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Message-ID: <853af8fa-0cef-b00b-3fd6-9780a2008050@bytedance.com>
Date:   Tue, 8 Aug 2023 19:17:31 +0800
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Subject: Re: [RFC v1 4/5] maple_tree: avoid bulk alloc/free to use percpu
 array more
To:     Vlastimil Babka <vbabka@suse.cz>
Cc:     Hyeonggon Yoo <42.hyeyoo@gmail.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Christoph Lameter <cl@linux.com>,
        "Liam R. Howlett" <Liam.Howlett@oracle.com>,
        Roman Gushchin <roman.gushchin@linux.dev>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, patches@lists.linux.dev,
        Matthew Wilcox <willy@infradead.org>
References: <20230808095342.12637-7-vbabka@suse.cz>
 <20230808095342.12637-11-vbabka@suse.cz>
From:   Peng Zhang <zhangpeng.00@bytedance.com>
In-Reply-To: <20230808095342.12637-11-vbabka@suse.cz>
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Precedence: bulk


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)