在安装函数内部，调用了kzalloc(分配内核内存并清零)。
Internal to the setup function, there's a call to kzalloc (which allocates kernel memory and zeroes it).
默认情况下，变量的范围是变量第一次被分配到的函数的内部，如果它的第一次分配出现在任何函数的外部，那么它就是全局变量。
By default, a variable is scoped to the function in which it is first assigned to, or is global if its first assignment occurs outside any function.
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