[PATCH] rust: bitmap: encourage using xarray/maple_tree instead of id_pool

[PATCH] rust: bitmap: encourage using xarray/maple_tree instead of id_pool

[Alice Ryhl]

The id_pool.rs file was added for use in the Binder driver, which is
using a bitmap rather than the normal IDR implementation due its
specialized needs and performance/spinlock requirements. However, its
current name as kernel::id_pool encourages using it over other IDR
solutions.

To discourage choosing this pool when you don't need it, move it to
kernel::bitmap and add a comment recommending the xarray or maple tree
for generic IDR use-cases.

Please see the below links for the discussion that prompted moving this
file.

Link: https://lore.kernel.org/rust-for-linux/2026070334-dollar-hexagram-e49c@gregkh/
Link: https://lore.kernel.org/rust-for-linux/84bc8bd2-e292-4b84-9580-a1b5df4c5bdc@nvidia.com/
Link: https://lore.kernel.org/rust-for-linux/20260711063602.426311-1-ynorov@nvidia.com/
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 MAINTAINERS                         |  2 +-
 drivers/android/binder/process.rs   |  2 +-
 rust/kernel/bitmap.rs               |  2 ++
 rust/kernel/{ => bitmap}/id_pool.rs | 26 +++++++++++++++++++-------
 rust/kernel/lib.rs                  |  1 -
 5 files changed, 23 insertions(+), 10 deletions(-)

diff --git a/MAINTAINERS b/MAINTAINERS
index 4a8b0fd665ce..d80377d06c3a 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -4639,7 +4639,7 @@ 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
+F:	rust/kernel/bitmap/
 
 BITOPS API
 M:	Yury Norov <yury.norov@gmail.com>
diff --git a/drivers/android/binder/process.rs b/drivers/android/binder/process.rs
index 96b8440ceac6..eec417604806 100644
--- a/drivers/android/binder/process.rs
+++ b/drivers/android/binder/process.rs
@@ -16,10 +16,10 @@
 
 use kernel::{
     bindings,
+    bitmap::id_pool::IdPool,
     cred::Credential,
     error::Error,
     fs::file::{self, File},
-    id_pool::IdPool,
     list::{List, ListArc, ListArcField, ListLinks},
     mm,
     prelude::*,
diff --git a/rust/kernel/bitmap.rs b/rust/kernel/bitmap.rs
index b27e0ec80d64..ecf8f194cf3a 100644
--- a/rust/kernel/bitmap.rs
+++ b/rust/kernel/bitmap.rs
@@ -12,6 +12,8 @@
 use crate::pr_err;
 use core::ptr::NonNull;
 
+pub mod id_pool;
+
 /// Represents a C bitmap. Wraps underlying C bitmap API.
 ///
 /// # Invariants
diff --git a/rust/kernel/id_pool.rs b/rust/kernel/bitmap/id_pool.rs
similarity index 93%
rename from rust/kernel/id_pool.rs
rename to rust/kernel/bitmap/id_pool.rs
index 384753fe0e44..d6440ec8f60f 100644
--- a/rust/kernel/id_pool.rs
+++ b/rust/kernel/bitmap/id_pool.rs
@@ -3,6 +3,14 @@
 // Copyright (C) 2025 Google LLC.
 
 //! Rust API for an ID pool backed by a [`BitmapVec`].
+//!
+//! The id pool provided by this file is designed for specialized use-cases
+//! that need an implementation backed by a bitmap. If you just want a generic
+//! ID allocator, please use [`XArray`] instead. Or for use-cases that require
+//! contiguous ranges of IDs, use the [`MapleTree`].
+//!
+//! [`XArray`]: crate::xarray::XArray
+//! [`MapleTree`]: crate::maple_tree::MapleTree
 
 use crate::alloc::{AllocError, Flags};
 use crate::bitmap::BitmapVec;
@@ -23,8 +31,10 @@
 /// Basic usage
 ///
 /// ```
-/// use kernel::alloc::AllocError;
-/// use kernel::id_pool::{IdPool, UnusedId};
+/// use kernel::{
+///     alloc::AllocError,
+///     bitmap::id_pool::{IdPool, UnusedId},
+/// };
 ///
 /// let mut pool = IdPool::with_capacity(64, GFP_KERNEL)?;
 /// for i in 0..64 {
@@ -47,7 +57,7 @@
 /// ```no_run
 /// use kernel::alloc::{AllocError, flags::GFP_KERNEL};
 /// use kernel::sync::{new_spinlock, SpinLock};
-/// use kernel::id_pool::IdPool;
+/// use kernel::bitmap::id_pool::IdPool;
 ///
 /// fn get_id_maybe_realloc(guarded_pool: &SpinLock<IdPool>) -> Result<usize, AllocError> {
 ///     let mut pool = guarded_pool.lock();
@@ -134,10 +144,12 @@ pub fn capacity(&self) -> usize {
     /// ```
     /// use kernel::{
     ///     alloc::AllocError,
-    ///     bitmap::BitmapVec,
-    ///     id_pool::{
-    ///         IdPool,
-    ///         ReallocRequest,
+    ///     bitmap::{
+    ///         id_pool::{
+    ///             IdPool,
+    ///             ReallocRequest,
+    ///         },
+    ///         BitmapVec,
     ///     },
     /// };
     ///
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 9512af7156df..8e7717bb8b9f 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -77,7 +77,6 @@
 pub mod gpu;
 #[cfg(CONFIG_I2C = "y")]
 pub mod i2c;
-pub mod id_pool;
 #[doc(hidden)]
 pub mod impl_flags;
 pub mod init;

---
base-commit: 8cdeaa50eae8dad34885515f62559ee83e7e8dda
change-id: 20260711-id-pool-move-bitmap-5b0dc0d2befc

Best regards,
-- 
Alice Ryhl <aliceryhl@google.com>