Re: general protection fault in oom_unkillable_task

[Tetsuo Handa]

On 2019/06/16 16:37, Tetsuo Handa wrote:
> On 2019/06/16 6:33, Tetsuo Handa wrote:
>> On 2019/06/16 3:50, Shakeel Butt wrote:
>>>> While dump_tasks() traverses only each thread group, mem_cgroup_scan_tasks()
>>>> traverses each thread.
>>>
>>> I think mem_cgroup_scan_tasks() traversing threads is not intentional
>>> and css_task_iter_start in it should use CSS_TASK_ITER_PROCS as the
>>> oom killer only cares about the processes or more specifically
>>> mm_struct (though two different thread groups can have same mm_struct
>>> but that is fine).
>>
>> We can't use CSS_TASK_ITER_PROCS from mem_cgroup_scan_tasks(). I've tried
>> CSS_TASK_ITER_PROCS in an attempt to evaluate only one thread from each
>> thread group, but I found that CSS_TASK_ITER_PROCS causes skipping whole
>> threads in a thread group (and trivially allowing "Out of memory and no
>> killable processes...\n" flood) if thread group leader has already exited.
> 
> Seems that CSS_TASK_ITER_PROCS from mem_cgroup_scan_tasks() is now working.


I found a reproducer and the commit.

----------------------------------------
#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>
#include <asm/unistd.h>

static const unsigned long size = 1048576 * 200;
static int thread(void *unused)
{
        int fd = open("/dev/zero", O_RDONLY);
        char *buf = mmap(NULL, size, PROT_WRITE | PROT_READ,
                         MAP_ANONYMOUS | MAP_SHARED, EOF, 0);
        sleep(1);
        read(fd, buf, size);
        return syscall(__NR_exit, 0);
}
int main(int argc, char *argv[])
{
        FILE *fp;
        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);
        clone(thread, malloc(8192) + 4096, CLONE_SIGHAND | CLONE_THREAD | CLONE_VM, NULL);
        return syscall(__NR_exit, 0);
}
----------------------------------------

Here is a patch to use CSS_TASK_ITER_PROCS.