mm: why cat /proc/pid/smaps | grep Rss is different from cat /proc/pid/statm?

[root@localhost c_test]# cat /proc/3948/smaps | grep Rss

Rss:                   4 kB
Rss:                   4 kB
Rss:                   4 kB
Rss:                 796 kB
Rss:                   0 kB
Rss:                  16 kB
Rss:                   8 kB
Rss:                  12 kB
Rss:                 132 kB
Rss:                  12 kB
Rss:                   4 kB
Rss:                   4 kB
Rss:                   4 kB
Rss:                   4 kB
Rss:                  12 kB
Rss:                   0 kB
Rss:                   4 kB
Rss:                   0 kB

[root@localhost c_test]# cat /proc/3948/statm
1042 173 154 1 0 48 0

173 means Rss is 173*4kb=692kb, right?
so why it is different from the sum(1020kb) of "cat /proc/pid/smaps | grep Rss"?

my test code is
...
int main()

{
	sleep(1000);
	return 0;
}

the kernel version is v4.1

Thanks,
Xishi Qiu

The /proc/[pid]/smaps read triggers the traversal of all of process's vmas and then page tables and accumulate RSS on each present page table entry.

[root@localhost c_test]# cat /proc/3948/statm

The files /proc/[pid]/statm and /proc/[pid]/status uses the counters (MM_ANONPAGES & MM_FILEPAGES) in mm_struct to report RSS of a process. These counters are modified on page table modifications. However the kernel implements an optimization where each thread keeps a local copy of these counters in its task_struct. These local counter are accumulated in the shared counter of mm_struct after some number of page faults (I think 32) faced by the thread and thus there will be mismatch with smaps file.

Shakeel

Hi Shakeel,

I malloc and memset 10M, then sleep. It seems that the problem is still exist, the kernel version is v4.1

[root@localhost c_test]# cat /proc/13746/statm 3603 2767 250 1 0 2609 0
[root@localhost c_test]# cat /proc/13746/smaps | grep Rss

Rss:                   4 kB
Rss:                   4 kB
Rss:                   4 kB
Rss:               10244 kB
Rss:                 924 kB
Rss:                   0 kB
Rss:                  16 kB
Rss:                   8 kB
Rss:                  12 kB
Rss:                 132 kB
Rss:                  12 kB
Rss:                   4 kB
Rss:                   4 kB
Rss:                   4 kB
Rss:                   4 kB
Rss:                   8 kB
Rss:                   0 kB
Rss:                   4 kB
Rss:                   0 kB

Hi Shakeel,

I misunderstand your meaning before. I know the reason now.

Thanks,
Xishi Qiu