Date:   Wed, 14 Sep 2016 15:50:11 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     David Rientjes <rientjes@...gle.com>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [RFC 3/4] mm, oom: do not rely on TIF_MEMDIE for exit_oom_victim

Hi Oleg,
does this sound sane/correct to you?

On Thu 01-09-16 11:51:03, Michal Hocko wrote:
> From: Michal Hocko <mhocko@...e.com>
> 
> mark_oom_victim and exit_oom_victim are used for oom_killer_disable
> which should block as long as there any any oom victims alive. Up to now
> we have relied on TIF_MEMDIE task flag to count how many oom victim
> we have. This is not optimal because only one thread receives this flag
> at the time while the whole process (thread group) is killed and should
> die. As a result we do not thaw the whole thread group and so a multi
> threaded process can leave some threads behind in the fridge. We really
> want to thaw all the threads.
> 
> This is not all that easy because there is no reliable way to count
> threads in the process as the oom killer might race with copy_process.
> So marking all threads with TIF_MEMDIE and increment oom_victims
> accordingly is not safe. Also TIF_MEMDIE flag should just die so
> we should better come up with a different approach.
> 
> All we need to guarantee is that exit_oom_victim is called at the time
> when no further access to (possibly suspended) devices or generate other
> IO (which would clobber suspended image and only once per process)
> is possible. It seems we can rely on exit_notify for that because we
> already have to detect the last thread to do a cleanup. Let's propagate
> that information up to do_exit and only call exit_oom_victim for such
> a thread. With this in place we can safely increment oom_victims only
> once per thread group and thaw all the threads from the process.
> freezing_slow_path can also rely on tsk_is_oom_victim as well now.
> 
> exit_io_context is currently called after exit_notify but it seems it is
> safe to call it right before exit_notify because that is passed
> exit_files.
> 
> Cc: Oleg Nesterov <oleg@...hat.com>
> Cc: Al Viro <viro@...iv.linux.org.uk>
> Signed-off-by: Michal Hocko <mhocko@...e.com>
> ---
>  include/linux/sched.h |  2 +-
>  kernel/exit.c         | 38 ++++++++++++++++++++++++++++----------
>  kernel/freezer.c      |  3 ++-
>  mm/oom_kill.c         | 29 +++++++++++++++++------------
>  4 files changed, 48 insertions(+), 24 deletions(-)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 770d01e7a68e..605e40b47992 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -2660,7 +2660,7 @@ static inline void kernel_signal_stop(void)
>  	schedule();
>  }
>  
> -extern void release_task(struct task_struct * p);
> +extern bool release_task(struct task_struct * p);
>  extern int send_sig_info(int, struct siginfo *, struct task_struct *);
>  extern int force_sigsegv(int, struct task_struct *);
>  extern int force_sig_info(int, struct siginfo *, struct task_struct *);
> diff --git a/kernel/exit.c b/kernel/exit.c
> index 914088e8c2ac..c762416dbed1 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -165,10 +165,11 @@ static void delayed_put_task_struct(struct rcu_head *rhp)
>  }
>  
>  
> -void release_task(struct task_struct *p)
> +bool release_task(struct task_struct *p)
>  {
>  	struct task_struct *leader;
>  	int zap_leader;
> +	bool last = false;
>  repeat:
>  	/* don't need to get the RCU readlock here - the process is dead and
>  	 * can't be modifying its own credentials. But shut RCU-lockdep up */
> @@ -197,8 +198,10 @@ void release_task(struct task_struct *p)
>  		 * then we are the one who should release the leader.
>  		 */
>  		zap_leader = do_notify_parent(leader, leader->exit_signal);
> -		if (zap_leader)
> +		if (zap_leader) {
>  			leader->exit_state = EXIT_DEAD;
> +			last = true;
> +		}
>  	}
>  
>  	write_unlock_irq(&tasklist_lock);
> @@ -208,6 +211,8 @@ void release_task(struct task_struct *p)
>  	p = leader;
>  	if (unlikely(zap_leader))
>  		goto repeat;
> +
> +	return last;
>  }
>  
>  /*
> @@ -434,8 +439,6 @@ static void exit_mm(struct task_struct *tsk)
>  	task_unlock(tsk);
>  	mm_update_next_owner(mm);
>  	mmput(mm);
> -	if (test_thread_flag(TIF_MEMDIE))
> -		exit_oom_victim();
>  }
>  
>  static struct task_struct *find_alive_thread(struct task_struct *p)
> @@ -584,12 +587,15 @@ static void forget_original_parent(struct task_struct *father,
>  /*
>   * Send signals to all our closest relatives so that they know
