Re: [PATCH 09/10] rust: xarray: add preload API

[Gary Guo]

On Wed Dec 3, 2025 at 10:26 PM GMT, Andreas Hindborg wrote:
> Add a preload API that allows preallocating memory for XArray
> insertions. This enables insertions to proceed without allocation
> failures in contexts where memory allocation is not desirable, such as
> in atomic contexts or where reliability is critical.
>
> The API includes:
>
> - `XArrayPreloadBuffer` for managing a pool of preallocated nodes.
> - `XArrayPreloadNode` representing a single preallocated node.
> - Integration with the entry API, allowing `VacantEntry::insert` and
>   `VacantEntry::insert_entry` to accept an optional preload buffer.
> - A new `Guard::insert_entry` method for inserting with preload support.
>
> The implementation uses a circular buffer to efficiently manage
> preallocated nodes. When an insertion would fail due to ENOMEM, the
> XArray state API automatically consumes a preallocated node from the
> buffer if available.

It looks like this code doesn't neede to be XArray specific. You can provide
a generic abstraction for kmem_cache and just use a specific instance for
XArray?

Best,
Gary

>
> Signed-off-by: Andreas Hindborg <a.hindborg@xxxxxxxxxx>
> ---
>  rust/bindings/bindings_helper.h |   3 +
>  rust/kernel/xarray.rs           |  58 ++++++++++-
>  rust/kernel/xarray/entry.rs     |  64 +++++++++---
>  rust/kernel/xarray/preload.rs   | 217 ++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 324 insertions(+), 18 deletions(-)
>
> diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
> index 86bca946faff0..8e9f8762d5e6e 100644
> --- a/rust/bindings/bindings_helper.h
> +++ b/rust/bindings/bindings_helper.h
> @@ -110,6 +110,9 @@ const gfp_t RUST_CONST_HELPER_XA_FLAGS_ALLOC = XA_FLAGS_ALLOC;
>  const gfp_t RUST_CONST_HELPER_XA_FLAGS_ALLOC1 = XA_FLAGS_ALLOC1;
>  const vm_flags_t RUST_CONST_HELPER_VM_MERGEABLE = VM_MERGEABLE;
>  const size_t RUST_CONST_HELPER_XAS_RESTART = (size_t)XAS_RESTART;
> +const size_t RUST_CONST_HELPER_XA_CHUNK_SHIFT = XA_CHUNK_SHIFT;
> +const size_t RUST_CONST_HELPER_XA_CHUNK_SIZE = XA_CHUNK_SIZE;
> +extern struct kmem_cache *radix_tree_node_cachep;
>  
>  #if IS_ENABLED(CONFIG_ANDROID_BINDER_IPC_RUST)
>  #include "../../drivers/android/binder/rust_binder.h"