[PATCH v13 0/5] rust: adds Bitmap API, ID pool and bindings

[PATCH v13 0/5] rust: adds Bitmap API, ID pool and bindings

[Burak Emir]

This series adds a Rust bitmap API 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 a safe abstraction to underlying bitmap
and bitops operations. For now, only includes method necessary for
dbitmap.h, more can be added later. We perform bounds checks for
hardening, violations are programmer errors that result in panics.

We include set_bit_atomic and clear_bit_atomic operations. One has
to avoid races with non-atomic operations, which is ensure by the
Rust type system: either callers have shared references &bitmap in
which case the mutations are atomic operations. Or there is a
exclusive reference &mut bitmap, in which case there is no concurrent
access.

This series includes an optimization to represent the bitmap inline,
as suggested by Yury.

We ran a simple microbenchmark which shows that overall the Rust API 
can be expected to be about 4.5% slower than C API.

We also introduce a Rust API in id_pool.rs that would replace 
dbitmap.h from the commit referenced above. This data structure is coupled 
with the bitmap API and adds support for growing and shrinking, along
with fine-grained control over when allocation happens. 
The Binder code needs this since it holds a spinlock at the time it
discovers that growing is necessary; this has to be release
for performing a memory allocation with GFP_KERNEL that may cause 
sleep.  We includes example doctests that demonstrate this usage.

This is v13, see [v12] for previous state. Thanks everyone for all the 
helpful comments, this series has improved significantly as a result of 
your work.

Changes v12 --> v13:
- Rebased on commit 75f5f23f8787 ("Merge tag 'block-6.16-20250619' of git://git.kernel.dk/linux")
- Renamed types CBitmap --> Bitmap, Bitmap --> BitmapVec.
- Rewrote unit tests to not use `unwrap()` but `Result` and `?`
- Replaced NonNull::as_mut() with NonNull::as_ptr()
- declared local BITS_PER_LONG usize constants, fixed rustdoc

Changes v11 --> v12:
- Added Reviewed-by tag, changed named of benchmark module
- Fixed config, rustdoc, clippy and bytes computation (Pekka).
- Added test that exercises CONFIG_RUST_BITMAP_HARDENED,
  verified it panics.
- Had to add a break to benchmark module, for the
  CONFIG_RUST_BITMAP_HARDENED case which enforces bounds check.

Changes v10 --> v11:
- Fix Kconfig dependency, Rust benchmark depends on CONFIG_RUST
- Disable clippy warning for len() without is_empty() in id_pool.rs

Changes v9 --> v10:
- change helper to use `unsigned long` for `nr` parameter (Boqun)
- add missing and fix safety comments (Boqun, Pekka)
- move benchmark module output and results to #4 commit msg.
- use pr_cont to avoid repeating find_bit_benchmark_rust log prefix.
- Disable clippy warning for len() without is_empty() in bitmap.rs

Changes v8 --> v9:
- added a new type `CBitmap` that makes any C bitmap accessible with
  the same API, and add Deref so both Bitmap and
  CBitmap can share the same implementation.  Full credit for this 
  goes to Alice who suggested idea and code.
- added config dependency on CONFIG_RUST that was missing from
  CONIG_FIND_BIT_BENCHMARK_RUST.
- implemented Send for Bitmap, it is actually needed by Binder.
- reworded commit msg for clarity.
- removed unsafe for atomic ops.
- renamed `bitmap_hardening_assert` to `bitmap_assert` and make operations
  do nothing and log error when RUST_BITMAP_HARDENED is off.
- update author information in find_bit_benchmark_rust.rs
- Various improvements to id_pool, better names and comments.

Changes v7 --> v8:
- added Acked-by for bindings patches
- add RUST_BITMAP_HARDENED config, making extra bound-checks configurable.
  This is added to security/Kconfig.hardening 
- changed checks of `index` return value to >=
- removed change to FIND_BIT_BENCHMARK

Changes v6 --> v7:
- Added separate unit tests in bitmap.rs and benchmark module,
  following the example in find_bit_benchmark.c
- Added discussion about using vendored bitset to commit message.
- Refined warning about naming convention

Changes v5 --> v6:
- Added SAFETY comment for atomic operations.
- Added missing volatile to bitops set_bit and clear_bit bindings.
- Fixed condition on `nbits` to be <= i32::MAX, update SAFETY comments.
- Readability improvements.
- Updated doc comments wording and indentation.

