linux-kernel - [PATCH v3 3/5] rust: id_pool: do not supply starting capacity

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Message-ID: <20251028-binder-bitmap-v3-3-32822d4b3207@google.com>
Date: Tue, 28 Oct 2025 10:55:16 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Yury Norov <yury.norov@...il.com>
Cc: "Arve Hjønnevåg" <arve@...roid.com>, Todd Kjos <tkjos@...roid.com>, 
	Martijn Coenen <maco@...roid.com>, Joel Fernandes <joelagnelf@...dia.com>, 
	Christian Brauner <brauner@...nel.org>, Carlos Llamas <cmllamas@...gle.com>, 
	Suren Baghdasaryan <surenb@...gle.com>, Burak Emir <bqe@...gle.com>, Miguel Ojeda <ojeda@...nel.org>, 
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	"Björn Roy Baron" <bjorn3_gh@...tonmail.com>, Benno Lossin <lossin@...nel.org>, 
	Andreas Hindborg <a.hindborg@...nel.org>, Trevor Gross <tmgross@...ch.edu>, 
	Danilo Krummrich <dakr@...nel.org>, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Alice Ryhl <aliceryhl@...gle.com>
Subject: [PATCH v3 3/5] rust: id_pool: do not supply starting capacity

Rust Binder wants to use inline bitmaps whenever possible to avoid
allocations, so introduce a constructor for an IdPool with arbitrary
capacity that stores the bitmap inline.

The existing constructor could be renamed to with_capacity() to match
constructors for other similar types, but it is removed as there is
currently no user for it.

Acked-by: Yury Norov (NVIDIA) <yury.norov@...il.com>
Reviewed-by: Burak Emir <bqe@...gle.com>
Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
---
 rust/kernel/id_pool.rs | 46 ++++++++++++++++++----------------------------
 1 file changed, 18 insertions(+), 28 deletions(-)

diff --git a/rust/kernel/id_pool.rs b/rust/kernel/id_pool.rs
index a41a3404213ca92d53b14c80101afff6ac8c416e..d53628a357ed84a6e00ef9dfd03a75e85a87532c 100644
--- a/rust/kernel/id_pool.rs
+++ b/rust/kernel/id_pool.rs
@@ -28,19 +28,21 @@
 /// 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 {
+/// let mut pool = IdPool::new();
+/// let cap = pool.capacity();
+///
+/// for i in 0..cap {
 ///     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)?);
+/// pool.release_id(5);
+/// assert_eq!(5, 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));
+/// assert_eq!(pool.acquire_next_id(0), Some(cap));
 /// # Ok::<(), Error>(())
 /// ```
 ///
@@ -96,16 +98,11 @@ 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`].
-    ///
-    /// [`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 })
+    pub fn new() -> Self {
+        Self {
+            map: BitmapVec::new_inline(),
+        }
     }
 
     /// Returns how many IDs this pool can currently have.
@@ -119,20 +116,6 @@ pub fn capacity(&self) -> usize {
     /// 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();
@@ -224,3 +207,10 @@ pub fn release_id(&mut self, id: usize) {
         self.map.clear_bit(id);
     }
 }
+
+impl Default for IdPool {
+    #[inline]
+    fn default() -> Self {
+        Self::new()
+    }
+}

-- 
2.51.1.838.g19442a804e-goog