swapper_pg_dir中如何添加用户进程页表，用代码来说明

在swapper_pg_dir中添加用户进程页表的步骤如下：

1. 创建页表

pgd_t *pgd = NULL;
pgd = (pgd_t *)get_free_page(); // 获取一块空闲物理页
if (!pgd) {
    return -1;
}
memset(pgd, 0, PAGE_SIZE); // 将页表清零

2. 将页表加入到swapper_pg_dir中

swapper_pg_dir[PGD_INDEX(address)] = virt_to_phys(pgd) | PG_PRESENT | PG_WRITE;

其中，address为用户进程的虚拟地址，PGD_INDEX(address)为该地址对应的pgd索引。

3. 添加映射关系

pte_t *pte = NULL;
pte = (pte_t *)get_free_page(); // 获取一块空闲物理页
if (!pte) {
    return -1;
}
memset(pte, 0, PAGE_SIZE); // 将页表清零

pgd[PGD_INDEX(address)] = virt_to_phys(pte) | PG_PRESENT | PG_WRITE;

pte[PT_INDEX(address)] = page_address | PG_PRESENT | PG_USER | PG_WRITE;

其中，page_address为要映射的物理页地址，PT_INDEX(address)为该地址对应的pte索引。

完整代码如下：

static int map_user_page(unsigned long address, unsigned long page_address)
{
    pgd_t *pgd = NULL;
    pgd = (pgd_t *)get_free_page(); // 获取一块空闲物理页
    if (!pgd) {
        return -1;
    }
    memset(pgd, 0, PAGE_SIZE); // 将页表清零

    swapper_pg_dir[PGD_INDEX(address)] = virt_to_phys(pgd) | PG_PRESENT | PG_WRITE;

    pte_t *pte = NULL;
    pte = (pte_t *)get_free_page(); // 获取一块空闲物理页
    if (!pte) {
        return -1;
    }
    memset(pte, 0, PAGE_SIZE); // 将页表清零

    pgd[PGD_INDEX(address)] = virt_to_phys(pte) | PG_PRESENT | PG_WRITE;

    pte[PT_INDEX(address)] = page_address | PG_PRESENT | PG_USER | PG_WRITE;

    return 0;
}