Changes v4 --> v5: (suggested by Yury and Alice)
- rebased on next-20250318
- split MAINTAINERS changes
- no dependencies on [1] and [2] anymore - Viresh,
  please do add a separate section if you want to maintain cpumask.rs
  separately.
- imports atomic and non-atomic variants, introduces a naming convention
  set_bit and set_bit_atomic on the Rust side.
- changed naming and comments. Keeping `new`.
- change dynamic_id_pool to id_pool
- represent bitmap inline when possible
- add some more tests
- add myself to M: line for the Rust abstractions

Changes v3 --> v4:
- Rebased on Viresh's v3 [2].
- split into multiple patches, separate Rust and bindings. (Yury)
- adds dynamic_id_pool.rs to show the Binder use case. (Yury)
- include example usage that requires release of spinlock (Alice)
- changed bounds checks to `assert!`, shorter (Yury)
- fix param names in binding helpers. (Miguel)
- proper rustdoc formatting, and use examples as kunit tests. (Miguel)
- reduce number of Bitmap methods, and simplify API through
  use Option<usize> to handle the "not found" case.
- make Bitmap pointer accessors private, so Rust Bitmap API
  provides an actual abstraction boundary (Tamir)
- we still return `AllocError` in `Bitmap::new` in case client code
  asks for a size that is too large. Intentionally
  different from other bounds checks because it is not about
  access but allocation, and we expect that client code need
  never handle AllocError and nbits > u32::MAX situations
  differently.

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 [1] and Viresh's v3 [2] changes related to
  bitmap.
- 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@gmail.com/
Link [2] https://lore.kernel.org/rust-for-linux/cover.1742296835.git.viresh.kumar@linaro.org/
Link [v12] https://lore.kernel.org/rust-for-linux/20250611194840.877308-1-bqe@google.com/




Burak Emir (5):
  rust: add bindings for bitmap.h
  rust: add bindings for bitops.h
  rust: add bitmap API.
  rust: add find_bit_benchmark_rust module.
  rust: add dynamic ID pool abstraction for bitmap

 MAINTAINERS                     |  15 +
 lib/Kconfig.debug               |  13 +
 lib/Makefile                    |   1 +
 lib/find_bit_benchmark_rust.rs  | 103 ++++++
 rust/bindings/bindings_helper.h |   2 +
 rust/helpers/bitmap.c           |   9 +
 rust/helpers/bitops.c           |  23 ++
 rust/helpers/helpers.c          |   2 +
 rust/kernel/bitmap.rs           | 597 ++++++++++++++++++++++++++++++++
 rust/kernel/id_pool.rs          | 226 ++++++++++++
 rust/kernel/lib.rs              |   2 +
 security/Kconfig.hardening      |  10 +
 12 files changed, 1003 insertions(+)
 create mode 100644 lib/find_bit_benchmark_rust.rs
 create mode 100644 rust/helpers/bitmap.c
 create mode 100644 rust/helpers/bitops.c
 create mode 100644 rust/kernel/bitmap.rs
 create mode 100644 rust/kernel/id_pool.rs


base-commit: 75f5f23f8787c5e184fcb2fbcd02d8e9317dc5e7
-- 
2.50.0.rc2.761.g2dc52ea45b-goog

[PATCH v13 1/5] rust: add bindings for bitmap.h

[Burak Emir]

Makes the bitmap_copy_and_extend inline function available to Rust.
Adds F: to existing MAINTAINERS section BITMAP API BINDINGS [RUST].

Suggested-by: Alice Ryhl <aliceryhl@google.com>
Suggested-by: Yury Norov <yury.norov@gmail.com>
Signed-off-by: Burak Emir <bqe@google.com>
Acked-by: Yury Norov [NVIDIA] <yury.norov@gmail.com>
---
 MAINTAINERS                     | 1 +
 rust/bindings/bindings_helper.h | 1 +
 rust/helpers/bitmap.c           | 9 +++++++++
 rust/helpers/helpers.c          | 1 +
 4 files changed, 12 insertions(+)
 create mode 100644 rust/helpers/bitmap.c

diff --git a/MAINTAINERS b/MAINTAINERS
index f584e170cfc3..f83386712a2d 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -4200,6 +4200,7 @@ F:	tools/lib/find_bit.c
 BITMAP API BINDINGS [RUST]
 M:	Yury Norov <yury.norov@gmail.com>
 S:	Maintained
+F:	rust/helpers/bitmap.c
 F:	rust/helpers/cpumask.c
 
