From 11eca92a2caebcc2b3b65ca290385ff4b0498946 Mon Sep 17 00:00:00 2001 From: Burak Emir Date: Mon, 8 Sep 2025 07:21:53 +0000 Subject: rust: add bitmap API. Provides an abstraction for C bitmap API and bitops operations. This commit enables a Rust implementation of an Android Binder data structure from commit 15d9da3f818c ("binder: use bitmap for faster descriptor lookup"), which can be found in drivers/android/dbitmap.h. It is a step towards upstreaming the Rust port of Android Binder driver. We follow the C Bitmap API closely in naming and semantics, with a few differences that take advantage of Rust language facilities and idioms. The main types are `BitmapVec` for owned bitmaps and `Bitmap` for references to C bitmaps. * We leverage Rust type system guarantees as follows: * all (non-atomic) mutating operations require a &mut reference which amounts to exclusive access. * the `BitmapVec` type implements Send. This enables transferring ownership between threads and is needed for Binder. * the `BitmapVec` type implements Sync, which enables passing shared references &Bitmap between threads. Atomic operations can be used to safely modify from multiple threads (interior mutability), though without ordering guarantees. * The Rust API uses `{set,clear}_bit` vs `{set,clear}_bit_atomic` as names for clarity, which differs from the C naming convention `set_bit` for atomic vs `__set_bit` for non-atomic. * we include enough operations for the API to be useful. Not all operations are exposed yet in order to avoid dead code. The missing ones can be added later. * We take a fine-grained approach to safety: * Low-level bit-ops get a safe API with bounds checks. Calling with an out-of-bounds arguments to {set,clear}_bit becomes a no-op and get logged as errors. * We also introduce a RUST_BITMAP_HARDENED config, which causes invocations with out-of-bounds arguments to panic. * methods correspond to find_* C methods tolerate out-of-bounds since the C implementation does. Also here, out-of-bounds arguments are logged as errors, or panic in RUST_BITMAP_HARDENED mode. * We add a way to "borrow" bitmaps from C in Rust, to make C bitmaps that were allocated in C directly usable in Rust code (`Bitmap`). * the Rust API is optimized to represent the bitmap inline if it would fit into a pointer. This saves allocations which is relevant in the Binder use case. The underlying C bitmap is *not* exposed for raw access in Rust. Doing so would permit bypassing the Rust API and lose static guarantees. 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. We use 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. Adds new MAINTAINERS section BITMAP API [RUST]. Suggested-by: Alice Ryhl Suggested-by: Yury Norov Signed-off-by: Burak Emir Reviewed-by: Alice Ryhl Signed-off-by: Yury Norov (NVIDIA) --- rust/kernel/bitmap.rs | 585 ++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 585 insertions(+) create mode 100644 rust/kernel/bitmap.rs (limited to 'rust/kernel/bitmap.rs') diff --git a/rust/kernel/bitmap.rs b/rust/kernel/bitmap.rs new file mode 100644 index 000000000000..f349b1eb59f9 --- /dev/null +++ b/rust/kernel/bitmap.rs @@ -0,0 +1,585 @@ +// 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; +#[cfg(not(CONFIG_RUST_BITMAP_HARDENED))] +use crate::pr_err; +use core::ptr::NonNull; + +const BITS_PER_LONG: usize = bindings::BITS_PER_LONG as usize; + +/// Represents a C bitmap. Wraps underlying C bitmap API. +/// +/// # Invariants +/// +/// Must reference a `[c_ulong]` long enough to fit `data.len()` bits. +#[cfg_attr(CONFIG_64BIT, repr(align(8)))] +#[cfg_attr(not(CONFIG_64BIT), repr(align(4)))] +pub struct Bitmap { + data: [()], +} + +impl Bitmap { + /// Borrows a C bitmap. + /// + /// # Safety + /// + /// * `ptr` holds a non-null address of an initialized array of `unsigned long` + /// that is large enough to hold `nbits` bits. + /// * the array must not be freed for the lifetime of this [`Bitmap`] + /// * concurrent access only happens through atomic operations + pub unsafe fn from_raw<'a>(ptr: *const usize, nbits: usize) -> &'a Bitmap { + let data: *const [()] = core::ptr::slice_from_raw_parts(ptr.cast(), nbits); + // INVARIANT: `data` references an initialized array that can hold `nbits` bits. + // SAFETY: + // The caller guarantees that `data` (derived from `ptr` and `nbits`) + // points to a valid, initialized, and appropriately sized memory region + // that will not be freed for the lifetime 'a. + // We are casting `*const [()]` to `*const Bitmap`. The `Bitmap` + // struct is a ZST with a `data: [()]` field. This means its layout + // is compatible with a slice of `()`, and effectively it's a "thin pointer" + // (its size is 0 and alignment is 1). The `slice_from_raw_parts` + // function correctly encodes the length (number of bits, not elements) + // into the metadata of the fat pointer. Therefore, dereferencing this + // pointer as `&Bitmap` is safe given the caller's guarantees. + unsafe { &*(data as *const Bitmap) } + } + + /// Borrows a C bitmap exclusively. + /// + /// # Safety + /// + /// * `ptr` holds a non-null address of an initialized array of `unsigned long` + /// that is large enough to hold `nbits` bits. + /// * the array must not be freed for the lifetime of this [`Bitmap`] + /// * no concurrent access may happen. + pub unsafe fn from_raw_mut<'a>(ptr: *mut usize, nbits: usize) -> &'a mut Bitmap { + let data: *mut [()] = core::ptr::slice_from_raw_parts_mut(ptr.cast(), nbits); + // INVARIANT: `data` references an initialized array that can hold `nbits` bits. + // SAFETY: + // The caller guarantees that `data` (derived from `ptr` and `nbits`) + // points to a valid, initialized, and appropriately sized memory region + // that will not be freed for the lifetime 'a. + // Furthermore, the caller guarantees no concurrent access will happen, + // which upholds the exclusivity requirement for a mutable reference. + // Similar to `from_raw`, casting `*mut [()]` to `*mut Bitmap` is + // safe because `Bitmap` is a ZST with a `data: [()]` field, + // making its layout compatible with a slice of `()`. + unsafe { &mut *(data as *mut Bitmap) } + } + + /// Returns a raw pointer to the backing [`Bitmap`]. + pub fn as_ptr(&self) -> *const usize { + core::ptr::from_ref::(self).cast::() + } + + /// Returns a mutable raw pointer to the backing [`Bitmap`]. + pub fn as_mut_ptr(&mut self) -> *mut usize { + core::ptr::from_mut::(self).cast::() + } + + /// Returns length of this [`Bitmap`]. + #[expect(clippy::len_without_is_empty)] + pub fn len(&self) -> usize { + self.data.len() + } +} + +/// Holds either a pointer to array of `unsigned long` or a small bitmap. +#[repr(C)] +union BitmapRepr { + bitmap: usize, + ptr: NonNull, +} + +macro_rules! bitmap_assert { + ($cond:expr, $($arg:tt)+) => { + #[cfg(CONFIG_RUST_BITMAP_HARDENED)] + assert!($cond, $($arg)*); + } +} + +macro_rules! bitmap_assert_return { + ($cond:expr, $($arg:tt)+) => { + #[cfg(CONFIG_RUST_BITMAP_HARDENED)] + assert!($cond, $($arg)*); + + #[cfg(not(CONFIG_RUST_BITMAP_HARDENED))] + if !($cond) { + pr_err!