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

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Date:	Mon, 8 Sep 2014 13:58:30 -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 1:54 PM, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> On Monday, September 08, 2014 10:40:17 AM Cong Wang wrote:
>>
>> We asked you to comment on either if this patch is safe for PM freeze
>> if we don't have the cgroup_freezing() check, or if it is not safe why (so that
>> I can put it in the comment).
>
> OK, which version of it?  Anything that has been posted as a complete patch you
> have a link to?  Or am I supposed to go through the whole thread and figure out
> how the patch *might* have looked had it been posted and then comment?

Sorry, I meant this version, the one we are discussing about.
In case you missed it too, let me show the related code for you:

+       /* It might not be safe to check TIF_MEMDIE for pm freeze. */
+       if (cgroup_freezing(current) && test_thread_flag(TIF_MEMDIE))
+               return true;
+

(return true means it will break the loop of freezing, that is thaw itself.)

So our question is if we really need the cgroup_freezing() check here?
IOW, is it safe to check TIF_MEMDIE and then thaw itself even if the
freeze request is from PM? Hope it is clear now.

>> OK, maybe just one or two sentences. Let me know if the following
>> comment is okay for you:
>>
>> /* OOM killer might decide to kill this process after it is frozen,
>>   in this case it should thaw and die. */
>
> I don't think it's sufficient.  In particular, what does "thaw and die" mean
> exactly?  It should be thawed immediately and then die or it should die right
> after it's thawed (at one point in the future)?
>

OK, let me try again:

/* 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. */

(Again, more detailed explanation is in the changelog, comment is just
for a quick reference, this seems what we all agree on.)

Thanks!
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