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





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