($($arg)*); + return + } + } +} + +/// Represents an owned bitmap. +/// +/// Wraps underlying C bitmap API. See [`Bitmap`] for available +/// methods. +/// +/// # Examples +/// +/// Basic usage +/// +/// ``` +/// use kernel::alloc::flags::GFP_KERNEL; +/// use kernel::bitmap::BitmapVec; +/// +/// let mut b = BitmapVec::new(16, GFP_KERNEL)?; +/// +/// assert_eq!(16, b.len()); +/// for i in 0..16 { +/// if i % 4 == 0 { +/// b.set_bit(i); +/// } +/// } +/// assert_eq!(Some(0), b.next_bit(0)); +/// assert_eq!(Some(1), b.next_zero_bit(0)); +/// assert_eq!(Some(4), b.next_bit(1)); +/// assert_eq!(Some(5), b.next_zero_bit(4)); +/// assert_eq!(Some(12), b.last_bit()); +/// # Ok::<(), Error>(()) +/// ``` +/// +/// # Invariants +/// +/// * `nbits` is `<= i32::MAX` and never changes. +/// * if `nbits <= bindings::BITS_PER_LONG`, then `repr` is a `usize`. +/// * otherwise, `repr` holds a non-null pointer to an initialized +/// array of `unsigned long` that is large enough to hold `nbits` bits. +pub struct BitmapVec { + /// Representation of bitmap. + repr: BitmapRepr, + /// Length of this bitmap. Must be `<= i32::MAX`. + nbits: usize, +} + +impl core::ops::Deref for BitmapVec { + type Target = Bitmap; + + fn deref(&self) -> &Bitmap { + let ptr = if self.nbits <= BITS_PER_LONG { + // SAFETY: Bitmap is represented inline. + unsafe { core::ptr::addr_of!(self.repr.bitmap) } + } else { + // SAFETY: Bitmap is represented as array of `unsigned long`. + unsafe { self.repr.ptr.as_ptr() } + }; + + // SAFETY: We got the right pointer and invariants of [`Bitmap`] hold. + // An inline bitmap is treated like an array with single element. + unsafe { Bitmap::from_raw(ptr, self.nbits) } + } +} + +impl core::ops::DerefMut for BitmapVec { + fn deref_mut(&mut self) -> &mut Bitmap { + let ptr = if self.nbits <= BITS_PER_LONG { + // SAFETY: Bitmap is represented inline. + unsafe { core::ptr::addr_of_mut!(self.repr.bitmap) } + } else { + // SAFETY: Bitmap is represented as array of `unsigned long`. + unsafe { self.repr.ptr.as_ptr() } + }; + + // SAFETY: We got the right pointer and invariants of [`BitmapVec`] hold. + // An inline bitmap is treated like an array with single element. + unsafe { Bitmap::from_raw_mut(ptr, self.nbits) } + } +} + +/// Enable ownership transfer to other threads. +/// +/// SAFETY: We own the underlying bitmap representation. +unsafe impl Send for BitmapVec {} + +/// Enable unsynchronized concurrent access to [`BitmapVec`] through shared references. +/// +/// SAFETY: `deref()` will return a reference to a [`Bitmap`]. Its methods +/// take immutable references are either atomic or read-only. +unsafe impl Sync for BitmapVec {} + +impl Drop for BitmapVec { + fn drop(&mut self) { + if self.nbits <= BITS_PER_LONG { + return; + } + // SAFETY: `self.ptr` was returned by the C `bitmap_zalloc`. + // + // INVARIANT: there is no other use of the `self.ptr` after this + // call and the value is being dropped so the broken invariant is + // not observable on function exit. + unsafe { bindings::bitmap_free(self.repr.ptr.as_ptr()) }; + } +} + +impl BitmapVec { + /// Constructs a new [`BitmapVec`]. + /// + /// Fails with [`AllocError`] when the [`BitmapVec`] could not be allocated. This + /// includes the case when `nbits` is greater than `i32::MAX`. + #[inline] + pub fn new(nbits: usize, flags: Flags) -> Result { + if nbits <= BITS_PER_LONG { + return Ok(BitmapVec { + repr: BitmapRepr { bitmap: 0 }, + nbits, + }); + } + if nbits > i32::MAX.try_into().unwrap() { + return Err(AllocError); + } + let nbits_u32 = u32::try_from(nbits).unwrap(); + // SAFETY: `BITS_PER_LONG < nbits` and `nbits <= i32::MAX`. + let ptr = unsafe { bindings::bitmap_zalloc(nbits_u32, flags.as_raw()) }; + let ptr = NonNull::new(ptr).ok_or(AllocError)?; + // INVARIANT: `ptr` returned by C `bitmap_zalloc` and `nbits` checked. + Ok(BitmapVec { + repr: BitmapRepr { ptr }, + nbits, + }) + } + + /// Returns length of this [`Bitmap`]. + #[allow(clippy::len_without_is_empty)] + #[inline] + pub fn len(&self) -> usize { + self.nbits + } +} + +impl Bitmap { + /// Set bit with index `index`. + /// + /// ATTENTION: `set_bit` is non-atomic, which differs from the naming + /// convention in C code. The corresponding C function is `__set_bit`. + /// + /// If CONFIG_RUST_BITMAP_HARDENED is not enabled and `index` is greater than + /// or equal to `self.nbits`, does nothing. + /// + /// # Panics + /// + /// Panics if CONFIG_RUST_BITMAP_HARDENED is enabled and `index` is greater than + /// or equal to `self.nbits`. + #[inline] + pub fn set_bit(&mut self, index: usize) { + bitmap_assert_return!( + index < self.len(), + "Bit `index` must be < {}, was {}", + self.len(), + index + ); + // SAFETY: Bit `index` is within bounds. + unsafe { bindings::__set_bit(index, self.as_mut_ptr()) }; + } + + /// Set bit with index `index`, atomically. + /// + /// This is a relaxed atomic operation (no implied memory barriers). + /// + /// ATTENTION: The naming convention differs from C, where the corresponding + /// function is called `set_bit`. + /// + /// If CONFIG_RUST_BITMAP_HARDENED is not enabled and `index` is greater than + /// or equal to `self.len()`, does nothing. + /// + /// # Panics + /// + /// Panics if CONFIG_RUST_BITMAP_HARDENED is enabled and `index` is greater than + /// or equal to `self.len()`. + #[inline] + pub fn set_bit_atomic(&self, index: usize) { + bitmap_assert_return!( + index < self.len(), + "Bit `index` must be < {}, was {}", + self.len(), + index + ); + // SAFETY: `index` is within bounds and the caller has ensured that + // there is no mix of non-atomic and atomic operations. + unsafe { bindings::set_bit(index, self.as_ptr().cast_mut()) }; + } + + /// Clear `index` bit. + /// + /// ATTENTION: `clear_bit` is non-atomic, which differs from the naming + /// convention in C code. The corresponding C function is `__clear_bit`. + /// + /// If CONFIG_RUST_BITMAP_HARDENED is not enabled and `index` is greater than + /// or equal to `self.len()`, does nothing. + /// + /// # Panics + /// + /// Panics if CONFIG_RUST_BITMAP_HARDENED is enabled and `index` is greater than + /// or equal to `self.len()`. + #[inline] + pub fn clear_bit(&mut self, index: usize) { + bitmap_assert_return!( + index < self.len(), + "Bit `index` must be < {}, was {}", + self.len(), + index + ); + // SAFETY: `index` is within bounds. + unsafe { bindings::__clear_bit(index, self.as_mut_ptr()) }; + } + + /// Clear `index` bit, atomically. + /// + /// This is a relaxed atomic operation (no implied memory barriers). + /// + /// ATTENTION: The naming convention differs from C, where the corresponding + /// function is called `clear_bit`. + /// + /// If CONFIG_RUST_BITMAP_HARDENED is not enabled and `index` is greater than + /// or equal to `self.len()`, does nothing. + /// + /// # Panics + /// + /// Panics if CONFIG_RUST_BITMAP_HARDENED is enabled and `index` is greater than + /// or equal to `self.len()`. + #[inline] + pub fn clear_bit_atomic(&self, index: usize) { + bitmap_assert_return!