[RFC] mm: why vfree() do not free page table memory?

[Xishi Qiu]

When calling vfree(), it calls unmap_vmap_area() to clear page table,
but do not free the memory of page table, why? just for performance?

If a driver use vmalloc() and vfree() frequently, we will lost much
page table memory, maybe oom later.

Thanks,
Xishi Qiu