>   * to properly mourn us..
> + *
> + * Returns true if this is the last thread from the thread group
>   */
> -static void exit_notify(struct task_struct *tsk, int group_dead)
> +static bool exit_notify(struct task_struct *tsk, int group_dead)
>  {
>  	bool autoreap;
>  	struct task_struct *p, *n;
>  	LIST_HEAD(dead);
> +	bool last = false;
>  
>  	write_lock_irq(&tasklist_lock);
>  	forget_original_parent(tsk, &dead);
> @@ -606,6 +612,7 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
>  	} else if (thread_group_leader(tsk)) {
>  		autoreap = thread_group_empty(tsk) &&
>  			do_notify_parent(tsk, tsk->exit_signal);
> +		last = thread_group_empty(tsk);
>  	} else {
>  		autoreap = true;
>  	}
> @@ -621,8 +628,11 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
>  
>  	list_for_each_entry_safe(p, n, &dead, ptrace_entry) {
>  		list_del_init(&p->ptrace_entry);
> -		release_task(p);
> +		if (release_task(p) && p == tsk)
> +			last = true;
>  	}
> +
> +	return last;
>  }
>  
>  #ifdef CONFIG_DEBUG_STACK_USAGE
> @@ -766,7 +776,18 @@ void do_exit(long code)
>  	TASKS_RCU(preempt_disable());
>  	TASKS_RCU(tasks_rcu_i = __srcu_read_lock(&tasks_rcu_exit_srcu));
>  	TASKS_RCU(preempt_enable());
> -	exit_notify(tsk, group_dead);
> +
> +	if (tsk->io_context)
> +		exit_io_context(tsk);
> +
> +	/*
> +	 * Notify oom_killer_disable that the last oom thread is exiting.
> +	 * We might have more threads running at this point but none of them
> +	 * will access any devices behind this point.
> +	 */
> +	if (exit_notify(tsk, group_dead) && tsk_is_oom_victim(current))
> +		exit_oom_victim();
> +
>  	proc_exit_connector(tsk);
>  	mpol_put_task_policy(tsk);
>  #ifdef CONFIG_FUTEX
> @@ -784,9 +805,6 @@ void do_exit(long code)
>  	 */
>  	tsk->flags |= PF_EXITPIDONE;
>  
> -	if (tsk->io_context)
> -		exit_io_context(tsk);
> -
>  	if (tsk->splice_pipe)
>  		free_pipe_info(tsk->splice_pipe);
>  
> diff --git a/kernel/freezer.c b/kernel/freezer.c
> index 6f56a9e219fa..c6a64474a408 100644
> --- a/kernel/freezer.c
> +++ b/kernel/freezer.c
> @@ -10,6 +10,7 @@
>  #include <linux/syscalls.h>
>  #include <linux/freezer.h>
>  #include <linux/kthread.h>
> +#include <linux/oom.h>
>  
>  /* total number of freezing conditions in effect */
>  atomic_t system_freezing_cnt = ATOMIC_INIT(0);
> @@ -42,7 +43,7 @@ bool freezing_slow_path(struct task_struct *p)
>  	if (p->flags & (PF_NOFREEZE | PF_SUSPEND_TASK))
>  		return false;
>  
> -	if (test_tsk_thread_flag(p, TIF_MEMDIE))
> +	if (tsk_is_oom_victim(p))
>  		return false;
>  
>  	if (pm_nosig_freezing || cgroup_freezing(p))
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index e26529edcee3..5dec6321ac7b 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -649,33 +649,38 @@ static inline void wake_oom_reaper(struct task_struct *tsk)
>  static void mark_oom_victim(struct task_struct *tsk)
>  {
>  	struct mm_struct *mm = tsk->mm;
> +	struct task_struct *t;
>  
>  	WARN_ON(oom_killer_disabled);
> -	/* OOM killer might race with memcg OOM */
> -	if (test_and_set_tsk_thread_flag(tsk, TIF_MEMDIE))
> -		return;
>  
>  	/* oom_mm is bound to the signal struct life time. */
> -	if (!cmpxchg(&tsk->signal->oom_mm, NULL, mm))
> +	if (!cmpxchg(&tsk->signal->oom_mm, NULL, mm)) {
>  		atomic_inc(&tsk->signal->oom_mm->mm_count);
>  
> +		/* Only count thread groups */
> +		atomic_inc(&oom_victims);
> +	}
> +
>  	/*
> -	 * Make sure that the task is woken up from uninterruptible sleep
> -	 * if it is frozen because OOM killer wouldn't be able to free
> -	 * any memory and livelock. freezing_slow_path will tell the freezer
> -	 * that TIF_MEMDIE tasks should be ignored.
> +	 * Make sure that the the whole thread groupd is woken up from
> +	 * uninterruptible sleep if it is frozen because the oom victim
> +	 * will free its memory completely only after exit.
> +	 * freezing_slow_path will tell the freezer that oom victims
> +	 * should be ignored.
>  	 */
> -	__thaw_task(tsk);
> -	atomic_inc(&oom_victims);
> +	rcu_read_lock();
> +	for_each_thread(tsk, t)
> +		__thaw_task(t);
> +	rcu_read_unlock();
>  }
>  
>  /**
>   * exit_oom_victim - note the exit of an OOM victim
> + *
> + * Has to be called only once per thread group.
>   */
>  void exit_oom_victim(void)
>  {
> -	clear_thread_flag(TIF_MEMDIE);
> -
>  	if (!atomic_dec_return(&oom_victims))
>  		wake_up_all(&oom_victims_wait);
>  }
> -- 
> 2.8.1
> 

-- 
Michal Hocko
SUSE Labs

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