( + index < self.len(), + "Bit `index` must be < {}, was {}", + self.len(), + index + ); + // SAFETY: `index` is within bounds and the caller has ensured that + // there is no mix of non-atomic and atomic operations. + unsafe { bindings::clear_bit(index, self.as_ptr().cast_mut()) }; + } + + /// Copy `src` into this [`Bitmap`] and set any remaining bits to zero. + /// + /// # Examples + /// + /// ``` + /// use kernel::alloc::{AllocError, flags::GFP_KERNEL}; + /// use kernel::bitmap::BitmapVec; + /// + /// let mut long_bitmap = BitmapVec::new(256, GFP_KERNEL)?; + /// + /// assert_eq!(None, long_bitmap.last_bit()); + /// + /// let mut short_bitmap = BitmapVec::new(16, GFP_KERNEL)?; + /// + /// short_bitmap.set_bit(7); + /// long_bitmap.copy_and_extend(&short_bitmap); + /// assert_eq!(Some(7), long_bitmap.last_bit()); + /// + /// # Ok::<(), AllocError>(()) + /// ``` + #[inline] + pub fn copy_and_extend(&mut self, src: &Bitmap) { + let len = core::cmp::min(src.len(), self.len()); + // SAFETY: access to `self` and `src` is within bounds. + unsafe { + bindings::bitmap_copy_and_extend( + self.as_mut_ptr(), + src.as_ptr(), + len as u32, + self.len() as u32, + ) + }; + } + + /// Finds last set bit. + /// + /// # Examples + /// + /// ``` + /// use kernel::alloc::{AllocError, flags::GFP_KERNEL}; + /// use kernel::bitmap::BitmapVec; + /// + /// let bitmap = BitmapVec::new(64, GFP_KERNEL)?; + /// + /// match bitmap.last_bit() { + /// Some(idx) => { + /// pr_info!("The last bit has index {idx}.\n"); + /// } + /// None => { + /// pr_info!("All bits in this bitmap are 0.\n"); + /// } + /// } + /// # Ok::<(), AllocError>(()) + /// ``` + #[inline] + pub fn last_bit(&self) -> Option { + // SAFETY: `_find_next_bit` access is within bounds due to invariant. + let index = unsafe { bindings::_find_last_bit(self.as_ptr(), self.len()) }; + if index >= self.len() { + None + } else { + Some(index) + } + } + + /// Finds next set bit, starting from `start`. + /// + /// Returns `None` if `start` is greater or equal to `self.nbits`. + #[inline] + pub fn next_bit(&self, start: usize) -> Option { + bitmap_assert!( + start < self.len(), + "`start` must be < {} was {}", + self.len(), + start + ); + // SAFETY: `_find_next_bit` tolerates out-of-bounds arguments and returns a + // value larger than or equal to `self.len()` in that case. + let index = unsafe { bindings::_find_next_bit(self.as_ptr(), self.len(), start) }; + if index >= self.len() { + None + } else { + Some(index) + } + } + + /// Finds next zero bit, starting from `start`. + /// Returns `None` if `start` is greater than or equal to `self.len()`. + #[inline] + pub fn next_zero_bit(&self, start: usize) -> Option { + bitmap_assert!( + start < self.len(), + "`start` must be < {} was {}", + self.len(), + start + ); + // SAFETY: `_find_next_zero_bit` tolerates out-of-bounds arguments and returns a + // value larger than or equal to `self.len()` in that case. + let index = unsafe { bindings::_find_next_zero_bit(self.as_ptr(), self.len(), start) }; + if index >= self.len() { + None + } else { + Some(index) + } + } +} + +use macros::kunit_tests; + +#[kunit_tests(rust_kernel_bitmap)] +mod tests { + use super::*; + use kernel::alloc::flags::GFP_KERNEL; + + #[test] + fn bitmap_borrow() { + let fake_bitmap: [usize; 2] = [0, 0]; + // SAFETY: `fake_c_bitmap` is an array of expected length. + let b = unsafe { Bitmap::from_raw(fake_bitmap.as_ptr(), 2 * BITS_PER_LONG) }; + assert_eq!(2 * BITS_PER_LONG, b.len()); + assert_eq!(None, b.next_bit(0)); + } + + #[test] + fn bitmap_copy() { + let fake_bitmap: usize = 0xFF; + // SAFETY: `fake_c_bitmap` can be used as one-element array of expected length. + let b = unsafe { Bitmap::from_raw(core::ptr::addr_of!