linux-kernel - Re: [PATCH 1/2] oom: do not live lock on frozen tasks

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Date:	Mon, 26 Sep 2011 17:51:40 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	David Rientjes <rientjes@...gle.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Konstantin Khlebnikov <khlebnikov@...nvz.org>,
	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Tejun Heo <tj@...nel.org>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH 1/2] oom: do not live lock on frozen tasks

On Monday, September 26, 2011, Michal Hocko wrote:
> On Mon 26-09-11 01:56:57, David Rientjes wrote:
> > On Mon, 26 Sep 2011, Michal Hocko wrote:
> > 
> > > diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> > > index 626303b..b9774f3 100644
> > > --- a/mm/oom_kill.c
> > > +++ b/mm/oom_kill.c
> > > @@ -32,6 +32,7 @@
> > >  #include <linux/mempolicy.h>
> > >  #include <linux/security.h>
> > >  #include <linux/ptrace.h>
> > > +#include <linux/freezer.h>
> > >  
> > >  int sysctl_panic_on_oom;
> > >  int sysctl_oom_kill_allocating_task;
> > > @@ -451,6 +452,9 @@ static int oom_kill_task(struct task_struct *p, struct mem_cgroup *mem)
> > >  				task_pid_nr(q), q->comm);
> > >  			task_unlock(q);
> > >  			force_sig(SIGKILL, q);
> > > +
> > > +			if (frozen(q))
> > > +				thaw_process(q);
> > >  		}
> > >  
> > >  	set_tsk_thread_flag(p, TIF_MEMDIE);
> > 
> > This is in the wrong place, oom_kill_task() iterates over all threads that 
> > are _not_ in the same thread group as the chosen thread and kills them 
> > without giving them access to memory reserves.  The chosen task, p, could 
> > still be frozen and may not exit.
> 
> Ahh, right you are. I ave missed that one. Updated patch bellow.
> 
> > 
> > Once that's fixed, feel free to add my
> > 
> > 	Acked-by: David Rientjes <rientjes@...gle.com>
> 
> Thanks
> 
> > 
> > once Rafael sends his acked-by or reviewed-by.
> ---
> From f935ed4558c2fb033ef5c14e02b28e12a615f80e Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@...e.cz>
> Date: Fri, 16 Sep 2011 11:23:15 +0200
> Subject: [PATCH] oom: do not live lock on frozen tasks
> 
> Konstantin Khlebnikov has reported (https://lkml.org/lkml/2011/8/23/45)
> that OOM can end up in a live lock if select_bad_process picks up a frozen
> task.
> Unfortunately we cannot mark such processes as unkillable to ignore them
> because we could panic the system even though there is a chance that
> somebody could thaw the process so we can make a forward process (e.g. a
> process from another cpuset or with a different nodemask).
> 
> Let's thaw an OOM selected frozen process right after we've sent fatal
> signal from oom_kill_task.
> Thawing is safe if the frozen task doesn't access any suspended device
> (e.g. by ioctl) on the way out to the userspace where we handle the
> signal and die. Note, we are not interested in the kernel threads because
> they are not oom killable.
> 
> Accessing suspended devices by a userspace processes shouldn't be an
> issue because devices are suspended only after userspace is already
> frozen and oom is disabled at that time.
> 
> run_guest (drivers/lguest/core.c) calls try_to_freeze with an user
> context but it seems it is able to cope with signals because it
> explicitly checks for pending signals so we should be safe.
> 
> Other than that userspace accesses the fridge only from the
> signal handling routines so we are able to handle SIGKILL without any
> negative side effects.
> 
> Signed-off-by: Michal Hocko <mhocko@...e.cz>
> Reported-by: Konstantin Khlebnikov <khlebnikov@...nvz.org>

Acked-by: Rafael J. Wysocki <rjw@...k.pl>

> ---
>  mm/oom_kill.c |    6 ++++++
>  1 files changed, 6 insertions(+), 0 deletions(-)
> 
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 626303b..c419a7e 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -32,6 +32,7 @@
>  #include <linux/mempolicy.h>
>  #include <linux/security.h>
>  #include <linux/ptrace.h>
> +#include <linux/freezer.h>
>  
>  int sysctl_panic_on_oom;
>  int sysctl_oom_kill_allocating_task;
> @@ -451,10 +452,15 @@ static int oom_kill_task(struct task_struct *p, struct mem_cgroup *mem)
>  				task_pid_nr(q), q->comm);
>  			task_unlock(q);
>  			force_sig(SIGKILL, q);
> +
> +			if (frozen(q))
> +				thaw_process(q);
>  		}
>  
>  	set_tsk_thread_flag(p, TIF_MEMDIE);
>  	force_sig(SIGKILL, p);
> +	if (frozen(p))
> +		thaw_process(p);
>  
>  	return 0;
>  }
> 

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