Date:	Tue, 29 Mar 2011 10:12:31 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Hiroyuki Kamezawa <kamezawa.hiroyuki@...il.com>,
	Michel Lespinasse <walken@...gle.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"rientjes@...gle.com" <rientjes@...gle.com>,
	Andrey Vagin <avagin@...nvz.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Hugh Dickins <hughd@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 0/4] forkbomb killer

On Tue, Mar 29, 2011 at 9:32 AM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@...fujitsu.com> wrote:
> On Tue, 29 Mar 2011 09:24:30 +0900
> Minchan Kim <minchan.kim@...il.com> wrote:
>
>> On Tue, Mar 29, 2011 at 8:50 AM, KAMEZAWA Hiroyuki
>> <kamezawa.hiroyu@...fujitsu.com> wrote:
>> > On Tue, 29 Mar 2011 01:21:37 +0900
>> > Minchan Kim <minchan.kim@...il.com> wrote:
>> >
>> >> On Sat, Mar 26, 2011 at 05:48:45PM +0900, Hiroyuki Kamezawa wrote:
>> >> > 2011/3/26 Michel Lespinasse <walken@...gle.com>:
>> >> > > On Fri, Mar 25, 2011 at 01:05:50PM +0900, Minchan Kim wrote:
>> >> > >> Okay. Each approach has a pros and cons and at least, now anyone
>> >> > >> doesn't provide any method and comments but I agree it is needed(ex,
>> >> > >> careless and lazy admin could need it strongly). Let us wait a little
>> >> > >> bit more. Maybe google guys or redhat/suse guys would have a opinion.
>> >> > >
>> >> > > I haven't heard of fork bombs being an issue for us (and it's not been
>> >> > > for me on my desktop, either).
>> >> > >
>> >> > > Also, I want to point out that there is a classical userspace solution
>> >> > > for this, as implemented by killall5 for example. One can do
>> >> > > kill(-1, SIGSTOP) to stop all processes that they can send
>> >> > > signals to (except for init and itself). Target processes
>> >> > > can never catch or ignore the SIGSTOP. This stops the fork bomb
>> >> > > from causing further damage. Then, one can look at the process
>> >> > > tree and do whatever is appropriate - including killing by uid,
>> >> > > by cgroup or whatever policies one wants to implement in userspace.
>> >> > > Finally, the remaining processes can be restarted using SIGCONT.
>> >> > >
>> >> >
>> >> > Can that solution work even under OOM situation without new login/commands ?
>> >> > Please show us your solution, how to avoid Andrey's Bomb  with your way.
>> >> > Then, we can add Documentation, at least. Or you can show us your tool.
>> >> >
>> >> > Maybe it is....
>> >> > - running as a daemon. (because it has to lock its work memory before OOM.)
>> >> > - mlockall its own memory to work under OOM.
>> >> > - It can show process tree of users/admin or do all in automatic way
>> >> > with user's policy.
>> >> > - tell us which process is guilty.
>> >> > - wakes up automatically when OOM happens.....IOW, OOM should have some notifier
>> >> >   to userland.
>> >> > - never allocate any memory at running. (maybe it can't use libc.)
>> >> > - never be blocked by any locks, for example, some other task's mmap_sem.
>> >> >   One of typical mistakes of admins at OOM is typing 'ps' to see what
>> >> > happens.....
>> >> > - Can be used even with GUI system, which can't show console.
>> >>
>> >> Hi Kame,
>> >>
>> >> I am worried about run-time cost.
>> >> Should we care of mistake of users for robustness of OS?
>> >> Mostly right but we can't handle all mistakes of user so we need admin.
>> >> For exampe, what happens if admin execute "rm -rf /"?
>> >> For avoiding it, we get a solution "backup" about critical data.
>> >>
>> >
>> > Then, my patch is configurable and has control knobs....never invasive for
>> > people who don't want it. And simple and very low cost. It will have
>> > no visible performance/resource usage impact for usual guys.
>> >
>> >
>> >
>> >> In the same manner, if the system is very critical of forkbomb,
>> >> admin should consider it using memcg, virtualization, ulimit and so on.
>> >> If he don't want it, he should become a hard worker who have to
>> >> cross over other building to reboot it. Although he is a diligent man,
>> >> Reboot isn't good. So I suggest following patch which is just RFC.
>> >> For making formal patch, I have to add more comment and modify sysrq.txt.
>> >>
>> >
>> > For me, sysrq is of-no-use as I explained.
>>
>> Go to other building and new login?
>>
> I cannot login when the system is near happens.

I understand so I said your solution would be a last resort.

