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

From: Michal Hocko
Date: Mon Sep 26 2011 - 14:29:09 EST


On Mon 26-09-11 17:51:40, Rafael J. Wysocki wrote:
> On Monday, September 26, 2011, Michal Hocko wrote:
[...]
> > From f935ed4558c2fb033ef5c14e02b28e12a615f80e Mon Sep 17 00:00:00 2001
> > From: Michal Hocko <mhocko@xxxxxxx>
> > 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@xxxxxxx>
> > Reported-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxx>
>
> Acked-by: Rafael J. Wysocki <rjw@xxxxxxx>

Thanks.

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