完整虚拟内存系统

完整虚拟内存系统要解决的问题

页表、TLB和替换算法只是基础。完整系统还得适应差异巨大的硬件、方便内核访问用户数据、提供调试支持并保证安全隔离。早期的VAX/VMS已经用软件技巧弥补硬件缺陷，这些思路至今仍在Linux等系统中沿用。

VAX的小页面与页表压缩方法

512字节的页面让每个进程的线性页表 theoretically 可达数千万项。VAX把32位空间分成P0、P1和系统段，只用两套用户页表覆盖实际使用的区域。更妙的是把这两套页表放在内核自己的虚拟地址空间里，内存紧张时可以把页表页换出去。

空指针检测与内核共享映射

地址空间的第0页被标记为无效，空指针立刻产生故障。内核代码和数据出现在每个进程的高地址，切换进程时系统段寄存器保持不变。硬件保护位确保用户程序无法读写内核页，内核因此可以像一个受保护的共享库一样工作。

常见误区

• 低估小页面带来的页表体积，导致内存被页表本身耗尽
• 不把内核映射进用户空间，系统调用时拷贝用户缓冲区会变得极其复杂
• 漏掉第0页的无效标记，空指针错误会静默破坏内存而不是立即被发现

最低标准 C11 · 完整程序 · 下载 .c

#include <stdio.h>
#include <stdint.h>

int main(void) {
    printf("VAX-like 32-bit address space simulation (512-byte pages)\n");
    printf("VPN = VA >> 9 (23 bits); segment from top 2 bits of VPN\n\n");
    const uint32_t examples[7] = {
        0x00000000u,
        0x00001000u,
        0x3FFFFFFFu,
        0x40000000u,
        0x7FFFFFFFu,
        0x80000000u,
        0xC0000000u
    };
    const char *names[7] = {
        "null page",
        "user code",
        "end of P0 heap",
        "start of P1 stack",
        "end of P1",
        "kernel start",
        "unused"
    };
    for (int i = 0; i < 7; i++) {
        uint32_t va = examples[i];
        uint32_t offset = va & 0x1FFu;
        uint32_t vpn = va >> 9;
        uint32_t seg = (vpn >> 21) & 3u;
        const char *segname;
        switch (seg) {
            case 0: segname = "P0 (process)"; break;
            case 1: segname = "P1 (process)"; break;
            case 2: segname = "S (system)"; break;
            default: segname = "unused"; break;
        }
        printf("VA=0x%08X  VPN=%7u  off=%3u  seg=%s  [%s]%s\n",
               va, vpn, offset, segname, names[i],
               (va < 512 ? " INVALID" : ""));
    }
    printf("\nNote: page 0 is marked invalid to catch null-pointer bugs.\n");
    printf("Kernel (S) is mapped in every process for easy pointer passing.\n");
    return 0;
}

在本地编译

gcc -std=c11 -Wall -Wextra -Wpedantic -Werror ostep-23-complete-vm.c -o example && ./example

预期结果

VAX-like 32-bit address space simulation (512-byte pages)
VPN = VA >> 9 (23 bits); segment from top 2 bits of VPN

VA=0x00000000  VPN=      0  off=  0  seg=P0 (process)  [null page] INVALID
VA=0x00001000  VPN=      8  off=  0  seg=P0 (process)  [user code]
VA=0x3FFFFFFF  VPN=2097151  off=511  seg=P0 (process)  [end of P0 heap]
VA=0x40000000  VPN=2097152  off=  0  seg=P1 (process)  [start of P1 stack]
VA=0x7FFFFFFF  VPN=4194303  off=511  seg=P1 (process)  [end of P1]
VA=0x80000000  VPN=4194304  off=  0  seg=S (system)  [kernel start]
VA=0xC0000000  VPN=6291456  off=  0  seg=unused  [unused]

Note: page 0 is marked invalid to catch null-pointer bugs.
Kernel (S) is mapped in every process for easy pointer passing.

CHECK YOUR UNDERSTANDING

合上答案，试着解释。

为什么VAX要把用户页表放在内核虚拟内存中而不是物理内存？这样做有什么额外好处？

继续查证

• OSTEP chapter PDF

标准草案与官方章节会更新；版本标记只说明示例最低要求。