Re: [PATCH v11 4/5] rust: id_pool: do not round capacity up to BitmapVec::MAX_INLINE_LEN

[Yury Norov <ynorov@nvidia.com>]

On Mon, Nov 09, 2026 at 05:32:52PM +0900, Eliot Courtney wrote:
> Current code in IdPool::with_capacity rounds the capacity up to
> BitmapVec::MAX_INLINE_LEN, but BitmapVec::new works fine with values
> smaller than this and still uses an inline representation. Remove this
> behaviour.
> 
> This allows specifying a real capacity of 0, which was not previously
> possible.

How real capacity 0 became possible? Did we eventually find a use case
for it? If not, it should not be possible, and the implementation
should not explicitly handle that case.

> This breaks `grow_request` in this case, so change it to grow
> to at least `BitmapVec::MAX_INLINE_LEN`, mirroring the capacity floor in
> `shrink_request`.

Not the grow_request only, I believe. I recall the discussion about
panic on most operations on a 0-bit pool.
 
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> Reviewed-by: Burak Emir <burak.emir@gmail.com>
> Signed-off-by: Eliot Courtney <ecourtney@nvidia.com>
> ---
>  rust/kernel/id_pool.rs | 38 ++++++++++++++++++++++++++++++++------
>  1 file changed, 32 insertions(+), 6 deletions(-)
> 
> diff --git a/rust/kernel/id_pool.rs b/rust/kernel/id_pool.rs
> index 92eeb7caa243..f77bd90325ff 100644
> --- a/rust/kernel/id_pool.rs
> +++ b/rust/kernel/id_pool.rs
> @@ -112,13 +112,8 @@ pub fn new() -> Self {
>      }
>  
>      /// Constructs a new [`IdPool`] with space for a specific number of bits.
> -    ///
> -    /// A capacity below [`MAX_INLINE_LEN`] is adjusted to [`MAX_INLINE_LEN`].
> -    ///
> -    /// [`MAX_INLINE_LEN`]: BitmapVec::MAX_INLINE_LEN
>      #[inline]
>      pub fn with_capacity(num_ids: usize, flags: Flags) -> Result<Self, AllocError> {
> -        let num_ids = usize::max(num_ids, BitmapVec::MAX_INLINE_LEN);
>          let map = BitmapVec::new(num_ids, flags)?;
>          Ok(Self { map })
>      }
> @@ -152,6 +147,13 @@ pub fn capacity(&self) -> usize {
>      /// let resizer = alloc_request.realloc(GFP_KERNEL)?;
>      /// pool.shrink(resizer);
>      /// assert_eq!(pool.capacity(), BitmapVec::MAX_INLINE_LEN);
> +    ///
> +    /// // A pool at the `MAX_INLINE_LEN` floor cannot shrink further.
> +    /// assert!(pool.shrink_request().is_none());
> +    ///
> +    /// // Neither can a pool with a capacity below `MAX_INLINE_LEN`.
> +    /// let small = IdPool::with_capacity(8, GFP_KERNEL)?;
> +    /// assert!(small.shrink_request().is_none());
>      /// # Ok::<(), AllocError>(())
>      /// ```
>      #[inline]
> @@ -198,12 +200,36 @@ pub fn shrink(&mut self, mut resizer: PoolResizer) {
>  
>      /// Returns a [`ReallocRequest`] for growing this [`IdPool`], if possible.
>      ///
> +    /// Grows to at least [`MAX_INLINE_LEN`].
>      /// The capacity of an [`IdPool`] cannot be grown above [`MAX_LEN`].
>      ///
> +    /// [`MAX_INLINE_LEN`]: BitmapVec::MAX_INLINE_LEN
>      /// [`MAX_LEN`]: BitmapVec::MAX_LEN
> +    ///
> +    /// # Examples
> +    ///
> +    /// ```
> +    /// use kernel::{
> +    ///     alloc::AllocError,
> +    ///     bitmap::BitmapVec,
> +    ///     id_pool::IdPool, //
> +    /// };
> +    ///
> +    /// // Grow goes to at least BitmapVec::MAX_INLINE_LEN.
> +    /// let mut pool = IdPool::with_capacity(1, GFP_KERNEL)?;
> +    /// let resizer = pool.grow_request().ok_or(AllocError)?.realloc(GFP_KERNEL)?;
> +    /// pool.grow(resizer);
> +    /// assert_eq!(pool.capacity(), BitmapVec::MAX_INLINE_LEN);
> +    ///
> +    /// // Grow doubles if at least BitmapVec::MAX_INLINE_LEN.
> +    /// let resizer = pool.grow_request().ok_or(AllocError)?.realloc(GFP_KERNEL)?;
> +    /// pool.grow(resizer);
> +    /// assert_eq!(pool.capacity(), 2 * BitmapVec::MAX_INLINE_LEN);
> +    /// # Ok::<(), AllocError>(())
> +    /// ```
>      #[inline]
>      pub fn grow_request(&self) -> Option<ReallocRequest> {
> -        let num_ids = self.capacity() * 2;
> +        let num_ids = usize::max(BitmapVec::MAX_INLINE_LEN, self.capacity() * 2);

You only need this max(...) thing to fix the behavior of 0-len bitmap.
We don't have any users with the request to support them. Please don't
add any extra code to handle unwanted and unsupported cases.

Thanks,
Yury

>          if num_ids > BitmapVec::MAX_LEN {
>              return None;
>          }
> 
> -- 
> 2.56.0
>