On Monday, June 2nd, 2025 at 16.53, Burak Emir <[email protected]> wrote:
> 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 <[email protected]>
> Suggested-by: Yury Norov <[email protected]>
> Signed-off-by: Burak Emir <[email protected]>
> ---
> MAINTAINERS | 1 +
> rust/kernel/id_pool.rs | 222 +++++++++++++++++++++++++++++++++++++++++
> rust/kernel/lib.rs | 1 +
> 3 files changed, 224 insertions(+)
> create mode 100644 rust/kernel/id_pool.rs
>
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 943d85ed1876..bc95d98f266b 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -4134,6 +4134,7 @@ R: Yury Norov <[email protected]>
> 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 <[email protected]>
> diff --git a/rust/kernel/id_pool.rs b/rust/kernel/id_pool.rs
> new file mode 100644
> index 000000000000..cf26d405d9bb
> --- /dev/null
> +++ b/rust/kernel/id_pool.rs
> @@ -0,0 +1,222 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +// Copyright (C) 2025 Google LLC.
> +
> +//! Rust API for an ID pool backed by a [`Bitmap`].
> +
> +use crate::alloc::{AllocError, Flags};
> +use crate::bitmap::Bitmap;
> +
> +/// Represents a dynamic ID pool backed by a [`Bitmap`].
> +///
> +/// 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: Bitmap,
> +}
> +
> +/// Indicates that an [`IdPool`] should change to a new target size.
> +pub struct ReallocRequest {
> + num_ids: usize,
> +}
> +
> +/// Contains a [`Bitmap`] of a size suitable for reallocating [`IdPool`].
> +pub struct PoolResizer {
> + new: Bitmap,
> +}
> +
> +impl ReallocRequest {
> + /// Allocates a new backing [`Bitmap`] 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 = Bitmap::new(self.num_ids, flags)?;
> + Ok(PoolResizer { new })
> + }
> +}
> +
> +impl IdPool {
> + /// Constructs a new `[IdPool]`.
The link is broken, it should be [`IdPool`].
> + ///
> + /// A capacity below [`BITS_PER_LONG`] is adjusted to [`BITS_PER_LONG`].
Since `BITS_PER_LONG` is defined in the bindings, it needs an explicit
path. Rustdoc won't link to it otherwise.
Maybe something like:
```
/// [`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, bindings::BITS_PER_LONG as
> usize);
> + let map = Bitmap::new(num_ids, flags)?;
> + Ok(Self { map })
> + }
> +
> + /// Returns how many IDs this pool can currently have.
> + #[inline]
> + pub fn len(&self) -> usize {
> + self.map.len()
> + }
This needs `#[expect(clippy::len_without_is_empty)]` or adding the
`is_empty` method.
> +
> + /// Returns a [`ReallocRequest`] if the [`IdPool`] can be shrunk,
> [`None`] otherwise.
> + ///
> + /// The capacity of an [`IdPool`] cannot be shrunk below
> [`BITS_PER_LONG`].
The `BITS_PER_LONG` link is broken here as well.
> + ///
> + /// # 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.len(), kernel::bindings::BITS_PER_LONG as usize);
> + /// # Ok::<(), AllocError>(())
> + /// ```
> + #[inline]
> + pub fn shrink_request(&self) -> Option<ReallocRequest> {
> + let len = self.map.len();
> + // Shrinking below [`BITS_PER_LONG`] is never possible.
> + if len <= bindings::BITS_PER_LONG as usize {
> + 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 last_bit = self.map.last_bit();
> + if last_bit.is_none() {
> + return Some(ReallocRequest {
> + num_ids: bindings::BITS_PER_LONG as usize,
> + });
> + }
> + let bit = last_bit.unwrap();
Instead of `unwrap`, consider using `let else`:
```
let Some(bit) = self.map.last_bit() else {
return Some(ReallocRequest { ... });
};
```
> + if bit >= (len / 4) {
> + return None;
> + }
> + let num_ids = core::cmp::max(bindings::BITS_PER_LONG as usize, len /
> 2);
> + Some(ReallocRequest { num_ids })
> + }
> +
> + /// Shrinks pool by using a new [`Bitmap`], 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 updated = self.shrink_request();
> + if updated.is_none() {
> + return;
> + }
> + if updated.unwrap().num_ids > resizer.new.len() {
Same here, the unwrap can be replaced with `let else`.
> + 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.map.len() * 2;
> + if num_ids > i32::MAX.try_into().unwrap() {
> + return None;
> + }
> + Some(ReallocRequest { num_ids })
> + }
> +
> + /// Grows pool by using a new [`Bitmap`], if still necessary.
> + ///
> + /// The `resizer` arguments has to be obtained by calling
> [`grow_request`]
> + /// on this object and performing a `realloc`.
The `grow_request` link doesn't work. Also `realloc` should be a link to
the `ReallocRequest::realloc` method.
> + #[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.map.len() {
> + 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 `grow_request` is needed.
`None` and `grow_request` should be changed to links.
> + #[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 8c4161cd82ac..d7def807900a 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -54,6 +54,7 @@
> #[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;
> --
> 2.49.0.1204.g71687c7c1d-goog
Cheers,
Pekka