>
>> I think if server is important on such problem, it should have a solution.
>> The solution can be careful admin step or console with serial for
>> sysrq step or your forkbomb killer. We have been used sysrq with local
>> solution of last resort. In such context, sysrq solution ins't bad, I
>> think.
>>
>
> Mine works with Sysrq-f and this works poorly than mine.
>
>> If you can't provide 1 and 2, your forkbomb killer would be a last resort.
>> But someone can solve the problem in just careful admin or sysrq.
>> In that case, the user can disable forkbomb killer then it doesn't
>> affect system performance at all.
>> So maybe It could be separate topic.
>>
>> >
>> >> From 51bec44086a6b6c0e56ea978a2eb47e995236b47 Mon Sep 17 00:00:00 2001
>> >> From: Minchan Kim <minchan.kim@...il.com>
>> >> Date: Tue, 29 Mar 2011 00:52:20 +0900
>> >> Subject: [PATCH] [RFC] Prevent livelock by forkbomb
>> >>
>> >> Recently, We discussed how to prevent forkbomb.
>> >> The thing is a trade-off between cost VS effect.
>> >>
>> >> Forkbomb is a _race_ case which happes by someone's mistake
>> >> so if we have to pay cost in fast path(ex, fork, exec, exit),
>> >> It's a not good.
>> >>
>> >> Now, sysrq + I kills all processes. When I tested it, I still
>> >> need rebooting to work my system really well(ex, x start)
>> >> although console works. I don't know why we need such sysrq(kill
>> >> all processes and then what we can do?)
>> >>
>> >> So I decide to change sysrq + I to meet our goal which prevent
>> >> forkbomb. The rationale is following as.
>> >>
>> >> Forkbomb means somethings makes repeately tasks in a short time so
>> >> system don't have a free page then it become almost livelock state.
>> >> This patch uses the characteristc of forkbomb.
>> >>
>> >> When you push sysrq + I, it kills recent created tasks.
>> >> (In this version, 1 minutes). Maybe all processes included
>> >> forkbomb tasks are killed. If you can't get normal state of system
>> >> after you push sysrq + I, you can try one more. It can kill futher
>> >> recent tasks(ex, 2 minutes).
>> >>
>> >> You can continue to do it until your system becomes normal state.
>> >>
>> >> Signed-off-by: Minchan Kim <minchan.kim@...il.com>
>> >> ---
>> >>  drivers/tty/sysrq.c   |   45 ++++++++++++++++++++++++++++++++++++++++++---
>> >>  include/linux/sched.h |    6 ++++++
>> >>  2 files changed, 48 insertions(+), 3 deletions(-)
>> >>
>> >> diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
>> >> index 81f1395..6fb7e18 100644
>> >> --- a/drivers/tty/sysrq.c
>> >> +++ b/drivers/tty/sysrq.c
>> >> @@ -329,6 +329,45 @@ static void send_sig_all(int sig)
>> >>       }
>> >>  }
>> >>
>> >> +static void send_sig_recent(int sig)
>> >> +{
>> >> +     struct task_struct *p;
>> >> +     unsigned long task_jiffies, last_jiffies = 0;
>> >> +     bool kill = false;
>> >> +
>> >> +retry:
>> >
>> > you need tasklist lock for scanning reverse.
>>
>> Okay. I will look at it.
>>
>> >
>> >> +     for_each_process_reverse(p) {
>> >> +             if (p->mm && !is_global_init(p) && !fatal_signal_pending(p)) {
>> >> +                     /* recent created task */
>> >> +                     last_jiffies = timeval_to_jiffies(p->real_start_time);
>> >> +                     force_sig(sig, p);
>> >> +                     break;
>> >
>> > why break ? you need to kill all youngers. And what is the relationship with below ?
>>
>> It's for selecting recent _youngest_ task which are not kthread, not
>> init, not handled by below loop. In below loop, it start to send KILL
>> signal processes which are created within 1 minutes from _youngest_
>> process creation time.
>>
>> >
>> >
>> >> +             }
>> >> +     }
>> >> +
>> >> +     for_each_process_reverse(p) {
>> >> +             if (p->mm && !is_global_init(p)) {
>> >> +                     task_jiffies = timeval_to_jiffies(p->real_start_time);
>> >> +                     /*
>> >> +                      * Kill all processes which are created recenlty
>> >> +                      * (ex, 1 minutes)
>> >> +                      */
>> >> +                     if (task_jiffies > (last_jiffies - 60 * HZ)) {
>> >> +                             force_sig(sig, p);
>> >> +                             kill = true;
>> >> +                     }
>> >> +                     else
>> >> +                             break;
>> >> +             }
>> >> +     }
>> >> +
>> >> +     /*
>> >> +      * If we can't kill anything, restart with next group.
>> >> +      */
>> >> +     if (!kill)
>> >> +             goto retry;
>> >> +}
>> >
>> > This is not useful under OOM situation, we cannot use 'jiffies' to find younger tasks
>> > because "memory reclaim-> livelock" can take some amount of minutes very easily.
>> > So, I used other metrics. I think you do the same mistake I made before,
>> > this doesn't work.
>>
>> As far as I understand right, p->real_start_time is create time, not jiffies.
>> What I want is that kill all processes created recently, not all
>> process like old sysrq + I.
>>
>> Am I miss something?
>>
> When you run 'make -j' or 'Andrey's case' with "swap". You'll see 1minutes is too
> short and no task will be killed.
>
> To determine this 60*HZ is diffuclut. I think no one cannot detemine this.
> 1 minute is too short, 10 minutes are too long. So, I used a different manner,
> which seems to work well.

Okay. I can handle it. How about this?

retry:
old_time = yougest_task->start_time;
for_each_process_reverse(p) {
	time = p->start_time;
	if (time > old_time - 60 * HZ)
		kill(p);
}

/*
 * If user push sysrq within 1 minutes from last again,
 * we kill processes more.
 */
if (call_time < (now - 60 * HZ))
	goto retry;	

call_time = now;
return;

So whenever user push sysrq, older tasks would be killed and at last,
root forkbomb task would be killed.


>
> Thanks,
> -Kmae
>
>
>
>
>



-- 
Kind regards,
Minchan Kim
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