 BITOPS API
diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index 8cbb660e2ec2..cc2ae3b172da 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -35,6 +35,7 @@
 #include <drm/drm_ioctl.h>
 #include <kunit/test.h>
 #include <linux/auxiliary_bus.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..a50e2f082e47
--- /dev/null
+++ b/rust/helpers/bitmap.c
@@ -0,0 +1,9 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/bitmap.h>
+
+void rust_helper_bitmap_copy_and_extend(unsigned long *to, const unsigned long *from,
+		unsigned int count, unsigned int size)
+{
+	bitmap_copy_and_extend(to, from, count, size);
+}
diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
index b15b3cddad4e..cb47c4708a3b 100644
--- a/rust/helpers/helpers.c
+++ b/rust/helpers/helpers.c
@@ -8,6 +8,7 @@
  */
 
 #include "auxiliary.c"
+#include "bitmap.c"
 #include "blk.c"
 #include "bug.c"
 #include "build_assert.c"
-- 
2.50.0.rc2.761.g2dc52ea45b-goog

[PATCH v13 2/5] rust: add bindings for bitops.h

[Burak Emir]

Makes atomic set_bit and clear_bit inline functions as well as the
non-atomic variants __set_bit and __clear_bit available to Rust.
Adds a new MAINTAINERS section BITOPS API BINDINGS [RUST].

Suggested-by: Alice Ryhl <aliceryhl@google.com>
Suggested-by: Yury Norov <yury.norov@gmail.com>
Signed-off-by: Burak Emir <bqe@google.com>
Acked-by: Yury Norov [NVIDIA] <yury.norov@gmail.com>
---
 MAINTAINERS            |  5 +++++
 rust/helpers/bitops.c  | 23 +++++++++++++++++++++++
 rust/helpers/helpers.c |  1 +
 3 files changed, 29 insertions(+)
 create mode 100644 rust/helpers/bitops.c

diff --git a/MAINTAINERS b/MAINTAINERS
index f83386712a2d..8acc32a59ff5 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -4217,6 +4217,11 @@ F:	include/linux/bitops.h
 F:	lib/test_bitops.c
 F:	tools/*/bitops*
 
+BITOPS API BINDINGS [RUST]
+M:	Yury Norov <yury.norov@gmail.com>
+S:	Maintained
+F:	rust/helpers/bitops.c
+
 BLINKM RGB LED DRIVER
 M:	Jan-Simon Moeller <jansimon.moeller@gmx.de>
 S:	Maintained
diff --git a/rust/helpers/bitops.c b/rust/helpers/bitops.c
new file mode 100644
index 000000000000..5d0861d29d3f
--- /dev/null
+++ b/rust/helpers/bitops.c
@@ -0,0 +1,23 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/bitops.h>
+
+void rust_helper___set_bit(unsigned long nr, unsigned long *addr)
+{
+	__set_bit(nr, addr);
+}
+
+void rust_helper___clear_bit(unsigned long nr, unsigned long *addr)
+{
+	__clear_bit(nr, addr);
+}
+
+void rust_helper_set_bit(unsigned long nr, volatile unsigned long *addr)
+{
+	set_bit(nr, addr);
+}
+
+void rust_helper_clear_bit(unsigned long nr, volatile unsigned long *addr)
+{
+	clear_bit(nr, addr);
+}
diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
index cb47c4708a3b..63dc1ffd31e1 100644
--- a/rust/helpers/helpers.c
+++ b/rust/helpers/helpers.c
@@ -9,6 +9,7 @@
 
 #include "auxiliary.c"
 #include "bitmap.c"
+#include "bitops.c"
 #include "blk.c"
 #include "bug.c"
 #include "build_assert.c"
-- 
2.50.0.rc2.761.g2dc52ea45b-goog

[PATCH v13 3/5] rust: add bitmap API.

[Burak Emir]

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 <aliceryhl@google.com>
Suggested-by: Yury Norov <yury.norov@gmail.com>
Signed-off-by: Burak Emir <bqe@google.com>
---
 MAINTAINERS                |   7 +
 rust/kernel/bitmap.rs      | 582 +++++++++++++++++++++++++++++++++++++
 rust/kernel/lib.rs         |   1 +
 security/Kconfig.hardening |  10 +
 4 files changed, 600 insertions(+)
 create mode 100644 rust/kernel/bitmap.rs

diff --git a/MAINTAINERS b/MAINTAINERS
index 8acc32a59ff5..b7944346afbf 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -4203,6 +4203,13 @@ S:	Maintained
 F:	rust/helpers/bitmap.c
 F:	rust/helpers/cpumask.c
 
