one physical page allocated when malloc() called - linux

Question

I was trying to find the virtual set size and resident set size of a c program. I wrote a kernel module to traverse the vm_areas and calculated vss and rss. I also wrote one c program to validate the changes in vss and rss.
// 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 = %d\n",_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 = %d\n",vss,rss);
unsigned int *ptr = malloc(1<<21); // 2 MB
printf("ptr = %p\n",ptr);
// second observatrion
arg1[0] = (int)(_pid);
res = syscall(333,arg1,&arg2);
vss = arg1[1];
rss = arg1[2];
printf("vss = %d rss = %d\n",vss,rss);
if(vss - prev_vss > 0) {
printf("chnage in vss = %d\n", vss - prev_vss);
}
if(rss - prev_rss > 0) {
printf("chnage in rss = %d\n", rss - prev_rss);
}
prev_vss = vss;
prev_rss = rss;
// ...
return 0;
}
The output of the above program:
pid of this process = 12964
vss = 4332 rss = 1308
ptr = 0x7f4077464010
vss = 6384 rss = 1312
chnage in vss = 2052
chnage in rss = 4
Here are the dmesg output :
First observation:
[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
Second observation:
[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
The virtual address of the ptr was found to be : ptr = 0x7f4077464010 which corresponds to the 5th vm_area in the second obervation.
[00007f4077464000-00007f4077665000] VSS=2052KB // shown from the VSS outputs
My quesitons are :
Why there is a difference of between desired malloc size (which was of 2048 KB) and the vss output for the 5th vm_area (2052 KB)?
We have not accessed the memory region pointed by ptr yet. So then why one physical page s allocated as shown in the rss result of the seocnd observation for the 5th vm_area? ( is it possibly because of the new vm_area_struct ?)
Thank You !

Answer 1

malloc(xxx) does not exactly allocate xxx size of memory. malloc is not a system call, but a library function.
In general, malloc has following steps.
extend the heap space via brk (if it needs)
do mmap to map virtual address with physical address
allocate some metadata (for managing heap space, usually linked list).
In step 3, the one page would be accessed. it means one physical page is accessed and results in increasing the RSS size by 4KB (a page size).