Re: [RFC v1 4/5] maple_tree: avoid bulk alloc/free to use percpu array more

[Peng Zhang]

在 2023/8/8 17:53, Vlastimil Babka 写道:
> Using bulk alloc/free on a cache with percpu array should not be
> necessary and the bulk alloc actually bypasses the array (the prefill
> functionality currently relies on this).
>
> The simplest change is just to convert the respective maple tree
> wrappers to do a loop of normal alloc/free.
> ---
>   lib/maple_tree.c | 11 +++++++++--
>   1 file changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/lib/maple_tree.c b/lib/maple_tree.c
> index 1196d0a17f03..7a8e7c467d7c 100644
> --- a/lib/maple_tree.c
> +++ b/lib/maple_tree.c
> @@ -161,12 +161,19 @@ static inline struct maple_node *mt_alloc_one(gfp_t gfp)
>   
>   static inline int mt_alloc_bulk(gfp_t gfp, size_t size, void **nodes)
>   {
> -	return kmem_cache_alloc_bulk(maple_node_cache, gfp, size, nodes);
> +	int allocated = 0;
> +	for (size_t i = 0; i < size; i++) {
> +		nodes[i] = kmem_cache_alloc(maple_node_cache, gfp);
> +		if (nodes[i])
If the i-th allocation fails, node[i] will be NULL. This is wrong. We'd
better guarantee that mt_alloc_bulk() allocates completely successfully,
or returns 0. The following cases are not allowed:
nodes: [addr1][addr2][NULL][addr3].
> +			allocated++;
> +	}
> +	return allocated;
>   }
>   
>   static inline void mt_free_bulk(size_t size, void __rcu **nodes)
>   {
> -	kmem_cache_free_bulk(maple_node_cache, size, (void **)nodes);
> +	for (size_t i = 0; i < size; i++)
> +		kmem_cache_free(maple_node_cache, nodes[i]);
>   }
>   
>   static void mt_free_rcu(struct rcu_head *head)