+BITMAP API [RUST]
+M:	Alice Ryhl <aliceryhl@google.com>
+M:	Burak Emir <bqe@google.com>
+R:	Yury Norov <yury.norov@gmail.com>
+S:	Maintained
+F:	rust/kernel/bitmap.rs
+
 BITOPS API
 M:	Yury Norov <yury.norov@gmail.com>
 R:	Rasmus Villemoes <linux@rasmusvillemoes.dk>
diff --git a/rust/kernel/bitmap.rs b/rust/kernel/bitmap.rs
new file mode 100644
index 000000000000..a3cd0d4f6970
--- /dev/null
+++ b/rust/kernel/bitmap.rs
@@ -0,0 +1,582 @@
+// 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 {
+        self as *const Bitmap as *const usize
+    }
+
+    /// Returns a mutable raw pointer to the backing [`Bitmap`].
+    pub fn as_mut_ptr(&mut self) -> *mut usize {
+        self as *mut Bitmap as *mut usize
+    }
+
+    /// 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<usize>,
+}
+
+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<Self, AllocError> {
+        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() as *mut usize) };
+    }
+
+    /// 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() as *mut usize) };
+    }
+
+    /// 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<usize> {
+        // 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<usize> {
+        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<usize> {
+        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(())
+    }
+
+    #[cfg(not(CONFIG_RUST_BITMAP_HARDENED))]
+    #[test]
+    fn owned_bitmap_out_of_bounds() -> Result<(), AllocError> {
+        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].
+    // #[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(())
+    }
+}
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 6b4774b2b1c3..8ae5f1ee0308 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -54,6 +54,7 @@
 pub mod alloc;
 #[cfg(CONFIG_AUXILIARY_BUS)]
 pub mod auxiliary;
+pub mod bitmap;
 #[cfg(CONFIG_BLOCK)]
 pub mod block;
 #[doc(hidden)]
diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
index fd1238753cad..63863f6de740 100644
--- a/security/Kconfig.hardening
+++ b/security/Kconfig.hardening
@@ -248,6 +248,16 @@ config LIST_HARDENED
 
 	  If unsure, say N.
 
+config RUST_BITMAP_HARDENED
+	bool "Check integrity of bitmap Rust API"
+	depends on RUST
+	help
+	  Enables additional assertions in the Rust Bitmap API to catch
+	  arguments that are not guaranteed to result in an immediate access
+	  fault.
+
+	  If unsure, say N.
+
 config BUG_ON_DATA_CORRUPTION
 	bool "Trigger a BUG when data corruption is detected"
 	select LIST_HARDENED
-- 
2.50.0.rc2.761.g2dc52ea45b-goog

[PATCH v13 4/5] rust: add find_bit_benchmark_rust module.

[Burak Emir]

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 <aliceryhl@google.com>
Suggested-by: Yury Norov [NVIDIA] <yury.norov@gmail.com>
Reviewed-by: Yury Norov [NVIDIA] <yury.norov@gmail.com>
Signed-off-by: Burak Emir <bqe@google.com>
---
 MAINTAINERS                     |   1 +
 lib/Kconfig.debug               |  13 ++++
 lib/Makefile                    |   1 +
 lib/find_bit_benchmark_rust.rs  | 103 ++++++++++++++++++++++++++++++++
 rust/bindings/bindings_helper.h |   1 +
 rust/kernel/bitmap.rs           |  15 +++++
 6 files changed, 134 insertions(+)
 create mode 100644 lib/find_bit_benchmark_rust.rs

diff --git a/MAINTAINERS b/MAINTAINERS
index b7944346afbf..92511c925db8 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -4208,6 +4208,7 @@ M:	Alice Ryhl <aliceryhl@google.com>
 M:	Burak Emir <bqe@google.com>
 R:	Yury Norov <yury.norov@gmail.com>
 S:	Maintained
+F:	lib/find_bit_benchmark_rust.rs
 F:	rust/kernel/bitmap.rs
 
 BITOPS API
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index ebe33181b6e6..19e8275d3f29 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -2598,6 +2598,19 @@ config FIND_BIT_BENCHMARK
 
 	  If unsure, say N.
 
