linux-kernel - Re: [PATCH 1/2] maple_tree: Use kfree_rcu in ma_free

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Message-ID: <l7kladnev3bfxcg2n2rk6hdi757vro5warlwp44ripj3qmnsfr@2jlwi7hhsfot>
Date: Tue, 5 Aug 2025 21:25:13 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Pedro Falcato <pfalcato@...e.de>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>, maple-tree@...ts.infradead.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] maple_tree: Use kfree_rcu in ma_free_rcu

* Pedro Falcato <pfalcato@...e.de> [250718 13:21]:
> kfree_rcu is an optimized version of call_rcu + kfree. It used to not be
> possible to call it on non-kmalloc objects, but this restriction was
> lifted ever since SLOB was dropped from the kernel, and since commit
> 6c6c47b063b5 ("mm, slab: call kvfree_rcu_barrier() from kmem_cache_destroy()").
> 
> Thus, replace call_rcu + mt_free_rcu with kfree_rcu.
> 
> Signed-off-by: Pedro Falcato <pfalcato@...e.de>
> ---
>  lib/maple_tree.c | 13 +++----------
>  1 file changed, 3 insertions(+), 10 deletions(-)
> 
> diff --git a/lib/maple_tree.c b/lib/maple_tree.c
> index b4ee2d29d7a9..91da2d9d00c3 100644
> --- a/lib/maple_tree.c
> +++ b/lib/maple_tree.c
> @@ -191,13 +191,6 @@ static inline void mt_free_bulk(size_t size, void __rcu **nodes)
>  	kmem_cache_free_bulk(maple_node_cache, size, (void **)nodes);
>  }
>  
> -static void mt_free_rcu(struct rcu_head *head)
> -{
> -	struct maple_node *node = container_of(head, struct maple_node, rcu);
> -
> -	kmem_cache_free(maple_node_cache, node);
> -}
> -
>  /*
>   * ma_free_rcu() - Use rcu callback to free a maple node
>   * @node: The node to free
> @@ -208,7 +201,7 @@ static void mt_free_rcu(struct rcu_head *head)
>  static void ma_free_rcu(struct maple_node *node)
>  {
>  	WARN_ON(node->parent != ma_parent_ptr(node));
> -	call_rcu(&node->rcu, mt_free_rcu);
> +	kfree_rcu(node, rcu);
>  }
>  
>  static void mt_set_height(struct maple_tree *mt, unsigned char height)
> @@ -5281,7 +5274,7 @@ static void mt_free_walk(struct rcu_head *head)
>  	mt_free_bulk(node->slot_len, slots);
>  
>  free_leaf:
> -	mt_free_rcu(&node->rcu);
> +	mt_free_one(node);

Why are we still using mt_free_one()?  Couldn't this also be dropped in
favour of kfree() or does kfree() not work for kmem_cache?

>  }
>  
>  static inline void __rcu **mte_destroy_descend(struct maple_enode **enode,
> @@ -5365,7 +5358,7 @@ static void mt_destroy_walk(struct maple_enode *enode, struct maple_tree *mt,
>  
>  free_leaf:
>  	if (free)
> -		mt_free_rcu(&node->rcu);
> +		mt_free_one(node);
>  	else
>  		mt_clear_meta(mt, node, node->type);
>  }
> -- 
> 2.50.1
>