[PATCH v6 1/6] rust: bitmap: add MAX_LEN and MAX_INLINE_LEN constants

Alice Ryhl
2025-11-25T13:59:56Z

To avoid hard-coding these values in drivers, define constants for them
that drivers can reference. Also, update all instances in bitmap.rs and
id_pool.rs that use these values to use the new constants.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
rust/kernel/bitmap.rs | 33 +++++++++++++++++++--------------
rust/kernel/id_pool.rs | 39 ++++++++++++++++++++++-----------------
2 files changed, 41 insertions(+), 31 deletions(-)

diff --git a/rust/kernel/bitmap.rs b/rust/kernel/bitmap.rs
index aa8fc7bf06fc99865ae755d8694e4bec3dc8e7f0..0705646c6251a49f213a45f1f013cb9eb2ed81de 100644
--- a/rust/kernel/bitmap.rs
+++ b/rust/kernel/bitmap.rs
@@ -12,8 +12,6 @@
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
@@ -149,14 +147,14 @@ macro_rules! bitmap_assert_return {
///
/// # Invariants
///
-/// * `nbits` is `<= i32::MAX` and never changes.
-/// * if `nbits <= bindings::BITS_PER_LONG`, then `repr` is a `usize`.
+/// * `nbits` is `<= MAX_LEN`.
+/// * if `nbits <= MAX_INLINE_LEN`, 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`.
+ /// Length of this bitmap. Must be `<= MAX_LEN`.
nbits: usize,
}

@@ -164,7 +162,7 @@ impl core::ops::Deref for BitmapVec {
type Target = Bitmap;

fn deref(&self) -> &Bitmap {
- let ptr = if self.nbits <= BITS_PER_LONG {
+ let ptr = if self.nbits <= BitmapVec::MAX_INLINE_LEN {
// SAFETY: Bitmap is represented inline.
#[allow(unused_unsafe, reason = "Safe since Rust 1.92.0")]
unsafe {
@@ -183,7 +181,7 @@ fn deref(&self) -> &Bitmap {

impl core::ops::DerefMut for BitmapVec {
fn deref_mut(&mut self) -> &mut Bitmap {
- let ptr = if self.nbits <= BITS_PER_LONG {
+ let ptr = if self.nbits <= BitmapVec::MAX_INLINE_LEN {
// SAFETY: Bitmap is represented inline.
#[allow(unused_unsafe, reason = "Safe since Rust 1.92.0")]
unsafe {
@@ -213,7 +211,7 @@ unsafe impl Sync for BitmapVec {}

impl Drop for BitmapVec {
fn drop(&mut self) {
- if self.nbits <= BITS_PER_LONG {
+ if self.nbits <= BitmapVec::MAX_INLINE_LEN {
return;
}
// SAFETY: `self.ptr` was returned by the C `bitmap_zalloc`.
@@ -226,23 +224,29 @@ fn drop(&mut self) {
}

impl BitmapVec {
+ /// The maximum possible length of a `BitmapVec`.
+ pub const MAX_LEN: usize = i32::MAX as usize;
+
+ /// The maximum length that uses the inline representation.
+ pub const MAX_INLINE_LEN: usize = usize::BITS as usize;
+
/// 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`.
+ /// includes the case when `nbits` is greater than `MAX_LEN`.
#[inline]
pub fn new(nbits: usize, flags: Flags) -> Result<Self, AllocError> {
- if nbits <= BITS_PER_LONG {
+ if nbits <= BitmapVec::MAX_INLINE_LEN {
return Ok(BitmapVec {
repr: BitmapRepr { bitmap: 0 },
nbits,
});
}
- if nbits > i32::MAX.try_into().unwrap() {
+ if nbits > Self::MAX_LEN {
return Err(AllocError);
}
let nbits_u32 = u32::try_from(nbits).unwrap();
- // SAFETY: `BITS_PER_LONG < nbits` and `nbits <= i32::MAX`.
+ // SAFETY: `MAX_INLINE_LEN < nbits` and `nbits <= MAX_LEN`.
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.
@@ -495,9 +499,10 @@ mod tests {
#[test]
fn bitmap_borrow() {
let fake_bitmap: [usize; 2] = [0, 0];
+ let fake_bitmap_len = 2 * usize::BITS as usize;
// 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());
+ let b = unsafe { Bitmap::from_raw(fake_bitmap.as_ptr(), fake_bitmap_len) };
+ assert_eq!(fake_bitmap_len, b.len());
assert_eq!(None, b.next_bit(0));
}

