Re: [PATCH v3] oom, oom_reaper: do not enqueue same task twice

[Michal Hocko]

On Sun 27-01-19 23:57:38, Tetsuo Handa wrote:
[...]
> >From 9c9e935fc038342c48461aabca666f1b544e32b1 Mon Sep 17 00:00:00 2001
> From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> Date: Sun, 27 Jan 2019 23:51:37 +0900
> Subject: [PATCH v3] oom, oom_reaper: do not enqueue same task twice
> 
> Arkadiusz reported that enabling memcg's group oom killing causes
> strange memcg statistics where there is no task in a memcg despite
> the number of tasks in that memcg is not 0. It turned out that there
> is a bug in wake_oom_reaper() which allows enqueuing same task twice
> which makes impossible to decrease the number of tasks in that memcg
> due to a refcount leak.
> 
> This bug existed since the OOM reaper became invokable from
> task_will_free_mem(current) path in out_of_memory() in Linux 4.7,
> but memcg's group oom killing made it easier to trigger this bug by
> calling wake_oom_reaper() on the same task from one out_of_memory()
> request.
> 
> Fix this bug using an approach used by commit 855b018325737f76
> ("oom, oom_reaper: disable oom_reaper for oom_kill_allocating_task").
> As a side effect of this patch, this patch also avoids enqueuing
> multiple threads sharing memory via task_will_free_mem(current) path.
> 
> Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> Reported-by: Arkadiusz MiÅkiewicz <arekm@xxxxxxxx>
> Tested-by: Arkadiusz MiÅkiewicz <arekm@xxxxxxxx>
> Fixes: af8e15cc85a25315 ("oom, oom_reaper: do not enqueue task if it is on the oom_reaper_list head")

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
>  include/linux/sched/coredump.h | 1 +
>  mm/oom_kill.c                  | 4 ++--
>  2 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/sched/coredump.h b/include/linux/sched/coredump.h
> index ec912d0..ecdc654 100644
> --- a/include/linux/sched/coredump.h
> +++ b/include/linux/sched/coredump.h
> @@ -71,6 +71,7 @@ static inline int get_dumpable(struct mm_struct *mm)
>  #define MMF_HUGE_ZERO_PAGE	23      /* mm has ever used the global huge zero page */
>  #define MMF_DISABLE_THP		24	/* disable THP for all VMAs */
>  #define MMF_OOM_VICTIM		25	/* mm is the oom victim */
> +#define MMF_OOM_REAP_QUEUED	26	/* mm was queued for oom_reaper */
>  #define MMF_DISABLE_THP_MASK	(1 << MMF_DISABLE_THP)
>  
>  #define MMF_INIT_MASK		(MMF_DUMPABLE_MASK | MMF_DUMP_FILTER_MASK |\
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index f0e8cd9..059e617 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -647,8 +647,8 @@ static int oom_reaper(void *unused)
>  
>  static void wake_oom_reaper(struct task_struct *tsk)
>  {
> -	/* tsk is already queued? */
> -	if (tsk == oom_reaper_list || tsk->oom_reaper_list)
> +	/* mm is already queued? */
> +	if (test_and_set_bit(MMF_OOM_REAP_QUEUED, &tsk->signal->oom_mm->flags))
>  		return;
>  
>  	get_task_struct(tsk);
> -- 
> 1.8.3.1

-- 
Michal Hocko
SUSE Labs