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 <alicer...@google.com>
Suggested-by: Yury Norov <yury.no...@gmail.com>
Reviewed-by: Alice Ryhl <alicer...@google.com>
Signed-off-by: Burak Emir <b...@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 f99e7f4eb4ca..e62bebfffa9c 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -4308,6 +4308,7 @@ R: Yury Norov <yury.no...@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.no...@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 586be7f246eb..9b8a6c386c52 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -90,6 +90,7 @@
pub mod firmware;
pub mod fmt;
pub mod fs;
+pub mod id_pool;
pub mod init;
pub mod io;
pub mod ioctl;
--
2.51.0.rc0.205.g4a044479a3-goog