(fake_bitmap), 8) }; + assert_eq!(8, b.len()); + assert_eq!(None, b.next_zero_bit(0)); + } + + #[test] + fn bitmap_vec_new() -> Result<(), AllocError> { + let b = BitmapVec::new(0, GFP_KERNEL)?; + assert_eq!(0, b.len()); + + let b = BitmapVec::new(3, GFP_KERNEL)?; + assert_eq!(3, b.len()); + + let b = BitmapVec::new(1024, GFP_KERNEL)?; + assert_eq!(1024, b.len()); + + // Requesting too large values results in [`AllocError`]. + let res = BitmapVec::new(1 << 31, GFP_KERNEL); + assert!(res.is_err()); + Ok(()) + } + + #[test] + fn bitmap_set_clear_find() -> Result<(), AllocError> { + let mut b = BitmapVec::new(128, GFP_KERNEL)?; + + // Zero-initialized + assert_eq!(None, b.next_bit(0)); + assert_eq!(Some(0), b.next_zero_bit(0)); + assert_eq!(None, b.last_bit()); + + b.set_bit(17); + + assert_eq!(Some(17), b.next_bit(0)); + assert_eq!(Some(17), b.next_bit(17)); + assert_eq!(None, b.next_bit(18)); + assert_eq!(Some(17), b.last_bit()); + + b.set_bit(107); + + assert_eq!(Some(17), b.next_bit(0)); + assert_eq!(Some(17), b.next_bit(17)); + assert_eq!(Some(107), b.next_bit(18)); + assert_eq!(Some(107), b.last_bit()); + + b.clear_bit(17); + + assert_eq!(Some(107), b.next_bit(0)); + assert_eq!(Some(107), b.last_bit()); + Ok(()) + } + + #[test] + fn owned_bitmap_out_of_bounds() -> Result<(), AllocError> { + // TODO: Kunit #[test]s do not support `cfg` yet, + // so we add it here in the body. + #[cfg(not(CONFIG_RUST_BITMAP_HARDENED))] + { + let mut b = BitmapVec::new(128, GFP_KERNEL)?; + b.set_bit(2048); + b.set_bit_atomic(2048); + b.clear_bit(2048); + b.clear_bit_atomic(2048); + assert_eq!(None, b.next_bit(2048)); + assert_eq!(None, b.next_zero_bit(2048)); + assert_eq!(None, b.last_bit()); + } + Ok(()) + } + + // TODO: uncomment once kunit supports [should_panic] and `cfg`. + // #[cfg(CONFIG_RUST_BITMAP_HARDENED)] + // #[test] + // #[should_panic] + // fn owned_bitmap_out_of_bounds() -> Result<(), AllocError> { + // let mut b = BitmapVec::new(128, GFP_KERNEL)?; + // + // b.set_bit(2048); + // } + + #[test] + fn bitmap_copy_and_extend() -> Result<(), AllocError> { + let mut long_bitmap = BitmapVec::new(256, GFP_KERNEL)?; + + long_bitmap.set_bit(3); + long_bitmap.set_bit(200); + + let mut short_bitmap = BitmapVec::new(32, GFP_KERNEL)?; + + short_bitmap.set_bit(17); + + long_bitmap.copy_and_extend(&short_bitmap); + + // Previous bits have been cleared. + assert_eq!(Some(17), long_bitmap.next_bit(0)); + assert_eq!(Some(17), long_bitmap.last_bit()); + Ok(()) + } +} -- cgit From 38cc91db2e87ab55bfca2b194e791867b77f9e55 Mon Sep 17 00:00:00 2001 From: Burak Emir Date: Mon, 8 Sep 2025 07:21:54 +0000 Subject: rust: add find_bit_benchmark_rust module. Microbenchmark protected by a config FIND_BIT_BENCHMARK_RUST, following `find_bit_benchmark.c` but testing the Rust Bitmap API. We add a fill_random() method protected by the config in order to maintain the abstraction. The sample output from the benchmark, both C and Rust version: find_bit_benchmark.c output: ``` Start testing find_bit() with random-filled bitmap [ 438.101937] find_next_bit: 860188 ns, 163419 iterations [ 438.109471] find_next_zero_bit: 912342 ns, 164262 iterations [ 438.116820] find_last_bit: 726003 ns, 163419 iterations [ 438.130509] find_nth_bit: 7056993 ns, 16269 iterations [ 438.139099] find_first_bit: 1963272 ns, 16270 iterations [ 438.173043] find_first_and_bit: 27314224 ns, 32654 iterations [ 438.180065] find_next_and_bit: 398752 ns, 73705 iterations [ 438.186689] Start testing find_bit() with sparse