Re: memcg creates an unkillable task in 3.11-rc2

[Michal Hocko]

On Tue 30-07-13 01:19:31, Eric W. Biederman wrote:
[...]
> Hmm. Looking farther I see what is going on. And it has nothing to do
> with the freezer. (I have commented out that code and reproduced it
> without the freezer to be doubly certain).
> 
> 
> On the exit path exit_robust_list is triggering a page fault to fault a
> page back in.  Which since we have no memory causes the exit path
> to get stuck in mem_cgroup_handle_oom.

Hmm, interesting. I assume the exit is caused by the SIGKILL, right?
If yes, then why it hasn't coughed early in __mem_cgroup_try_charge

> Which means the following change should fix the hang.  I will test it in just
> a second.
> 
> The problem is that we only handled pending fatal signals and exiting
> processes when the OOM logic was enabled. Sigh.

If this is about PF_EXITING then we should handle that earlier in the
code chain (__mem_cgroup_try_charge sounds like a proper place). Let me
think about that.

Thanks for debugging this!

> Eric
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 00a7a66..5998a57 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1792,16 +1792,6 @@ static void mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask,
>         unsigned int points = 0;
>         struct task_struct *chosen = NULL;
>  
> -       /*
> -        * If current has a pending SIGKILL or is exiting, then automatically
> -        * select it.  The goal is to allow it to allocate so that it may
> -        * quickly exit and free its memory.
> -        */
> -       if (fatal_signal_pending(current) || current->flags & PF_EXITING) {
> -               set_thread_flag(TIF_MEMDIE);
> -               return;
> -       }
> -
>         check_panic_on_oom(CONSTRAINT_MEMCG, gfp_mask, order, NULL);
>         totalpages = mem_cgroup_get_limit(memcg) >> PAGE_SHIFT ? : 1;
>         for_each_mem_cgroup_tree(iter, memcg) {
> @@ -2220,7 +2210,15 @@ static bool mem_cgroup_handle_oom(struct mem_cgroup *memcg, gfp_t mask,
>                 mem_cgroup_oom_notify(memcg);
>         spin_unlock(&memcg_oom_lock);
>  
> -       if (need_to_kill) {
> +       /*
> +        * If current has a pending SIGKILL or is exiting, then automatically
> +        * select it.  The goal is to allow it to allocate so that it may
> +        * quickly exit and free its memory.
> +        */
> +       if (fatal_signal_pending(current) || current->flags & PF_EXITING) {
> +               set_thread_flag(TIF_MEMDIE);
> +               finish_wait(&memcg_oom_waitq, &owait.wait);
> +       } else if (need_to_kill) {
>                 finish_wait(&memcg_oom_waitq, &owait.wait);
>                 mem_cgroup_out_of_memory(memcg, mask, order);
>         } else {

-- 
Michal Hocko
SUSE Labs
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