+config FIND_BIT_BENCHMARK_RUST
+	tristate "Test find_bit functions in Rust"
+	depends on RUST
+	help
+	  This builds the "find_bit_benchmark_rust" module. It is a micro
+          benchmark that measures the performance of Rust functions that
+          correspond to the find_*_bit() operations in C. It follows the
+          FIND_BIT_BENCHMARK closely but will in general not yield same
+          numbers due to extra bounds checks and overhead of foreign
+          function calls.
+
+	  If unsure, say N.
+
 config TEST_FIRMWARE
 	tristate "Test firmware loading via userspace interface"
 	depends on FW_LOADER
diff --git a/lib/Makefile b/lib/Makefile
index c38582f187dd..d0cf5c1fedf1 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -62,6 +62,7 @@ obj-y += hexdump.o
 obj-$(CONFIG_TEST_HEXDUMP) += test_hexdump.o
 obj-y += kstrtox.o
 obj-$(CONFIG_FIND_BIT_BENCHMARK) += find_bit_benchmark.o
+obj-$(CONFIG_FIND_BIT_BENCHMARK_RUST) += find_bit_benchmark_rust.o
 obj-$(CONFIG_TEST_BPF) += test_bpf.o
 test_dhry-objs := dhry_1.o dhry_2.o dhry_run.o
 obj-$(CONFIG_TEST_DHRY) += test_dhry.o
diff --git a/lib/find_bit_benchmark_rust.rs b/lib/find_bit_benchmark_rust.rs
new file mode 100644
index 000000000000..adcd14988383
--- /dev/null
+++ b/lib/find_bit_benchmark_rust.rs
@@ -0,0 +1,103 @@
+// SPDX-License-Identifier: GPL-2.0
+//! Benchmark for find_bit-like methods in Bitmap Rust API.
+
+use kernel::alloc::flags::GFP_KERNEL;
+use kernel::bindings;
+use kernel::bitmap::BitmapVec;
+use kernel::error::{code, Result};
+use kernel::prelude::module;
+use kernel::time::Ktime;
+use kernel::ThisModule;
+use kernel::{pr_cont, pr_err};
+
+const BITMAP_LEN: usize = 4096 * 8 * 10;
+// Reciprocal of the fraction of bits that are set in sparse bitmap.
+const SPARSENESS: usize = 500;
+
+/// Test module that benchmarks performance of traversing bitmaps.
+struct Benchmark();
+
+fn test_next_bit(bitmap: &BitmapVec) {
+    let mut time = Ktime::ktime_get();
+    let mut cnt = 0;
+    let mut i = 0;
+
+    while let Some(index) = bitmap.next_bit(i) {
+        cnt += 1;
+        i = index + 1;
+        // CONFIG_RUST_BITMAP_HARDENED enforces strict bounds.
+        if i == BITMAP_LEN {
+            break;
+        }
+    }
+
+    time = Ktime::ktime_get() - time;
+    pr_cont!(
+        "next_bit:           {:18} ns, {:6} iterations\n",
+        time.to_ns(),
+        cnt
+    );
+}
+
+fn test_next_zero_bit(bitmap: &BitmapVec) {
+    let mut time = Ktime::ktime_get();
+    let mut cnt = 0;
+    let mut i = 0;
+
+    while let Some(index) = bitmap.next_zero_bit(i) {
+        cnt += 1;
+        i = index + 1;
+        // CONFIG_RUST_BITMAP_HARDENED enforces strict bounds.
+        if i == BITMAP_LEN {
+            break;
+        }
+    }
+
+    time = Ktime::ktime_get() - time;
+    pr_cont!(
+        "next_zero_bit:      {:18} ns, {:6} iterations\n",
+        time.to_ns(),
+        cnt
+    );
+}
+
+fn find_bit_test() {
+    pr_err!("\n");
+    pr_cont!("Start testing find_bit() Rust with random-filled bitmap\n");
+
+    let mut bitmap = BitmapVec::new(BITMAP_LEN, GFP_KERNEL).expect("alloc bitmap failed");
+    bitmap.fill_random();
+
+    test_next_bit(&bitmap);
+    test_next_zero_bit(&bitmap);
+
+    pr_cont!("Start testing find_bit() Rust with sparse bitmap\n");
+
+    let mut bitmap = BitmapVec::new(BITMAP_LEN, GFP_KERNEL).expect("alloc sparse bitmap failed");
+    let nbits = BITMAP_LEN / SPARSENESS;
+    for _i in 0..nbits {
+        // SAFETY: __get_random_u32_below is safe to call with any u32 argument.
+        let bit: usize =
+            unsafe { bindings::__get_random_u32_below(BITMAP_LEN.try_into().unwrap()) as _ };
+        bitmap.set_bit(bit);
+    }
+
+    test_next_bit(&bitmap);
+    test_next_zero_bit(&bitmap);
+}
+
+impl kernel::Module for Benchmark {
+    fn init(_module: &'static ThisModule) -> Result<Self> {
+        find_bit_test();
+        // Return error so test module can be inserted again without rmmod.
+        Err(code::EINVAL)
+    }
+}
+
+module! {
+    type: Benchmark,
+    name: "find_bit_benchmark_rust",
+    authors: ["Burak Emir <bqe@google.com>"],
+    description: "Module with benchmark for bitmap Rust API",
+    license: "GPL v2",
+}
diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index cc2ae3b172da..b1334261be6e 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -65,6 +65,7 @@
 #include <linux/pm_opp.h>
 #include <linux/poll.h>
 #include <linux/property.h>
+#include <linux/random.h>
 #include <linux/refcount.h>
 #include <linux/sched.h>
 #include <linux/security.h>
diff --git a/rust/kernel/bitmap.rs b/rust/kernel/bitmap.rs
index a3cd0d4f6970..f1bb742e601b 100644
--- a/rust/kernel/bitmap.rs
+++ b/rust/kernel/bitmap.rs
@@ -252,6 +252,21 @@ pub fn new(nbits: usize, flags: Flags) -> Result<Self, AllocError> {
     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() as *mut ffi::c_void,
+                usize::div_ceil(self.nbits, bindings::BITS_PER_LONG as usize)
+                    * bindings::BITS_PER_LONG as usize
+                    / 8,
+            );
+        }
+    }
 }
 
