-atomic __set_bit and __clear_bit from Bitmap Rust API (Yury)
>
> Link [1] https://lore.kernel.org/all/20250224233938.3158-1-yury.norov@xxxxxxxxx/
> Link [2] https://lore.kernel.org/all/cover.1740726226.git.viresh.kumar@xxxxxxxxxx/
> Link [v2]: https://lore.kernel.org/rust-for-linux/20250303114037.3259804-2-bqe@xxxxxxxxxx/
> ---
> MAINTAINERS | 8 ++
> rust/bindings/bindings_helper.h | 1 +
> rust/helpers/bitmap.c | 8 ++
> rust/helpers/bitops.c | 13 +++
> rust/helpers/helpers.c | 2 +
> rust/kernel/bitmap.rs | 190 ++++++++++++++++++++++++++++++++
> rust/kernel/lib.rs | 1 +
> 7 files changed, 223 insertions(+)
> create mode 100644 rust/helpers/bitmap.c
> create mode 100644 rust/helpers/bitops.c
> create mode 100644 rust/kernel/bitmap.rs
>
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 6d6e55d8593b..8f42fb1f24c6 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -4032,12 +4032,15 @@ F: tools/lib/find_bit.c
> BITMAP API BINDINGS [RUST]
> M: Yury Norov <yury.norov@xxxxxxxxx>
> S: Maintained
> +F: rust/helpers/bitmap.c
> F: rust/helpers/cpumask.c
>
> BITMAP API [RUST]
> M: Viresh Kumar <viresh.kumar@xxxxxxxxxx> (cpumask)
> +M: Alice Ryhl <aliceryhl@xxxxxxxxxx> (bitmap)
> R: Yury Norov <yury.norov@xxxxxxxxx>
> S: Maintained
> +F: rust/kernel/bitmap.rs
> F: rust/kernel/cpumask.rs
>
> BITOPS API
> @@ -4054,6 +4057,11 @@ F: include/linux/bitops.h
> F: lib/test_bitops.c
> F: tools/*/bitops*
>
> +BITOPS API BINDINGS [RUST]
> +M: Yury Norov <yury.norov@xxxxxxxxx>
> +S: Maintained
> +F: rust/helpers/bitops.c
> +
> BLINKM RGB LED DRIVER
> M: Jan-Simon Moeller <jansimon.moeller@xxxxxx>
> S: Maintained
> diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
> index 673b1daa9a58..50416c1a3de9 100644
> --- a/rust/bindings/bindings_helper.h
> +++ b/rust/bindings/bindings_helper.h
> @@ -7,6 +7,7 @@
> */
>
> #include <kunit/test.h>
> +#include <linux/bitmap.h>
> #include <linux/blk-mq.h>
> #include <linux/blk_types.h>
> #include <linux/blkdev.h>
> diff --git a/rust/helpers/bitmap.c b/rust/helpers/bitmap.c
> new file mode 100644
> index 000000000000..1cc88b34d716
> --- /dev/null
> +++ b/rust/helpers/bitmap.c
> @@ -0,0 +1,8 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/bitmap.h>
> +
> +void rust_helper_bitmap_copy_and_extend(unsigned long *dst, const unsigned long *src, unsigned int count, unsigned int size)
> +{
> + bitmap_copy_and_extend(dst, src, count, size);
> +}
> diff --git a/rust/helpers/bitops.c b/rust/helpers/bitops.c
> new file mode 100644
> index 000000000000..986dafb45184
> --- /dev/null
> +++ b/rust/helpers/bitops.c
> @@ -0,0 +1,13 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/bitops.h>
> +
> +void rust_helper___set_bit(unsigned int nr, volatile unsigned long *addr)
> +{
> + __set_bit(nr, addr);
> +}
> +
> +void rust_helper___clear_bit(unsigned int nr, volatile unsigned long *addr)
> +{
> + __clear_bit(nr, addr);
> +}
> diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
> index de2341cfd917..541d8cb30195 100644
> --- a/rust/helpers/helpers.c
> +++ b/rust/helpers/helpers.c
> @@ -7,6 +7,8 @@
> * Sorted alphabetically.
> */
>
> +#include "bitmap.c"
> +#include "bitops.c"
> #include "blk.c"
> #include "bug.c"
> #include "build_assert.c"
> diff --git a/rust/kernel/bitmap.rs b/rust/kernel/bitmap.rs
> new file mode 100644
> index 000000000000..b8fe18dff832
> --- /dev/null
> +++ b/rust/kernel/bitmap.rs
> @@ -0,0 +1,190 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +// Copyright (C) 2025 Google LLC.
> +
> +//! Rust API for bitmap.
> +//!
> +//! C headers: [`include/linux/bitmap.h`](srctree/include/linux/bitmap.h).
> +
> +use crate::alloc::{AllocError, Flags};
> +use crate::bindings;
> +use core::ptr::NonNull;
> +
> +/// Wraps underlying C bitmap structure.

