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Message-ID: <20aab1af-c183-db94-90d7-5e5425e3fd80@bytedance.com>
Date:   Mon, 31 Jul 2023 20:39:08 +0800
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Subject: Re: [PATCH 06/11] maple_tree: Introduce mas_replace_entry() to
 directly replace an entry
To:     "Liam R. Howlett" <Liam.Howlett@Oracle.com>
References: <20230726080916.17454-1-zhangpeng.00@bytedance.com>
 <20230726080916.17454-7-zhangpeng.00@bytedance.com>
 <20230726160843.hpl4razxiikqbuxy@revolver>
Cc:     Peng Zhang <zhangpeng.00@bytedance.com>, avagin@gmail.com,
        npiggin@gmail.com, mathieu.desnoyers@efficios.com,
        peterz@infradead.org, michael.christie@oracle.com,
        surenb@google.com, brauner@kernel.org, willy@infradead.org,
        akpm@linux-foundation.org, corbet@lwn.net,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-mm@kvack.org, linux-doc@vger.kernel.org
From:   Peng Zhang <zhangpeng.00@bytedance.com>
In-Reply-To: <20230726160843.hpl4razxiikqbuxy@revolver>
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在 2023/7/27 00:08, Liam R. Howlett 写道:
> * 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.
I've thought about this, and I feel like this is something the user
should be guaranteed. If the user is not sure whether to use it,
mas_store() can be used instead. And we should provide this interface
because it has better performance.
> 
>>
>> 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
>>