调用 malloc() 时分配的一个物理页面

Question

我试图找到c程序的virtual set size和resident set size。我编写了一个内核模块来遍历vm_areas并计算vss和rss。我还编写了一个c程序来验证vss和rss的变化。

// sample test program
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#define N 10000000
int main() {
// setup...
int *arg1 = malloc(5*sizeof(int));
char *arg2 = malloc(sizeof(char)*1024);
pid_t _pid = getpid();
int rss , prev_rss , vss, prev_vss;
printf("pid of this process = %dn",_pid);
//...

// first observatrion
arg1[0] = (int)(_pid);
long res = syscall(333,arg1,&arg2);
vss = prev_vss = arg1[1]; // agr1[1] stores the vss from the kernel module
rss = prev_rss = arg1[2]; // agr1[2] stores the rss from the kernel module
printf("vss = %d rss = %dn",vss,rss);

unsigned int *ptr = malloc(1<<21); // 2 MB 
printf("ptr = %pn",ptr);

// second observatrion
arg1[0] = (int)(_pid);  
res = syscall(333,arg1,&arg2);
vss = arg1[1];
rss = arg1[2];
printf("vss = %d rss = %dn",vss,rss);
if(vss - prev_vss > 0) {
printf("chnage in vss = %dn", vss - prev_vss);
}
if(rss - prev_rss > 0) {
printf("chnage in rss = %dn", rss - prev_rss);
}   
prev_vss = vss;
prev_rss = rss;
// ...
return 0;
}

上述程序的输出：

pid of this process = 12964
vss = 4332 rss = 1308
ptr = 0x7f4077464010
vss = 6384 rss = 1312
chnage in vss = 2052
chnage in rss = 4

以下是dmesg输出： 第一个观察：

[11374.065527]  1 = [0000000000400000-0000000000401000]      RSS=4KB     sample
[11374.065529]  2 = [0000000000600000-0000000000601000]      RSS=4KB     sample
[11374.065530]  3 = [0000000000601000-0000000000602000]      RSS=4KB     sample
[11374.065532]  4 = [0000000000c94000-0000000000cb5000]      RSS=4KB     
[11374.065539]  5 = [00007f4077665000-00007f407781f000]      RSS=1064KB      libc-2.19.so
[11374.065546]  6 = [00007f407781f000-00007f4077a1f000]      RSS=0KB     libc-2.19.so
[11374.065547]  7 = [00007f4077a1f000-00007f4077a23000]      RSS=16KB    libc-2.19.so
[11374.065549]  8 = [00007f4077a23000-00007f4077a25000]      RSS=8KB     libc-2.19.so
[11374.065551]  9 = [00007f4077a25000-00007f4077a2a000]      RSS=16KB    
[11374.065553]  10 = [00007f4077a2a000-00007f4077a4d000]     RSS=140KB   ld-2.19.so
[11374.065554]  11 = [00007f4077c33000-00007f4077c36000]     RSS=12KB    
[11374.065556]  12 = [00007f4077c49000-00007f4077c4c000]     RSS=12KB    
[11374.065557]  13 = [00007f4077c4c000-00007f4077c4d000]     RSS=4KB     ld-2.19.so
[11374.065559]  14 = [00007f4077c4d000-00007f4077c4e000]     RSS=4KB     ld-2.19.so
[11374.065561]  15 = [00007f4077c4e000-00007f4077c4f000]     RSS=4KB     
[11374.065563]  16 = [00007ffcdf974000-00007ffcdf995000]     RSS=8KB     
[11374.065565]  17 = [00007ffcdf9c3000-00007ffcdf9c6000]     RSS=0KB     
[11374.065566]  18 = [00007ffcdf9c6000-00007ffcdf9c8000]     RSS=4KB

第二点意见：

[11374.065655]  1 = [0000000000400000-0000000000401000]      RSS=4KB     sample
[11374.065657]  2 = [0000000000600000-0000000000601000]      RSS=4KB     sample
[11374.065658]  3 = [0000000000601000-0000000000602000]      RSS=4KB     sample
[11374.065660]  4 = [0000000000c94000-0000000000cb5000]      RSS=4KB     
[11374.065667]  5 = [00007f4077464000-00007f4077665000]      RSS=4KB     
[11374.065673]  6 = [00007f4077665000-00007f407781f000]      RSS=1064KB      libc-2.19.so
[11374.065679]  7 = [00007f407781f000-00007f4077a1f000]      RSS=0KB     libc-2.19.so
[11374.065681]  8 = [00007f4077a1f000-00007f4077a23000]      RSS=16KB    libc-2.19.so
[11374.065683]  9 = [00007f4077a23000-00007f4077a25000]      RSS=8KB     libc-2.19.so
[11374.065685]  10 = [00007f4077a25000-00007f4077a2a000]     RSS=16KB    
[11374.065687]  11 = [00007f4077a2a000-00007f4077a4d000]     RSS=140KB   ld-2.19.so
[11374.065688]  12 = [00007f4077c33000-00007f4077c36000]     RSS=12KB    
[11374.065690]  13 = [00007f4077c49000-00007f4077c4c000]     RSS=12KB    
[11374.065691]  14 = [00007f4077c4c000-00007f4077c4d000]     RSS=4KB     ld-2.19.so
[11374.065693]  15 = [00007f4077c4d000-00007f4077c4e000]     RSS=4KB     ld-2.19.so
[11374.065695]  16 = [00007f4077c4e000-00007f4077c4f000]     RSS=4KB     
[11374.065697]  17 = [00007ffcdf974000-00007ffcdf995000]     RSS=8KB     
[11374.065699]  18 = [00007ffcdf9c3000-00007ffcdf9c6000]     RSS=0KB     
[11374.065701]  19 = [00007ffcdf9c6000-00007ffcdf9c8000]     RSS=4KB

ptr的虚拟地址是：ptr = 0x7f4077464010对应于第二次观测中的第5个vm_area。

[00007f4077464000-00007f4077665000]      VSS=2052KB // shown from the VSS outputs

我的问题是：

1. 为什么所需的malloc大小(2048 KB)与 2052 KB)5vm_area (vss输出之间存在差异？

2. 我们尚未访问ptr指向的内存区域。那么，为什么分配一个物理页面，如第5个vm_area的 seocnd 观察结果rss所示？(可能是因为新vm_area_struct吗？

谢谢！

Answer 1

> malloc(xxx) 不会完全分配 xxx 大小的内存。 malloc 不是系统调用，而是库函数。

一般来说，malloc 有以下步骤。

1. 通过 BRK 扩展堆空间(如果需要)
2. 执行 mmap 以将虚拟地址与物理地址映射
3. 分配一些元数据(用于管理堆空间，通常是链表)。

在步骤 3 中，将访问一个页面。 这意味着访问一个物理页面，并导致 RSS 大小增加 4KB(页面大小)。