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

[Cong Wang]

On Mon, Sep 15, 2014 at 4:22 AM, Michal Hocko <mhocko@xxxxxxx> wrote:
> On Thu 04-09-14 15:30:41, Cong Wang wrote:
>> There is a race condition between OOM killer and freezer when
>> they try to operate on the same process, something like below:
>>
>>         Process A       Process B               Process C
>> trigger page fault
>> then trigger oom
>> B=oom_scan_process_thread()
>>                                         cgroup freezer freeze(A, B)
>>                         ...
>>                         try_to_freeze()
>>                         stay in D state
>> oom_kill_process(B)
>> restart page fault
>> ...
>>
>> In this case, process A triggered a page fault in user-space,
>> and the kernel page fault handler triggered OOM, then kernel
>> selected process B as the victim, right before being killed
>> process B was frozen by process C therefore went to D state,
>> then kernel sent SIGKILL but it is already too late as
>> process B will not care about pending signals any more.
>
> I have just got back to this patch again and I guess that the
> description is a bit misleading. Sure there is a race but the main
> problem is that frozen tasks are OOM unkillable in principle. So a task
> might hide into the fridge and livelock OOM killer. This has been broken
> since __thaw_task doesn't thaw anything (I guess it was a3201227f803
> which broke it).
>
> What do you think about the following changelog instead?
> "
> Since f660daac474c6f (oom: thaw threads if oom killed thread is frozen
> before deferring) OOM killer relies on being able to thaw a frozen task
> to handle OOM situation but a3201227f803 (freezer: make freezing() test
> freeze conditions in effect instead of TIF_FREEZE) has reorganized the
> code and stopped clearing freeze flag in __thaw_task. This means that
> the target task only wakes up and goes into the fridge again because the
> freezing condition hasn't changed for it. This reintroduces the bug
> fixed by f660daac474c6f.
>
> Fix the issue by checking for TIF_MEMDIE thread flag and get away from
> the fridge if it is set. oom_scan_process_thread doesn't have to check
> for the frozen task anymore because do_send_sig_info will wake up the
> thread and TIF_MEMDIE is already set by that time.
>
> Fixes: a3201227f803 (freezer: make freezing() test freeze conditions in effect instead of TIF_FREEZE)
> Cc: stable@xxxxxxxxxxxxxxx # 3.3+
> "


Yes, looks better to me.


>
> + cgroup_freezing check can go away.

With your patch, yes.

Thanks.
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