Re: [PATCH v2 11/11] rust: xarray: add preload API

[Andreas Hindborg]

Andreas Hindborg <a.hindborg@xxxxxxxxxx> writes:

> 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.
>
> Export `radix_tree_node_ctor` from C to enable Rust code to work with the
> radix tree node cache.
>
> Cc: "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx>
> Cc: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
> Signed-off-by: Andreas Hindborg <a.hindborg@xxxxxxxxxx>

I somehow managed to not include this last bit of detail:

commit 09bdfa18f6f879eb42df2e032ad5224eed29eb25
Author: Andreas Hindborg <a.hindborg@xxxxxxxxxx>
Date:   Fri Feb 6 22:38:09 2026 +0100

    radix-tree: enable sheaf suppport for kmem_cache
    
    The rust null block driver plans to rely on preloading xarray nodes from the
    radix_tree_node_cachep kmem_cache.
    
    Signed-off-by: Andreas Hindborg <a.hindborg@xxxxxxxxxx>

diff --git a/lib/radix-tree.c b/lib/radix-tree.c
index b642f2775e89c..ddd67ce672f5c 100644
--- a/lib/radix-tree.c
+++ b/lib/radix-tree.c
@@ -1599,10 +1599,16 @@ void __init radix_tree_init(void)
 	BUILD_BUG_ON(RADIX_TREE_MAX_TAGS + __GFP_BITS_SHIFT > 32);
 	BUILD_BUG_ON(ROOT_IS_IDR & ~GFP_ZONEMASK);
 	BUILD_BUG_ON(XA_CHUNK_SIZE > 255);
-	radix_tree_node_cachep = kmem_cache_create("radix_tree_node",
-			sizeof(struct radix_tree_node), 0,
-			SLAB_PANIC | SLAB_RECLAIM_ACCOUNT,
-			radix_tree_node_ctor);
+
+	struct kmem_cache_args args = {
+		.ctor = radix_tree_node_ctor,
+		.sheaf_capacity = 64,
+	};
+
+	radix_tree_node_cachep = kmem_cache_create(
+		"radix_tree_node", sizeof(struct radix_tree_node), &args,
+		SLAB_PANIC | SLAB_RECLAIM_ACCOUNT);
+
 	ret = cpuhp_setup_state_nocalls(CPUHP_RADIX_DEAD, "lib/radix:dead",
 					NULL, radix_tree_cpu_dead);
 	WARN_ON(ret < 0);


Best regards,
Andreas Hindborg