bitmap [ 438.193375] find_next_bit: 9675 ns, 656 iterations [ 438.201765] find_next_zero_bit: 1766136 ns, 327025 iterations [ 438.208429] find_last_bit: 9017 ns, 656 iterations [ 438.217816] find_nth_bit: 2749742 ns, 655 iterations [ 438.225168] find_first_bit: 721799 ns, 656 iterations [ 438.231797] find_first_and_bit: 2819 ns, 1 iterations [ 438.238441] find_next_and_bit: 3159 ns, 1 iterations ``` find_bit_benchmark_rust.rs output: ``` [ 451.182459] find_bit_benchmark_rust: [ 451.186688] Start testing find_bit() Rust with random-filled bitmap [ 451.194450] next_bit: 777950 ns, 163644 iterations [ 451.201997] next_zero_bit: 918889 ns, 164036 iterations [ 451.208642] Start testing find_bit() Rust with sparse bitmap [ 451.214300] next_bit: 9181 ns, 654 iterations [ 451.222806] next_zero_bit: 1855504 ns, 327026 iterations ``` Here are the results from 32 samples, with 95% confidence interval. The microbenchmark was built with RUST_BITMAP_HARDENED=n and run on a machine that did not execute other processes. Random-filled bitmap: +-----------+-------+-----------+--------------+-----------+-----------+ | Benchmark | Lang | Mean (ms) | Std Dev (ms) | 95% CI Lo | 95% CI Hi | +-----------+-------+-----------+--------------+-----------+-----------+ | find_bit/ | C | 825.07 | 53.89 | 806.40 | 843.74 | | next_bit | Rust | 870.91 | 46.29 | 854.88 | 886.95 | +-----------+-------+-----------+--------------+-----------+-----------+ | find_zero/| C | 933.56 | 56.34 | 914.04 | 953.08 | | next_zero | Rust | 945.85 | 60.44 | 924.91 | 966.79 | +-----------+-------+-----------+--------------+-----------+-----------+ Rust appears 5.5% slower for next_bit, 1.3% slower for next_zero. Sparse bitmap: +-----------+-------+-----------+--------------+-----------+-----------+ | Benchmark | Lang | Mean (ms) | Std Dev (ms) | 95% CI Lo | 95% CI Hi | +-----------+-------+-----------+--------------+-----------+-----------+ | find_bit/ | C | 13.17 | 6.21 | 11.01 | 15.32 | | next_bit | Rust | 14.30 | 8.27 | 11.43 | 17.17 | +-----------+-------+-----------+--------------+-----------+-----------+ | find_zero/| C | 1859.31 | 82.30 | 1830.80 | 1887.83 | | next_zero | Rust | 1908.09 | 139.82 | 1859.65 | 1956.54 | +-----------+-------+-----------+--------------+-----------+-----------+ Rust appears 8.5% slower for next_bit, 2.6% slower for next_zero. In summary, taking the arithmetic mean of all slow-downs, we can say the Rust API has a 4.5% slowdown. Suggested-by: Alice Ryhl Suggested-by: Yury Norov (NVIDIA) Reviewed-by: Yury Norov (NVIDIA) Reviewed-by: Alice Ryhl Signed-off-by: Burak Emir Signed-off-by: Yury Norov (NVIDIA) --- rust/kernel/bitmap.rs | 15 +++++++++++++++ 1 file changed, 15 insertions(+) (limited to 'rust/kernel/bitmap.rs') diff --git a/rust/kernel/bitmap.rs b/rust/kernel/bitmap.rs index f349b1eb59f9..f45915694454 100644 --- a/rust/kernel/bitmap.rs +++ b/rust/kernel/bitmap.rs @@ -252,6 +252,21 @@ impl BitmapVec { pub fn len(&self) -> usize { self.nbits } + + /// Fills this `Bitmap` with random bits. + #[cfg(CONFIG_FIND_BIT_BENCHMARK_RUST)] + pub fn fill_random(&mut self) { + // SAFETY: `self.as_mut_ptr` points to either an array of the + // appropriate length or one usize. + unsafe { + bindings::get_random_bytes( + self.as_mut_ptr().cast::(), + usize::div_ceil(self.nbits, bindings::BITS_PER_LONG as usize) + * bindings::BITS_PER_LONG as usize + / 8, + ); + } + } } impl Bitmap { -- cgit From 0f5878834d6ce97426219b64c02a2c4081419d53 Mon Sep 17 00:00:00 2001 From: Miguel Ojeda Date: Mon, 13 Oct 2025 02:14:22 +0200 Subject: rust: bitmap: clean Rust 1.92.0 `unused_unsafe` warning Starting with Rust 1.92.0 (expected 2025-12-11), Rust allows to safely take the address of a union field [1][2]: CLIPPY L rust/kernel.o error: unnecessary `unsafe` block --> rust/kernel/bitmap.rs:169:13 | 169 | unsafe { core::ptr::addr_of!