Missing article here.

> +///
> +/// # Invariants
> +///
> +/// * `ptr` is obtained from a successful call to `bitmap_zalloc` and
> +/// holds the address of an initialized array of unsigned long
> +/// that is large enough to hold `nbits` bits.
> +pub struct Bitmap {
> + /// Pointer to an array of unsigned long.
> + ptr: NonNull<usize>,
> + /// How many bits this bitmap stores. Must be < 2^32.
> + nbits: usize,

How come this isn't held as u32? There's a lot of conversion to u32
sprinkled around.

> +}
> +
> +impl Drop for Bitmap {
> + fn drop(&mut self) {
> + // SAFETY: `self.ptr` was returned by bitmap_zalloc.

Ticks around `bitmap_zalloc` for consistency. I think this should also
have an INVARIANT section since after this call the invariant
described on Bitmap is broken.

> + unsafe { bindings::bitmap_free(self.as_mut_ptr()) };
> + }
> +}
> +
> +#[cold]
> +fn panic_not_in_bounds_lt(arg: &'static str, len: usize, val: usize) -> ! {
> + panic!("{arg} must be less than length {len}, was {val}");
> +}

Have you considered using build_error or returning an error?

> +
> +#[cold]
> +fn panic_not_in_bounds_le(arg: &'static str, len: usize, val: usize) -> ! {
> + panic!("{arg} must be less than or equal to length {len}, was {val}");
> +}
> +
> +impl Bitmap {
> + /// Constructs a new [`Bitmap`].
> + ///
> + /// Fails with AllocError if `nbits` is greater than or equal to 2^32,
> + /// or when the bitmap could not be allocated.
> + ///
> + /// # Example
> + ///
> + /// ```
> + /// # use kernel::bitmap::Bitmap;
> + ///
> + /// fn new_bitmap() -> Bitmap {
> + /// Bitmap::new(128, GFP_KERNEL).unwrap()
> + /// }
> + /// ```
> + #[inline]
> + pub fn new(nbits: usize, flags: Flags) -> Result<Self, AllocError> {
> + if let Ok(nbits_u32) = u32::try_from(nbits) {
> + // SAFETY: nbits == 0 is permitted and nbits fits in u32.
> + let ptr = unsafe { bindings::bitmap_zalloc(nbits_u32, flags.as_raw()) };
> + // Zero-size allocation is ok and yields a dangling pointer.
> + let ptr = NonNull::new(ptr).ok_or(AllocError)?;
> + Ok(Bitmap { ptr, nbits })
> + } else {
> + Err(AllocError)
> + }
> + }

Similar question to above: why return an error here but panic in the setters?

> +
> + /// Returns how many bits this bitmap holds.
> + #[inline]
> + pub fn len(&self) -> usize {
> + self.nbits
> + }
> +
> + /// Returns true if this bitmap has length 0.
> + #[inline]
> + pub fn is_empty(&self) -> bool {
> + self.nbits == 0
> + }
> +
> + /// Returns a mutable raw pointer to the backing bitmap.
> + #[inline]
> + pub fn as_mut_ptr(&mut self) -> *mut usize {
> + self.ptr.as_ptr()
> + }
> +
> + /// Returns a raw pointer to the backing bitmap.
> + #[inline]
> + pub fn as_ptr(&self) -> *const usize {
> + self.ptr.as_ptr()
> + }

Could you describe the need for these functions in the commit message?
I would expect that leaking the internal pointer would be an anti-goal
of an abstraction like this.

