linux-kernel - Re: [Patch v4 1/2] freezer: check OOM kill while being frozen

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Date:	Mon, 8 Sep 2014 16:29:24 -0700
From:	Cong Wang <xiyou.wangcong@...il.com>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	Tejun Heo <tj@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
	David Rientjes <rientjes@...gle.com>,
	Michal Hocko <mhocko@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [Patch v4 1/2] freezer: check OOM kill while being frozen

On Mon, Sep 8, 2014 at 4:42 PM, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> On Monday, September 08, 2014 04:16:15 PM Cong Wang wrote:
>> On Mon, Sep 8, 2014 at 4:23 PM, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
>> >
>> > The reason why it matters for the suspend-time freezing is that we freeze tasks
>> > to take them out of the picture entirely until they are thawed.  Therefore we
>> > can't allow them to go back to the picture just for a while until they are
>> > killed.  Frozen tasks are not supposed to get back to the picture at all.
>> >
>>
>>
>> Ok, then checking TIF_MEMDIE is unsafe for PM freeze, we should
>> keep the cgroup_freezing() test to make sure freeze request is from
>> cgroup not PM. Question got answered. :)
>
> Do I think correctly that cgroups freezing and system suspend are
> mutually exclusive?  If not, then this still is problematic.

Good point! Although rare, but it is possible we freeze a process both from
cgroup and PM. Hmm, this means we have to explicitly exclude PM rather
just checking cgroup freeze? Interesting, but I am not familiar with PM.

>
>> I will put the following as a comment:
>>
>> /* OOM killer may decide to kill this process after it is frozen,
>>    in this case SIGKILL can never be handled, so we should check
>>   TIF_MEMDIE and if it is set, thaw and let SIGKILL kill it.
>>   But for PM freeze, it is not allowed to get out even for a while,
>>   so we have to keep it being frozen. */
>>
>> Let me know if it looks good to you.
>
> Well, it reflects the reality, so it's good enough.  But please adhere to the
> coding style rules for comments.
>

Sure thing.

Thanks!
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