linux内存管理-虚拟地址转换成物理地址的过程

          我们知道一个虚拟地址要依次经过分段机制转为线性地址，再经过分页机制从线性地址

转换为物理地址。虚拟地址是由段标识符：段内偏移来表示。

linux为了便于管理，将所有段标识符都设置为0，这样段内偏移就相当于线性地址。

之前简单了介绍了分段，分页的原理，接下来通过代码实例进一步加深对地址转换过程的理解，下面的示例是以arm64 来说明的：

static unsigned long test_vaddr2paddr(unsigned long vaddr)

{
pgd_t *pgd;
pud_t *pud;
pmd_t *pmd;
pte_t *pte;
unsigned long paddr = 0;
unsigned long page_addr = 0;
unsigned long page_offset = 0;

/*init_mm.pgd 是内核空间的页目录，整个内核空间使用此pgd；
每个用户进程都有自己的pgd,struct task_struct->struct mm_struct->pgd
因为地址是在内核空间分配的，因此使用内核的pgd；*/
//这一步是跟进pgd首地址，跟线性地址里面包含的偏移量得到下一级页表pud的物理地址，
pgd = pgd_offset(&init_mm,vaddr);

    
printk("pgd_val = 0x%lx pgd addr:0x%lx\n", (unsigned long int)pgd_val(*pgd),(unsigned long int)pgd_val(pgd));
printk("init_mm pgd val:0x%lx,pgd addr:0x%lx\n",(unsigned long )pgd_val(*(init_mm.pgd)),pgd_val((init_mm.pgd)));
printk("pgd_index = %ld\n", pgd_index(vaddr));
if (pgd_none(*pgd)) {
printk("not mapped in pgd\n");
return -1;

}
printk("\n");

//计算pmd页表地址
pud = pud_offset(pgd, vaddr);
printk("pud_val = 0x%lx \n", (unsigned long int)pud_val(*pud));
if (pud_none(*pud)) {
printk("not mapped in pud\n");
return -1;
}
printk("\n");
//计算pte
pmd = pmd_offset(pud, vaddr);
printk("pmd_val = 0x%lx\n", (unsigned long int)pmd_val(*pmd));
printk("pmd_index = %lu\n", pmd_index(vaddr));
if (pmd_none(*pmd)) {
printk("not mapped in pmd\n");
return -1;
}
printk("\n");

pte = pte_offset_kernel(pmd, vaddr);
printk("pte_val = 0x%lx\n", (unsigned long int)pte_val(*pte));
printk("pte_index = %lu\n", pte_index(vaddr));
if (pte_none(*pte)) {
printk("not mapped in pte\n");
return -1;
}

//先将pte值转换为struct page，然后在转换未物理地址
page_addr=page_to_phys(pte_page(*pte));
page_offset=pte_index(vaddr); //页内偏移量
paddr = page_addr | page_offset;

printk("page_addr = %lx, page_offset = %lx\n", page_addr, page_offset);
printk("vaddr = %lx, paddr = %lx\n", vaddr, paddr);

return paddr;

}

static int  test_addr_convet(void)

{
unsigned long vaddr = 0;
vaddr = (unsigned long)vmalloc(1000 * sizeof(char));
if (vaddr == 0) {
printk("vmalloc failed..\n");
return 0;
}

printk("vmalloc_vaddr=0x%lx\n", vaddr);
test_vaddr2paddr(vaddr);

vaddr = __get_free_page(GFP_KERNEL);
if (vaddr == 0) {
printk("__get_free_page failed..\n");
return 0;
}
printk("alloc page addr:0x%lx\n",vaddr);
test_vaddr2paddr(vaddr);

vaddr=(unsigned long)kmalloc(4096,GFP_KERNEL);
if(vaddr ==0)
{
printk(KERN_ERR "kmall fail\n");
return 0;
}
printk("kmalloc addr=0x%lx\n", vaddr);
test_vaddr2paddr(vaddr);
printk("\n\n");

return 0;

}

这段代码可以用于演示地址转换的详细过程，运行的结果如下(不同的平台运行结果会有差异，仅供参考)：

[   76.354838] @0 vmalloc_vaddr=0xffffff8001d90000

[   76.358514] @0 pgd_val = 0x1aac06003 pgd addr:0xffffffc001a04000

[   76.364722] @0 init_mm pgd val:0x1aac06003,pgd addr:0xffffffc001a04000

[   76.373907] @0 pgd_index = 0

[   76.375932] @0 

[   76.377746] @0 pud_val = 0x1aac06003 

[   76.382005] @3 

[   76.382886] @3 pmd_val = 0x1a980f003

[   76.386454] @3 pmd_index = 14

[   76.389398] @3 

[   76.391282] @3 pte_val = 0x2e000017da44713

[   76.395260] @3 pte_index = 400

[   76.398246] @3 pte_val = 0x2e000017da44713

[   76.402447] @3 pte_index = 400

[   76.405392] @3 

[   76.407134] @3 page_addr = 17da44000, page_offset = 190

[   76.412404] @3 vaddr = ffffff8001d90000, paddr = 17da44190

[   76.417798] @3 alloc page addr:0xffffffc13da7e000

[   76.422562] @3 pgd_val = 0x7fffa003 pgd addr:0xffffffc001a04820

[   76.428371] @3 init_mm pgd val:0x1aac06003,pgd addr:0xffffffc001a04000

[   76.434942] @3 pgd_index = 260

[   76.437918] @3 

[   76.439652] @3 pud_val = 0x7fffa003 

[   76.443271] @3 

[   76.444964] @3 pmd_val = 0x1bcec2003

[   76.448503] @3 pmd_index = 493

[   76.451655] @3 

[   76.453285] @3 pte_val = 0x2c000017da7e713

[   76.457531] @3 pte_index = 126

[   76.460540] @3 pte_val = 0x2c000017da7e713

[   76.464483] @3 pte_index = 126

[   76.467519] @3 

[   76.469258] @3 page_addr = 17da7e000, page_offset = 7e

[   76.474514] @3 vaddr = ffffffc13da7e000, paddr = 17da7e07e

[   76.479885] @3 kmalloc addr=0xffffffc16424e000

[   76.484329] @3 pgd_val = 0x7fff9003 pgd addr:0xffffffc001a04828

[   76.490296] @3 init_mm pgd val:0x1aac06003,pgd addr:0xffffffc001a04000

[   76.496711] @3 pgd_index = 261

[   76.499723] @3 

[   76.501562] @3 pud_val = 0x7fff9003 

[   76.505077] @3 

[   76.506824] @3 pmd_val = 0x1bcd8e003

[   76.510450] @3 pmd_index = 289

[   76.513421] @3 

[   76.515091] @3 pte_val = 0x2c00001a424e713

[   76.519171] @3 pte_index = 78

[   76.522241] @3 pte_val = 0x2c00001a424e713

[   76.526243] @3 pte_index = 78

[   76.529204] @3 

[   76.531028] @3 page_addr = 1a424e000, page_offset = 4e

[   76.536062] @3 vaddr = ffffffc16424e000, paddr = 1a424e04e