(self.repr.bitmap) } | ^^^^^^ unnecessary `unsafe` block | = note: `-D unused-unsafe` implied by `-D warnings` = help: to override `-D warnings` add `#[allow(unused_unsafe)]` error: unnecessary `unsafe` block --> rust/kernel/bitmap.rs:185:13 | 185 | unsafe { core::ptr::addr_of_mut!(self.repr.bitmap) } | ^^^^^^ unnecessary `unsafe` block Thus allow both instances to clean the warning in newer compilers. Link: https://github.com/rust-lang/rust/issues/141264 [1] Link: https://github.com/rust-lang/rust/pull/141469 [2] Signed-off-by: Miguel Ojeda Reviewed-by: Alice Ryhl Signed-off-by: Yury Norov (NVIDIA) --- rust/kernel/bitmap.rs | 2 ++ 1 file changed, 2 insertions(+) (limited to 'rust/kernel/bitmap.rs') diff --git a/rust/kernel/bitmap.rs b/rust/kernel/bitmap.rs index f45915694454..711b8368b38f 100644 --- a/rust/kernel/bitmap.rs +++ b/rust/kernel/bitmap.rs @@ -166,6 +166,7 @@ impl core::ops::Deref for BitmapVec { fn deref(&self) -> &Bitmap { let ptr = if self.nbits <= BITS_PER_LONG { // SAFETY: Bitmap is represented inline. + #[allow(unused_unsafe, reason = "Safe since Rust 1.92.0")] unsafe { core::ptr::addr_of!(self.repr.bitmap) } } else { // SAFETY: Bitmap is represented as array of `unsigned long`. @@ -182,6 +183,7 @@ impl core::ops::DerefMut for BitmapVec { fn deref_mut(&mut self) -> &mut Bitmap { let ptr = if self.nbits <= BITS_PER_LONG { // SAFETY: Bitmap is represented inline. + #[allow(unused_unsafe, reason = "Safe since Rust 1.92.0")] unsafe { core::ptr::addr_of_mut!(self.repr.bitmap) } } else { // SAFETY: Bitmap is represented as array of `unsigned long`. -- cgit From 1f1d3e1d094db732d22b892227bf1e1ac3a8ca04 Mon Sep 17 00:00:00 2001 From: Miguel Ojeda Date: Fri, 17 Oct 2025 00:57:54 +0200 Subject: rust: bitmap: fix formatting We do our best to keep the repository `rustfmt`-clean, thus run the tool to fix the formatting issue. Link: https://docs.kernel.org/rust/coding-guidelines.html#style-formatting Link: https://rust-for-linux.com/contributing#submit-checklist-addendum Fixes: 0f5878834d6c ("rust: bitmap: clean Rust 1.92.0 `unused_unsafe` warning") Reviewed-by: Burak Emir Signed-off-by: Miguel Ojeda --- rust/kernel/bitmap.rs | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) (limited to 'rust/kernel/bitmap.rs') diff --git a/rust/kernel/bitmap.rs b/rust/kernel/bitmap.rs index 711b8368b38f..aa8fc7bf06fc 100644 --- a/rust/kernel/bitmap.rs +++ b/rust/kernel/bitmap.rs @@ -167,7 +167,9 @@ impl core::ops::Deref for BitmapVec { let ptr = if self.nbits <= BITS_PER_LONG { // SAFETY: Bitmap is represented inline. #[allow(unused_unsafe, reason = "Safe since Rust 1.92.0")] - unsafe { core::ptr::addr_of!(self.repr.bitmap) } + unsafe { + core::ptr::addr_of!(self.repr.bitmap) + } } else { // SAFETY: Bitmap is represented as array of `unsigned long`. unsafe { self.repr.ptr.as_ptr() } @@ -184,7 +186,9 @@ impl core::ops::DerefMut for BitmapVec { let ptr = if self.nbits <= BITS_PER_LONG { // SAFETY: Bitmap is represented inline. #[allow(unused_unsafe, reason = "Safe since Rust 1.92.0")] - unsafe { core::ptr::addr_of_mut!(self.repr.bitmap) } + unsafe { + core::ptr::addr_of_mut!(self.repr.bitmap) + } } else { // SAFETY: Bitmap is represented as array of `unsigned long`. unsafe { self.repr.ptr.as_ptr() } -- cgit