不管arm(arm32支持两级页表)提供几级页表,linux是支持四级页表的
在arch/arm处理的时候,将四级转换为两级

• pgd

__pgd
pgd_bad
pgd_clear
pgd_none
pgd_present
pgd_val
pgd_free

• pud

pud_bad
pud_clear
pud_page
pud_populate
pud_present
pud_write
set_pud

- pmd
```c

__pmd
pmd_alloc_one
pmd_free
pmd_leaf
pmd_offset
pmd_trans_huge
pmd_val
pmd_write

• pte

__pte
pte_accessible
pte_access_permitted
pte_same
pte_val
pte_alloc_one
pte_alloc_one_kernel