linux-kernel - Re: [PATCH 1/2] oom: don't assume that a coredumping thread will exit soon

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Date:	Tue, 2 Dec 2014 10:19:47 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Cong Wang <xiyou.wangcong@...il.com>,
	David Rientjes <rientjes@...gle.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>, Tejun Heo <tj@...nel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] oom: don't assume that a coredumping thread will
 exit soon

On Fri 28-11-14 00:04:05, Oleg Nesterov wrote:
> oom_kill.c assumes that PF_EXITING task should exit and free the memory
> soon. This is wrong in many ways and one important case is the coredump.
> A task can sleep in exit_mm() "forever" while the coredumping sub-thread
> can need more memory.
> 
> Change the PF_EXITING checks to take SIGNAL_GROUP_COREDUMP into account,
> we add the new trivial helper for that.
> 
> Note: this is only the first step, this patch doesn't try to solve other
> problems. For example it doesn't try to clear the wrongly set TIF_MEMDIE
> (SIGNAL_GROUP_COREDUMP check is obviously racy),

I am not sure I understand this. What do you mean by wrongly set
TIF_MEMDIE? That we give a process access to reserves even though it is
already done with the coredumping?

> fatal_signal_pending() can be false positive, etc.

When can this happen?

> Signed-off-by: Oleg Nesterov <oleg@...hat.com>

I guess the patch as is makes sense and it is an improvement. We need
to call the helper in mem_cgroup_out_of_memory as well, though.

With that feel free to add
Acked-by: Michal Hocko <mhocko@...e.cz>

Also the original fix for the coredumping (edd45544c6f0 "oom: avoid
deferring oom killer if exiting task is being traced") doesn't work
really as per http://marc.info/?l=linux-kernel&m=141711049013620 then
this and the follow up patch should be marked for stable I guess.

> ---
>  mm/oom_kill.c |   12 +++++++++---
>  1 files changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 5340f6b..7af33b5 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -254,6 +254,12 @@ static enum oom_constraint constrained_alloc(struct zonelist *zonelist,
>  }
>  #endif
>  
> +static inline bool task_will_free_mem(struct task_struct *task)
> +{
> +	return (task->flags & PF_EXITING) &&
> +		!(task->signal->flags & SIGNAL_GROUP_COREDUMP);
> +}
> +
>  enum oom_scan_t oom_scan_process_thread(struct task_struct *task,
>  		unsigned long totalpages, const nodemask_t *nodemask,
>  		bool force_kill)
> @@ -281,7 +287,7 @@ enum oom_scan_t oom_scan_process_thread(struct task_struct *task,
>  	if (oom_task_origin(task))
>  		return OOM_SCAN_SELECT;
>  
> -	if (task->flags & PF_EXITING && !force_kill) {
> +	if (task_will_free_mem(task) && !force_kill) {
>  		/*
>  		 * If this task is not being ptraced on exit, then wait for it
>  		 * to finish before killing some other task unnecessarily.
> @@ -443,7 +449,7 @@ void oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
>  	 * If the task is already exiting, don't alarm the sysadmin or kill
>  	 * its children or threads, just set TIF_MEMDIE so it can die quickly
>  	 */
> -	if (p->flags & PF_EXITING) {
> +	if (task_will_free_mem(p)) {
>  		set_tsk_thread_flag(p, TIF_MEMDIE);
>  		put_task_struct(p);
>  		return;
> @@ -649,7 +655,7 @@ void out_of_memory(struct zonelist *zonelist, gfp_t gfp_mask,
>  	 * 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) {
> +	if (fatal_signal_pending(current) || task_will_free_mem(current)) {
>  		set_thread_flag(TIF_MEMDIE);
>  		return;
>  	}
> -- 
> 1.5.5.1
> 

-- 
Michal Hocko
SUSE Labs
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