 impl Bitmap {
-- 
2.50.0.rc2.761.g2dc52ea45b-goog

[PATCH v13 5/5] rust: add dynamic ID pool abstraction for bitmap

[Burak Emir]

This is a port of the Binder data structure introduced in commit
15d9da3f818c ("binder: use bitmap for faster descriptor lookup") to
Rust.

Like drivers/android/dbitmap.h, the ID pool abstraction lets
clients acquire and release IDs. The implementation uses a bitmap to
know what IDs are in use, and gives clients fine-grained control over
the time of allocation. This fine-grained control is needed in the
Android Binder. We provide an example that release a spinlock for
allocation and unit tests (rustdoc examples).

The implementation does not permit shrinking below capacity below
BITS_PER_LONG.

Suggested-by: Alice Ryhl <aliceryhl@google.com>
Suggested-by: Yury Norov <yury.norov@gmail.com>
Signed-off-by: Burak Emir <bqe@google.com>
---
 MAINTAINERS            |   1 +
 rust/kernel/id_pool.rs | 226 +++++++++++++++++++++++++++++++++++++++++
 rust/kernel/lib.rs     |   1 +
 3 files changed, 228 insertions(+)
 create mode 100644 rust/kernel/id_pool.rs

diff --git a/MAINTAINERS b/MAINTAINERS
index 92511c925db8..49acd6d327dc 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -4210,6 +4210,7 @@ R:	Yury Norov <yury.norov@gmail.com>
 S:	Maintained
 F:	lib/find_bit_benchmark_rust.rs
 F:	rust/kernel/bitmap.rs
+F:	rust/kernel/id_pool.rs
 
