rust: add dynamic ID pool abstraction for bitmap

This is a port of the Binder data structure introduced in commit
15d9da3f81 ("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>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Burak Emir <bqe@google.com>
Signed-off-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>

MAINTAINERS

@@ -4309,6 +4309,7 @@ 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>

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);
+    }
+}

rust/kernel/lib.rs

@@ -90,6 +90,7 @@ pub mod faux;
 pub mod firmware;
 pub mod fmt;
 pub mod fs;
+pub mod id_pool;
 pub mod init;
 pub mod io;
 pub mod ioctl;