Is it a bug (about share memory)?

["Zheng, Jeff" <jeff.zheng@intel.com>]

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Hi,

I tried to use share memory to check vm overcommit by check Committed_AS in /proc/meminfo. It seems that attach of share momory will add the value of Committed_AS but detach of share memory does not reduce the value of Committed_AS. Below is the result (attached file is source of the command)
[root@hpi overcommit_basic]# ./shm 5000000 m
The value of Committed_AS is 100436 KB before attach
The value of Committed_AS is 105384 KB After attach
[root@hpi overcommit_basic]# ./shm 5000000 m
The value of Committed_AS is 105520 KB before attach
The value of Committed_AS is 105520 KB After attach
[root@hpi overcommit_basic]# ./shm 5000000 n
The value of Committed_AS is 105416 KB before attach
The value of Committed_AS is 110364 KB After attach

the first parm is size of share memory, the second parm is proj of API ftok. When I allocate 5M, Committed_AS add 5M. The issue is: when I detach the share memory, and there is no other process attach to the share memory, Committed_AS does not reduce 5M.

Inactive of /proc/meminfo also changed.

My kernel is 2.5.70. And it seems that all kernel is the same.


Thanks
Jeff                        Jeff.Zheng@intel.com
BTW, I speak for myself, not for Intel Corp.

[-- Attachment #2: shm.c --]
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#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>

#define SHM_SIZE 1024  /* make it a 1K shared memory segment */

int GetValue(char *n) {
   char s[200],*p;
   FILE *f;
   int value,len;
   f=fopen("/proc/meminfo", "r");
   while ((p=fgets(s, 200, f)) != NULL) {
	   len=strlen(n);
	   if (strncmp(n, p, len)==0) {
		   p+=len+1;
		   while (isspace(*p)) p++;
		   value=atol(p);
		   fclose(f);
		   return(value);
	   }
   }
   fclose(f);
   return 0;
}

int main(int argc, char *argv[])
{
    key_t key;
    int shmid;
    char *data,*p;
    int mode;
    int memsize,committed_as;
    int i;

    if (argc > 3) {
        fprintf(stderr, "usage: shmdemo [data_to_write] [proj_of_ftok]\n");
        exit(1);
    }
    
    committed_as=GetValue("Committed_AS");
    printf("The value of Committed_AS is %d KB before attach\n", committed_as);
    memsize = atol(argv[1]);
    /* make the key: */
    if ((key = ftok("/tmp/shm.tmp", *argv[2])) == -1) {
        perror("ftok");
        exit(1);
    }

    /* connect to (and possibly create) the segment: */
    if ((shmid = shmget(key, memsize, 0644 | IPC_CREAT)) == -1) {
        perror("shmget");
        exit(1);
    }

    /* attach to the segment to get a pointer to it: */
    p = data = shmat(shmid, (void *)0, 0);
    if (data == (char *)(-1)) {
        perror("shmat");
        exit(1);
    }
    for (i=0; i< memsize-10; i++)
	    *p++ = 'a';

    committed_as=GetValue("Committed_AS");
    printf("The value of Committed_AS is %d KB After attach\n", committed_as);
    /* detach from the segment: */
    if (shmdt(data) == -1) {
        perror("shmdt");
        exit(1);
    }

    return 0;
}