linux-kernel - Re: [PATCH -mm v2 1/3] mm/oom_kill: remove the wrong fatal_signal_pending() check in oom_kill

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Date:	Fri, 2 Oct 2015 20:32:41 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	oleg@...hat.com, mhocko@...nel.org
Cc:	akpm@...ux-foundation.org, rientjes@...gle.com, kwalker@...hat.com,
	skozina@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -mm v2 1/3] mm/oom_kill: remove the wrong fatal_signal_pending() check in oom_kill_process()

Oleg Nesterov wrote:
> On 10/01, Michal Hocko wrote:
> >
> > zap_process will add SIGKILL to all threads but the
> > current which will go on without being killed and if this is not a
> > thread group leader then we would miss it.
> 
> Yes. And note that de_thread() does the same. Speaking of oom-killer
> this is mostly fine, the execing thread is going to release its old
> ->mm and it has already passed the copy_strings() stage which can use
> a lot more memory.

So, we have the same wrong fatal_signal_pending() check in out_of_memory()

        /*
         * 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.
         *
         * But don't select if current has already released its mm and cleared
         * TIF_MEMDIE flag at exit_mm(), otherwise an OOM livelock may occur.
         */
        if (current->mm &&
            (fatal_signal_pending(current) || task_will_free_mem(current))) {
                mark_oom_victim(current);
                return true;
        }

because it is possible that T starts the coredump, T sends SIGKILL to P,
P calls out_of_memory() on GFP_FS allocation, P misses to set SIGKILL on T?

Since T sends SIGKILL to all clone(CLONE_VM) tasks upon coredump, P needs
to do

        rcu_read_lock();
        for_each_process(p) {
                if (!process_shares_mm(p, current->mm))
                        continue;
                if (unlikely(p->flags & PF_KTHREAD))
                        continue;
                if (p->signal->oom_score_adj == OOM_SCORE_ADJ_MIN)
                        continue;

                do_send_sig_info(SIGKILL, SEND_SIG_FORCED, p, true);
        }
        rcu_read_unlock();

after mark_oom_victim(current) in case T is not in the same thread group?

If yes, what happens if some task failed to receive SIGKILL due to
p->signal->oom_score_adj == OOM_SCORE_ADJ_MIN condition?
Will we hit mm->mmap_sem livelock?
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