 BITOPS API
 M:	Yury Norov <yury.norov@gmail.com>
diff --git a/rust/kernel/id_pool.rs b/rust/kernel/id_pool.rs
new file mode 100644
index 000000000000..a41a3404213c
--- /dev/null
+++ b/rust/kernel/id_pool.rs
@@ -0,0 +1,226 @@
+// SPDX-License-Identifier: GPL-2.0
+
+// Copyright (C) 2025 Google LLC.
+
+//! Rust API for an ID pool backed by a [`BitmapVec`].
+
+use crate::alloc::{AllocError, Flags};
+use crate::bitmap::BitmapVec;
+
+const BITS_PER_LONG: usize = bindings::BITS_PER_LONG as usize;
+
+/// Represents a dynamic ID pool backed by a [`BitmapVec`].
+///
+/// Clients acquire and release IDs from unset bits in a bitmap.
+///
+/// The capacity of the ID pool may be adjusted by users as
+/// needed. The API supports the scenario where users need precise control
+/// over the time of allocation of a new backing bitmap, which may require
+/// release of spinlock.
+/// Due to concurrent updates, all operations are re-verified to determine
+/// if the grow or shrink is sill valid.
+///
+/// # Examples
+///
+/// Basic usage
+///
+/// ```
+/// use kernel::alloc::{AllocError, flags::GFP_KERNEL};
+/// use kernel::id_pool::IdPool;
+///
+/// let mut pool = IdPool::new(64, GFP_KERNEL)?;
+/// for i in 0..64 {
+///     assert_eq!(i, pool.acquire_next_id(i).ok_or(ENOSPC)?);
+/// }
+///
+/// pool.release_id(23);
+/// assert_eq!(23, pool.acquire_next_id(0).ok_or(ENOSPC)?);
+///
+/// assert_eq!(None, pool.acquire_next_id(0));  // time to realloc.
+/// let resizer = pool.grow_request().ok_or(ENOSPC)?.realloc(GFP_KERNEL)?;
+/// pool.grow(resizer);
+///
+/// assert_eq!(pool.acquire_next_id(0), Some(64));
+/// # Ok::<(), Error>(())
+/// ```
+///
+/// Releasing spinlock to grow the pool
+///
+/// ```no_run
+/// use kernel::alloc::{AllocError, flags::GFP_KERNEL};
+/// use kernel::sync::{new_spinlock, SpinLock};
+/// use kernel::id_pool::IdPool;
+///
+/// fn get_id_maybe_realloc(guarded_pool: &SpinLock<IdPool>) -> Result<usize, AllocError> {
+///     let mut pool = guarded_pool.lock();
+///     loop {
+///         match pool.acquire_next_id(0) {
+///             Some(index) => return Ok(index),
+///             None => {
+///                 let alloc_request = pool.grow_request();
+///                 drop(pool);
+///                 let resizer = alloc_request.ok_or(AllocError)?.realloc(GFP_KERNEL)?;
+///                 pool = guarded_pool.lock();
+///                 pool.grow(resizer)
+///             }
+///         }
+///     }
+/// }
+/// ```
+pub struct IdPool {
+    map: BitmapVec,
+}
+
+/// Indicates that an [`IdPool`] should change to a new target size.
+pub struct ReallocRequest {
+    num_ids: usize,
+}
+
+/// Contains a [`BitmapVec`] of a size suitable for reallocating [`IdPool`].
+pub struct PoolResizer {
+    new: BitmapVec,
+}
+
+impl ReallocRequest {
+    /// Allocates a new backing [`BitmapVec`] for [`IdPool`].
+    ///
+    /// This method only prepares reallocation and does not complete it.
+    /// Reallocation will complete after passing the [`PoolResizer`] to the
+    /// [`IdPool::grow`] or [`IdPool::shrink`] operation, which will check
+    /// that reallocation still makes sense.
+    pub fn realloc(&self, flags: Flags) -> Result<PoolResizer, AllocError> {
+        let new = BitmapVec::new(self.num_ids, flags)?;
+        Ok(PoolResizer { new })
+    }
+}
+
+impl IdPool {
+    /// Constructs a new [`IdPool`].
+    ///
+    /// A capacity below [`BITS_PER_LONG`] is adjusted to
+    /// [`BITS_PER_LONG`].
+    ///
+    /// [`BITS_PER_LONG`]: srctree/include/asm-generic/bitsperlong.h
+    #[inline]
+    pub fn new(num_ids: usize, flags: Flags) -> Result<Self, AllocError> {
+        let num_ids = core::cmp::max(num_ids, BITS_PER_LONG);
+        let map = BitmapVec::new(num_ids, flags)?;
+        Ok(Self { map })
+    }
+
+    /// Returns how many IDs this pool can currently have.
+    #[inline]
+    pub fn capacity(&self) -> usize {
+        self.map.len()
+    }
+
+    /// Returns a [`ReallocRequest`] if the [`IdPool`] can be shrunk, [`None`] otherwise.
+    ///
+    /// The capacity of an [`IdPool`] cannot be shrunk below [`BITS_PER_LONG`].
+    ///
+    /// [`BITS_PER_LONG`]: srctree/include/asm-generic/bitsperlong.h
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use kernel::alloc::{AllocError, flags::GFP_KERNEL};
+    /// use kernel::id_pool::{ReallocRequest, IdPool};
+    ///
+    /// let mut pool = IdPool::new(1024, GFP_KERNEL)?;
+    /// let alloc_request = pool.shrink_request().ok_or(AllocError)?;
+    /// let resizer = alloc_request.realloc(GFP_KERNEL)?;
+    /// pool.shrink(resizer);
+    /// assert_eq!(pool.capacity(), kernel::bindings::BITS_PER_LONG as usize);
+    /// # Ok::<(), AllocError>(())
+    /// ```
+    #[inline]
+    pub fn shrink_request(&self) -> Option<ReallocRequest> {
+        let cap = self.capacity();
+        // Shrinking below [`BITS_PER_LONG`] is never possible.
+        if cap <= BITS_PER_LONG {
+            return None;
+        }
+        // Determine if the bitmap can shrink based on the position of
+        // its last set bit. If the bit is within the first quarter of
+        // the bitmap then shrinking is possible. In this case, the
+        // bitmap should shrink to half its current size.
+        let Some(bit) = self.map.last_bit() else {
+            return Some(ReallocRequest {
+                num_ids: BITS_PER_LONG,
+            });
+        };
+        if bit >= (cap / 4) {
+            return None;
+        }
+        let num_ids = usize::max(BITS_PER_LONG, cap / 2);
+        Some(ReallocRequest { num_ids })
+    }
+
+    /// Shrinks pool by using a new [`BitmapVec`], if still possible.
+    #[inline]
+    pub fn shrink(&mut self, mut resizer: PoolResizer) {
+        // Between request to shrink that led to allocation of `resizer` and now,
+        // bits may have changed.
+        // Verify that shrinking is still possible. In case shrinking to
+        // the size of `resizer` is no longer possible, do nothing,
+        // drop `resizer` and move on.
+        let Some(updated) = self.shrink_request() else {
+            return;
+        };
+        if updated.num_ids > resizer.new.len() {
+            return;
+        }
+
+        resizer.new.copy_and_extend(&self.map);
+        self.map = resizer.new;
+    }
+
+    /// Returns a [`ReallocRequest`] for growing this [`IdPool`], if possible.
+    ///
+    /// The capacity of an [`IdPool`] cannot be grown above [`i32::MAX`].
+    #[inline]
+    pub fn grow_request(&self) -> Option<ReallocRequest> {
+        let num_ids = self.capacity() * 2;
+        if num_ids > i32::MAX.try_into().unwrap() {
+            return None;
+        }
+        Some(ReallocRequest { num_ids })
+    }
+
+    /// Grows pool by using a new [`BitmapVec`], if still necessary.
+    ///
+    /// The `resizer` arguments has to be obtained by calling [`Self::grow_request`]
+    /// on this object and performing a [`ReallocRequest::realloc`].
+    #[inline]
+    pub fn grow(&mut self, mut resizer: PoolResizer) {
+        // Between request to grow that led to allocation of `resizer` and now,
+        // another thread may have already grown the capacity.
+        // In this case, do nothing, drop `resizer` and move on.
+        if resizer.new.len() <= self.capacity() {
+            return;
+        }
+
+        resizer.new.copy_and_extend(&self.map);
+        self.map = resizer.new;
+    }
+
+    /// Acquires a new ID by finding and setting the next zero bit in the
+    /// bitmap.
+    ///
+    /// Upon success, returns its index. Otherwise, returns [`None`]
+    /// to indicate that a [`Self::grow_request`] is needed.
+    #[inline]
+    pub fn acquire_next_id(&mut self, offset: usize) -> Option<usize> {
+        let next_zero_bit = self.map.next_zero_bit(offset);
+        if let Some(nr) = next_zero_bit {
+            self.map.set_bit(nr);
+        }
+        next_zero_bit
+    }
+
+    /// Releases an ID.
+    #[inline]
+    pub fn release_id(&mut self, id: usize) {
+        self.map.clear_bit(id);
+    }
+}
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 8ae5f1ee0308..569c8b87031f 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -79,6 +79,7 @@
 #[cfg(CONFIG_RUST_FW_LOADER_ABSTRACTIONS)]
 pub mod firmware;
 pub mod fs;
+pub mod id_pool;
 pub mod init;
 pub mod io;
 pub mod ioctl;
-- 
2.50.0.rc2.761.g2dc52ea45b-goog

