Re: [PATCH v2] mm, oom: Tolerate processes sharing mm with different view of oom_score_adj.

[Michal Hocko]

On Thu 31-01-19 07:49:35, Tetsuo Handa wrote:
> This patch reverts both commit 44a70adec910d692 ("mm, oom_adj: make sure
> processes sharing mm have same view of oom_score_adj") and commit
> 97fd49c2355ffded ("mm, oom: kill all tasks sharing the mm") in order to
> close a race and reduce the latency at __set_oom_adj(), and reduces the
> warning at __oom_kill_process() in order to minimize the latency.
> 
> Commit 36324a990cf578b5 ("oom: clear TIF_MEMDIE after oom_reaper managed
> to unmap the address space") introduced the worst case mentioned in
> 44a70adec910d692. But since the OOM killer skips mm with MMF_OOM_SKIP set,
> only administrators can trigger the worst case.
> 
> Since 44a70adec910d692 did not take latency into account, we can "hold RCU
> for minutes and trigger RCU stall warnings" by calling printk() on many
> thousands of thread groups. Also, current code becomes a DoS attack vector
> which will allow "stalling for more than one month in unkillable state"
> simply printk()ing same messages when many thousands of thread groups
> tried to iterate __set_oom_adj() on each other.
> 
> I also noticed that 44a70adec910d692 is racy [1], and trying to fix the
> race will require a global lock which is too costly for rare events. And
> Michal Hocko is thinking to change the oom_score_adj implementation to per
> mm_struct (with shadowed score stored in per task_struct in order to
> support vfork() => __set_oom_adj() => execve() sequence) so that we don't
> need the global lock.
> 
> If the worst case in 44a70adec910d692 happened, it is an administrator's
> request. Therefore, before changing the oom_score_adj implementation,
> let's eliminate the DoS attack vector first.

This is really ridiculous. I have already nacked the previous version
and provided two ways around. The simplest one is to drop the printk.
The second one is to move oom_score_adj to the mm struct. Could you
explain why do you still push for this?

> [1] https://lkml.kernel.org/r/20181008011931epcms1p82dd01b7e5c067ea99946418bc97de46a@epcms1p8
> 
> Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> Reported-by: Yong-Taek Lee <ytk.lee@xxxxxxxxxxx>
> Nacked-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
>  fs/proc/base.c     | 46 ----------------------------------------------
>  include/linux/mm.h |  2 --
>  mm/oom_kill.c      | 10 ++++++----
>  3 files changed, 6 insertions(+), 52 deletions(-)
> 
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 633a634..41ece8f 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -1020,7 +1020,6 @@ static ssize_t oom_adj_read(struct file *file, char __user *buf, size_t count,
>  static int __set_oom_adj(struct file *file, int oom_adj, bool legacy)
>  {
>  	static DEFINE_MUTEX(oom_adj_mutex);
> -	struct mm_struct *mm = NULL;
>  	struct task_struct *task;
>  	int err = 0;
>  
> @@ -1050,55 +1049,10 @@ static int __set_oom_adj(struct file *file, int oom_adj, bool legacy)
>  		}
>  	}
>  
> -	/*
> -	 * Make sure we will check other processes sharing the mm if this is
> -	 * not vfrok which wants its own oom_score_adj.
> -	 * pin the mm so it doesn't go away and get reused after task_unlock
> -	 */
> -	if (!task->vfork_done) {
> -		struct task_struct *p = find_lock_task_mm(task);
> -
> -		if (p) {
> -			if (atomic_read(&p->mm->mm_users) > 1) {
> -				mm = p->mm;
> -				mmgrab(mm);
> -			}
> -			task_unlock(p);
> -		}
> -	}
> -
>  	task->signal->oom_score_adj = oom_adj;
>  	if (!legacy && has_capability_noaudit(current, CAP_SYS_RESOURCE))
>  		task->signal->oom_score_adj_min = (short)oom_adj;
>  	trace_oom_score_adj_update(task);
> -
> -	if (mm) {
> -		struct task_struct *p;
> -
> -		rcu_read_lock();
> -		for_each_process(p) {
> -			if (same_thread_group(task, p))
> -				continue;
> -
> -			/* do not touch kernel threads or the global init */
> -			if (p->flags & PF_KTHREAD || is_global_init(p))
> -				continue;
> -
> -			task_lock(p);
> -			if (!p->vfork_done && process_shares_mm(p, mm)) {
> -				pr_info("updating oom_score_adj for %d (%s) from %d to %d because it shares mm with %d (%s). Report if this is unexpected.\n",
> -						task_pid_nr(p), p->comm,
> -						p->signal->oom_score_adj, oom_adj,
> -						task_pid_nr(task), task->comm);
> -				p->signal->oom_score_adj = oom_adj;
> -				if (!legacy && has_capability_noaudit(current, CAP_SYS_RESOURCE))
> -					p->signal->oom_score_adj_min = (short)oom_adj;
> -			}
> -			task_unlock(p);
> -		}
> -		rcu_read_unlock();
> -		mmdrop(mm);
> -	}
>  err_unlock:
>  	mutex_unlock(&oom_adj_mutex);
>  	put_task_struct(task);
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 80bb640..28879c1 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2690,8 +2690,6 @@ static inline int in_gate_area(struct mm_struct *mm, unsigned long addr)
>  }
>  #endif	/* __HAVE_ARCH_GATE_AREA */
>  
> -extern bool process_shares_mm(struct task_struct *p, struct mm_struct *mm);
> -
>  #ifdef CONFIG_SYSCTL
>  extern int sysctl_drop_caches;
>  int drop_caches_sysctl_handler(struct ctl_table *, int,
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index f0e8cd9..c7005b1 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -478,7 +478,7 @@ static void dump_header(struct oom_control *oc, struct task_struct *p)
>   * task's threads: if one of those is using this mm then this task was also
>   * using it.
>   */
> -bool process_shares_mm(struct task_struct *p, struct mm_struct *mm)
> +static bool process_shares_mm(struct task_struct *p, struct mm_struct *mm)
>  {
>  	struct task_struct *t;
>  
> @@ -896,12 +896,14 @@ static void __oom_kill_process(struct task_struct *victim)
>  			continue;
>  		if (same_thread_group(p, victim))
>  			continue;
> -		if (is_global_init(p)) {
> +		if (is_global_init(p) ||
> +		    p->signal->oom_score_adj == OOM_SCORE_ADJ_MIN) {
>  			can_oom_reap = false;
> -			set_bit(MMF_OOM_SKIP, &mm->flags);
> -			pr_info("oom killer %d (%s) has mm pinned by %d (%s)\n",
> +			if (!test_bit(MMF_OOM_SKIP, &mm->flags))
> +				pr_info("oom killer %d (%s) has mm pinned by %d (%s)\n",
>  					task_pid_nr(victim), victim->comm,
>  					task_pid_nr(p), p->comm);
> +			set_bit(MMF_OOM_SKIP, &mm->flags);
>  			continue;
>  		}
>  		/*
> -- 
> 1.8.3.1

-- 
Michal Hocko
SUSE Labs