分配初始化零页

1.从bootmem分配一页

/*这里zero_page指向虚拟地址*/
void *zero_page = alloc_bootmem_low_pages(PAGE_SIZE);

2.找出zero_page对应的页框描述符page结构体

struct page * empty_zero_page = virt_to_page(zero_page);

3.flush_dcache_page(empty_zero_page);

 这个函数里面涉及一些cache即buffer的硬件操作，有时间可以详细分析，有助于加深多不同类型cache的理解。

flush_dcache_page(empty_zero_page);
    -->struct address_space *mapping = page_mapping(page);/*这里应该返回NULL*/
        -->#define PageHighMem(__p) is_highmem(page_zone(__p))
    -->__flush_dcache_page(mapping, page);
        -->__cpuc_flush_dcache_page(page_address(page));
            /*即v6_flush_kern_dcache_page in arm/arm/mm/cache-v6.S*/
            -->proc_info_list->cache->flush_kern_dcache_page

这里汇编代码的作用是把缓存中的数据写到物理内存中，确保数据的一致性。

/*
 *    v6_flush_kern_dcache_page(kaddr)
 *
 *    Ensure that the data held in the page kaddr is written back
 *    to the page in question.
 *
 *    - kaddr   - kernel address (guaranteed to be page aligned)
 */
ENTRY(v6_flush_kern_dcache_page)
    add    r1, r0, #PAGE_SZ
1:
#ifdef HARVARD_CACHE
    mcr    p15, 0, r0, c7, c14, 1        @ clean & invalidate D line
#else
    mcr    p15, 0, r0, c7, c15, 1        @ clean & invalidate unified line
#endif    
    add    r0, r0, #D_CACHE_LINE_SIZE
    cmp    r0, r1
    blo    1b
#ifdef HARVARD_CACHE
    mov    r0, #0
    mcr    p15, 0, r0, c7, c10, 4
#endif
    mov    pc, lr