Re: [RFC PATCH] memcg, oom: throttle dump_header for memcg ooms without eligible tasks

[Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>]

On 2018/10/15 17:19, Michal Hocko wrote:
> As so many dozens of times before, I will point you to an incremental
> nature of changes we really prefer in the mm land. We are also after a
> simplicity which your proposal lacks in many aspects. You seem to ignore
> that general approach and I have hard time to consider your NAK as a
> relevant feedback. Going to an extreme and basing a complex solution on
> it is not going to fly. No killable process should be a rare event which
> requires a seriously misconfigured memcg to happen so wildly. If you can
> trigger it with a normal user privileges then it would be a clear bug to
> address rather than work around with printk throttling.
> 

I can trigger 200+ times / 900+ lines / 69KB+ of needless OOM messages
with a normal user privileges. This is a lot of needless noise/delay.
No killable process is not a rare event, even without root privileges.

[root@ccsecurity kumaneko]# time ./a.out
Killed

real    0m2.396s
user    0m0.000s
sys     0m2.970s
[root@ccsecurity ~]# dmesg | grep 'no killable' | wc -l
202
[root@ccsecurity ~]# dmesg | wc
    942    7335   70716
[root@ccsecurity ~]#

----------------------------------------
#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <sched.h>
#include <sys/mman.h>

#define NUMTHREADS 256
#define MMAPSIZE 4 * 10485760
#define STACKSIZE 4096
static int pipe_fd[2] = { EOF, EOF };
static int memory_eater(void *unused)
{
        int fd = open("/dev/zero", O_RDONLY);
        char *buf = mmap(NULL, MMAPSIZE, PROT_WRITE | PROT_READ,
                         MAP_ANONYMOUS | MAP_SHARED, EOF, 0);
        read(pipe_fd[0], buf, 1);
        read(fd, buf, MMAPSIZE);
        pause();
        return 0;
}
int main(int argc, char *argv[])
{
        int i;
        char *stack;
        FILE *fp;
        const unsigned long size = 1048576 * 200;
        mkdir("/sys/fs/cgroup/memory/test1", 0755);
        fp = fopen("/sys/fs/cgroup/memory/test1/memory.limit_in_bytes", "w");
        fprintf(fp, "%lu\n", size);
        fclose(fp);
        fp = fopen("/sys/fs/cgroup/memory/test1/tasks", "w");
        fprintf(fp, "%u\n", getpid());
        fclose(fp);
        if (setgid(-2) || setuid(-2))
                return 1;
        stack = mmap(NULL, STACKSIZE * NUMTHREADS, PROT_WRITE | PROT_READ,
                     MAP_ANONYMOUS | MAP_SHARED, EOF, 0);
        for (i = 0; i < NUMTHREADS; i++)
                if (clone(memory_eater, stack + (i + 1) * STACKSIZE,
                          CLONE_SIGHAND | CLONE_THREAD | CLONE_VM | CLONE_FS | CLONE_FILES, NULL) == -1)
                        break;
        sleep(1);
        close(pipe_fd[1]);
        pause();
        return 0;
}
----------------------------------------