Re: [PATCH v13 0/5] rust: adds Bitmap API, ID pool and bindings

[Alice Ryhl]

On Fri, Jun 20, 2025 at 10:29 AM Burak Emir <bqe@google.com> wrote:
>
> This series adds a Rust bitmap API 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 a safe abstraction to underlying bitmap
> and bitops operations. For now, only includes method necessary for
> dbitmap.h, more can be added later. We perform bounds checks for
> hardening, violations are programmer errors that result in panics.
>
> We include set_bit_atomic and clear_bit_atomic operations. One has
> to avoid races with non-atomic operations, which is ensure by the
> Rust type system: either callers have shared references &bitmap in
> which case the mutations are atomic operations. Or there is a
> exclusive reference &mut bitmap, in which case there is no concurrent
> access.
>
> This series includes an optimization to represent the bitmap inline,
> as suggested by Yury.
>
> We ran a simple microbenchmark which shows that overall the Rust API
> can be expected to be about 4.5% slower than C API.
>
> We also introduce a Rust API in id_pool.rs that would replace
> dbitmap.h from the commit referenced above. This data structure is coupled
> with the bitmap API and adds support for growing and shrinking, along
> with fine-grained control over when allocation happens.
> The Binder code needs this since it holds a spinlock at the time it
> discovers that growing is necessary; this has to be release
> for performing a memory allocation with GFP_KERNEL that may cause
> sleep.  We includes example doctests that demonstrate this usage.
>
> This is v13, see [v12] for previous state. Thanks everyone for all the
> helpful comments, this series has improved significantly as a result of
> your work.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>