> +
> + /// Sets bit with number `nr`.
> + ///
> + /// # Panics
> + ///
> + /// Panics if `nr` is greater than or equal to `self.nbits`.
> + #[inline]
> + pub fn set_bit(&mut self, nr: usize) {
> + if self.nbits <= nr {
> + panic_not_in_bounds_lt("nr", self.nbits, nr)
> + }
> + // SAFETY: Bit nr is within bounds.
> + unsafe { bindings::__set_bit(nr as u32, self.as_mut_ptr()) };
> + }
> +
> + /// Clears bit with number `nr`.
> + ///
> + /// # Panics
> + ///
> + /// Panics if `nr` is greater than or equal to `self.nbits`.
> + #[inline]
> + pub fn clear_bit(&mut self, nr: usize) {
> + if self.nbits <= nr {
> + panic_not_in_bounds_lt("nr", self.nbits, nr);
> + }
> + // SAFETY: Bit nr is within bounds.
> + unsafe { bindings::__clear_bit(nr as u32, self.as_mut_ptr()) };
> + }
> +
> + /// Copies all bits from `src` and sets any remaining bits to zero.
> + ///
> + /// # Panics
> + ///
> + /// Panics if `src.nbits` has more bits than this bitmap.
> + #[inline]
> + pub fn copy_from_bitmap_and_extend(&mut self, src: &Bitmap) {

nit: `Bitmap` could be `Self` here.

> + if self.nbits < src.nbits {
> + panic_not_in_bounds_le("src.nbits", self.nbits, src.nbits);
> + }
> + // SAFETY: nbits == 0 is supported and access to `self` and `src` is within bounds.
> + unsafe {
> + bindings::bitmap_copy_and_extend(self.as_mut_ptr(), src.as_ptr(), src.nbits as u32, self.nbits as u32)
> + };
> + }
> +
> + /// Finds the last bit.
> + #[inline]
> + pub fn find_last_bit(&self) -> usize {
> + // SAFETY: nbits == 0 is supported and access is within bounds.
> + unsafe { bindings::_find_last_bit(self.as_ptr(), self.nbits) }
> + }
> +
> + /// Finds the last bit, searching up to `nbits` bits.
> + ///
> + /// # Panics
> + ///
> + /// Panics if `nbits` is too large for this bitmap.
> + #[inline]
> + pub fn find_last_bit_upto(&self, nbits: usize) -> usize {
> + if self.nbits < nbits {
> + panic_not_in_bounds_le("nbits", self.nbits, nbits);
> + }
> + // SAFETY: nbits == 0 is supported and access is within bounds.
> + unsafe { bindings::_find_last_bit(self.as_ptr(), nbits) }
> + }
> +
> + /// Finds the next zero bit, searching up to `nbits`.
> + ///
> + /// # Panics
> + ///
> + /// Panics if `nbits` is too large for this bitmap.
> + #[inline]
> + pub fn find_next_zero_bit_upto(&self, nbits: usize) -> usize {
> + if self.nbits < nbits {
> + panic_not_in_bounds_le("nbits", self.nbits, nbits);
> + }
> + // SAFETY: nbits == 0 is supported and access is within bounds.
> + unsafe {
> + bindings::_find_next_zero_bit(self.as_ptr(), nbits, 0 /* offset */)
> + }
> + }

How come `find_last_bit` exists but `find_next_zero_bit` does not? I
wonder if the APIs could be unified by taking optional sizes and
offsets.

> +
> + /// Finds the next zero bit, searching up to `nbits` bits, with offset `offset`.
> + ///
> + /// # Panics
> + ///
> + /// Panics if `nbits` is too large for this bitmap.
> + #[inline]
> + pub fn find_next_zero_bit_upto_offset(&self, nbits: usize, offset: usize) -> usize {
> + if self.nbits < nbits {
> + panic_not_in_bounds_le("nbits", self.nbits, nbits);
> + }
> + // SAFETY: nbits == 0 and out-of-bounds offset is supported, and access is within bounds.
> + unsafe { bindings::_find_next_zero_bit(self.as_ptr(), nbits, offset) }
> + }
> +}
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index efbd7be98dab..be06ffc47473 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -36,6 +36,7 @@
> pub use ffi;
>
> pub mod alloc;
> +pub mod bitmap;
> #[cfg(CONFIG_BLOCK)]
> pub mod block;
> #[doc(hidden)]
> --
> 2.49.0.rc0.332.g42c0ae87b1-goog
>
>

Cheers.
Tamir

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In reply to: Burak Emir: "[PATCH v3] rust: add bindings and API for bitmap.h and bitops.h."
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