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

[Michal Hocko <mhocko@suse.cz>]

On Mon 26-09-11 18:25:36, KAMEZAWA Hiroyuki wrote:
> On Mon, 26 Sep 2011 11:14:40 +0200
> Michal Hocko <mhocko@suse.cz> 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@google.com>
> > 
> > Thanks
> > 
> > > 
> > > once Rafael sends his acked-by or reviewed-by.
> > ---
> > From f935ed4558c2fb033ef5c14e02b28e12a615f80e Mon Sep 17 00:00:00 2001
> > From: Michal Hocko <mhocko@suse.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@suse.cz>
> > Reported-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
> 
> Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

Thanks

-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic

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