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Date:   Wed, 26 Jul 2023 12:08:43 -0400
From:   "Liam R. Howlett" <Liam.Howlett@Oracle.com>
To:     Peng Zhang <zhangpeng.00@bytedance.com>
Cc:     corbet@lwn.net, akpm@linux-foundation.org, willy@infradead.org,
        brauner@kernel.org, surenb@google.com, michael.christie@oracle.com,
        peterz@infradead.org, mathieu.desnoyers@efficios.com,
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Subject: Re: [PATCH 06/11] maple_tree: Introduce mas_replace_entry() to
 directly replace an entry
Message-ID: <20230726160843.hpl4razxiikqbuxy@revolver>
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References: <20230726080916.17454-1-zhangpeng.00@bytedance.com>
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* Peng Zhang <zhangpeng.00@bytedance.com> [230726 04:10]:
> If mas has located a specific entry, it may be need to replace this
> entry, so introduce mas_replace_entry() to do this. mas_replace_entry()
> will be more efficient than mas_store*() because it doesn't do many
> unnecessary checks.
> 
> This function should be inline, but more functions need to be moved to
> the header file, so I didn't do it for the time being.

I am really nervous having no checks here.  I get that this could be
used for duplicating the tree more efficiently, but having a function
that just swaps a value in is very dangerous - especially since it is
decoupled from the tree duplication code.

> 
> Signed-off-by: Peng Zhang <zhangpeng.00@bytedance.com>
> ---
>  include/linux/maple_tree.h |  1 +
>  lib/maple_tree.c           | 25 +++++++++++++++++++++++++
>  2 files changed, 26 insertions(+)
> 
> diff --git a/include/linux/maple_tree.h b/include/linux/maple_tree.h
> index 229fe78e4c89..a05e9827d761 100644
> --- a/include/linux/maple_tree.h
> +++ b/include/linux/maple_tree.h
> @@ -462,6 +462,7 @@ struct ma_wr_state {
>  
>  void *mas_walk(struct ma_state *mas);
>  void *mas_store(struct ma_state *mas, void *entry);
> +void mas_replace_entry(struct ma_state *mas, void *entry);
>  void *mas_erase(struct ma_state *mas);
>  int mas_store_gfp(struct ma_state *mas, void *entry, gfp_t gfp);
>  void mas_store_prealloc(struct ma_state *mas, void *entry);
> diff --git a/lib/maple_tree.c b/lib/maple_tree.c
> index efac6761ae37..d58572666a00 100644
> --- a/lib/maple_tree.c
> +++ b/lib/maple_tree.c
> @@ -5600,6 +5600,31 @@ void *mas_store(struct ma_state *mas, void *entry)
>  }
>  EXPORT_SYMBOL_GPL(mas_store);
>  
> +/**
> + * mas_replace_entry() - Replace an entry that already exists in the maple tree
> + * @mas: The maple state
> + * @entry: The entry to store
> + *
> + * Please note that mas must already locate an existing entry, and the new entry
> + * must not be NULL. If these two points cannot be guaranteed, please use
> + * mas_store*() instead, otherwise it will cause an internal error in the maple
> + * tree. This function does not need to allocate memory, so it must succeed.
> + */
> +void mas_replace_entry(struct ma_state *mas, void *entry)
> +{
> +	void __rcu **slots;
> +
> +#ifdef CONFIG_DEBUG_MAPLE_TREE
> +	MAS_WARN_ON(mas, !mte_is_leaf(mas->node));
> +	MAS_WARN_ON(mas, !entry);
> +	MAS_WARN_ON(mas, mas->offset >= mt_slots[mte_node_type(mas->node)]);
> +#endif
> +
> +	slots = ma_slots(mte_to_node(mas->node), mte_node_type(mas->node));
> +	rcu_assign_pointer(slots[mas->offset], entry);
> +}
> +EXPORT_SYMBOL_GPL(mas_replace_entry);
> +
>  /**
>   * mas_store_gfp() - Store a value into the tree.
>   * @mas: The maple state
> -- 
> 2.20.1
>