Again, I do not know how the code looks without the patch, but
On 08/14, David Rientjes wrote:
>
> static int oom_kill_task(struct task_struct *p, struct mem_cgroup *mem)
> {
> + struct task_struct *g, *q;
> + struct mm_struct *mm;
> +
> p = find_lock_task_mm(p);
> if (!p) {
> task_unlock(p);
> return 1;
> }
> +
> + /* mm cannot be safely dereferenced after task_unlock(p) */
Yes. But also we can't trust this pointer, see below.
> + mm = p->mm;
> +
> pr_err("Killed process %d (%s) total-vm:%lukB, anon-rss:%lukB, file-rss:%lukB\n",
> task_pid_nr(p), p->comm, K(p->mm->total_vm),
> K(get_mm_counter(p->mm, MM_ANONPAGES)),
> K(get_mm_counter(p->mm, MM_FILEPAGES)));
> task_unlock(p);
>
> -
> set_tsk_thread_flag(p, TIF_MEMDIE);
> force_sig(SIGKILL, p);
So, we killed this process. It is very possible it was the only user
of this ->mm. exit_mm() can free this mmemory. After that another task
execs, exec_mmap() can allocate the same memory again.
Now,
> @@ -438,6 +444,20 @@ static int oom_kill_task(struct task_struct *p, struct mem_cgroup *mem)
> */
> boost_dying_task_prio(p, mem);
>
> + /*
> + * Kill all threads sharing p->mm in other thread groups, if any. They
> + * don't get access to memory reserves or a higher scheduler priority,
> + * though, to avoid depletion of all memory or task starvation. This
> + * prevents mm->mmap_sem livelock when an oom killed task cannot exit
> + * because it requires the semaphore and its contended by another
> + * thread trying to allocate memory itself. That thread will now get
> + * access to memory reserves since it has a pending fatal signal.
> + */
> + do_each_thread(g, q) {
> + if (q->mm == mm && !same_thread_group(q, p))
> + force_sig(SIGKILL, q);
> + } while_each_thread(g, q);
We can kill the wrong task. "q->mm == mm" doesn't necessarily mean
we found the task which shares ->mm with p (see above).
This needs atomic_inc(mm_users). And please do not use do_each_thread.
David, I apologize in advance if I won't reply to your futher emails.
I don't have the time for the kernel hacking at all.
Oleg.
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