diff --git a/rust/kernel/id_pool.rs b/rust/kernel/id_pool.rs
index a41a3404213ca92d53b14c80101afff6ac8c416e..0b1f720a1f7df8766a8481f3c0b332d4cff3b4ad 100644
--- a/rust/kernel/id_pool.rs
+++ b/rust/kernel/id_pool.rs
@@ -7,8 +7,6 @@
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.
@@ -97,13 +95,12 @@ pub fn realloc(&self, flags: Flags) -> Result<PoolResizer, AllocError> {
impl IdPool {
/// Constructs a new [`IdPool`].
///
- /// A capacity below [`BITS_PER_LONG`] is adjusted to
- /// [`BITS_PER_LONG`].
+ /// A capacity below [`MAX_INLINE_LEN`] is adjusted to [`MAX_INLINE_LEN`].
///
- /// [`BITS_PER_LONG`]: srctree/include/asm-generic/bitsperlong.h
+ /// [`MAX_INLINE_LEN`]: BitmapVec::MAX_INLINE_LEN
#[inline]
pub fn new(num_ids: usize, flags: Flags) -> Result<Self, AllocError> {
- let num_ids = core::cmp::max(num_ids, BITS_PER_LONG);
+ let num_ids = usize::max(num_ids, BitmapVec::MAX_INLINE_LEN);
let map = BitmapVec::new(num_ids, flags)?;
Ok(Self { map })
}
@@ -116,28 +113,34 @@ pub fn capacity(&self) -> usize {

/// Returns a [`ReallocRequest`] if the [`IdPool`] can be shrunk, [`None`] otherwise.
///
- /// The capacity of an [`IdPool`] cannot be shrunk below [`BITS_PER_LONG`].
+ /// The capacity of an [`IdPool`] cannot be shrunk below [`MAX_INLINE_LEN`].
///
- /// [`BITS_PER_LONG`]: srctree/include/asm-generic/bitsperlong.h
+ /// [`MAX_INLINE_LEN`]: BitmapVec::MAX_INLINE_LEN
///
/// # Examples
///
/// ```
- /// use kernel::alloc::{AllocError, flags::GFP_KERNEL};
- /// use kernel::id_pool::{ReallocRequest, IdPool};
+ /// use kernel::{
+ /// alloc::AllocError,
+ /// bitmap::BitmapVec,
+ /// id_pool::{
+ /// IdPool,
+ /// ReallocRequest,
+ /// },
+ /// };
///
/// 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);
+ /// assert_eq!(pool.capacity(), BitmapVec::MAX_INLINE_LEN);
/// # 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 {
+ // Shrinking below `MAX_INLINE_LEN` is never possible.
+ if cap <= BitmapVec::MAX_INLINE_LEN {
return None;
}
// Determine if the bitmap can shrink based on the position of
@@ -146,13 +149,13 @@ pub fn shrink_request(&self) -> Option<ReallocRequest> {
// bitmap should shrink to half its current size.
let Some(bit) = self.map.last_bit() else {
return Some(ReallocRequest {
- num_ids: BITS_PER_LONG,
+ num_ids: BitmapVec::MAX_INLINE_LEN,
});
};
if bit >= (cap / 4) {
return None;
}
- let num_ids = usize::max(BITS_PER_LONG, cap / 2);
+ let num_ids = usize::max(BitmapVec::MAX_INLINE_LEN, cap / 2);
Some(ReallocRequest { num_ids })
}

@@ -177,11 +180,13 @@ pub fn shrink(&mut self, mut resizer: PoolResizer) {

/// Returns a [`ReallocRequest`] for growing this [`IdPool`], if possible.
///
- /// The capacity of an [`IdPool`] cannot be grown above [`i32::MAX`].
+ /// The capacity of an [`IdPool`] cannot be grown above [`MAX_LEN`].
+ ///
+ /// [`MAX_LEN`]: BitmapVec::MAX_LEN
#[inline]
pub fn grow_request(&self) -> Option<ReallocRequest> {
let num_ids = self.capacity() * 2;
- if num_ids > i32::MAX.try_into().unwrap() {
+ if num_ids > BitmapVec::MAX_LEN {
return None;
}
Some(ReallocRequest { num_ids })

--
2.52.0.460.gd25c4c69ec-goog