What's the difference between kmem_arena and kernel_arena?

[btw at mail.ustc.edu.cn]

Hi All,

There are two similar variables declared in vm/vm_kern.h, they are kernel_arena
and kmem_arena. Both of them are used in kmem_malloc():

        rv = kmem_back((vmem == kmem_arena) ? kmem_object : kernel_object,

I'm wondering what's the difference between them. Why both of them are needed?
I have done a lot of searching, but I still can not find an answer.

Thanks in advance!

- twb