Re: [PATCH] memcg: Do not hang on OOM when killed by userspace OOMaccess to memory reserves

[Michal Hocko]

On Thu 09-01-14 13:40:10, David Rientjes wrote:
> On Thu, 9 Jan 2014, Michal Hocko wrote:
> 
> > Eric has reported that he can see task(s) stuck in memcg OOM handler
> > regularly. The only way out is to
> > 	echo 0 > $GROUP/memory.oom_controll
> > His usecase is:
> > - Setup a hierarchy with memory and the freezer
> >   (disable kernel oom and have a process watch for oom).
> > - In that memory cgroup add a process with one thread per cpu.
> > - In one thread slowly allocate once per second I think it is 16M of ram
> >   and mlock and dirty it (just to force the pages into ram and stay there).
> > - When oom is achieved loop:
> >   * attempt to freeze all of the tasks.
> >   * if frozen send every task SIGKILL, unfreeze, remove the directory in
> >     cgroupfs.
> > 
> > Eric has then pinpointed the issue to be memcg specific.
> > 
> > All tasks are sitting on the memcg_oom_waitq when memcg oom is disabled.
> > Those that have received fatal signal will bypass the charge and should
> > continue on their way out. The tricky part is that the exit path might
> > trigger a page fault (e.g. exit_robust_list), thus the memcg charge,
> > while its memcg is still under OOM because nobody has released any
> > charges yet.
> > Unlike with the in-kernel OOM handler the exiting task doesn't get
> > TIF_MEMDIE set so it doesn't shortcut futher charges of the killed task
> > and falls to the memcg OOM again without any way out of it as there are
> > no fatal signals pending anymore.
> > 
> > This patch fixes the issue by checking PF_EXITING early in
> > __mem_cgroup_try_charge and bypass the charge same as if it had fatal
> > signal pending or TIF_MEMDIE set.
> > 
> > Normally exiting tasks (aka not killed) will bypass the charge now but
> > this should be OK as the task is leaving and will release memory and
> > increasing the memory pressure just to release it in a moment seems
> > dubious wasting of cycles. Besides that charges after exit_signals
> > should be rare.
> > 
> > Reported-by: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
> > Signed-off-by: Michal Hocko <mhocko@xxxxxxx>
> 
> Is this tested?

By Eric? No AFAIK. I wasn't able to reproduce the issue myself.

> > ---
> >  mm/memcontrol.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index b8dfed1b9d87..b86fbb04b7c6 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -2685,7 +2685,8 @@ static int __mem_cgroup_try_charge(struct mm_struct *mm,
> >  	 * MEMDIE process.
> >  	 */
> >  	if (unlikely(test_thread_flag(TIF_MEMDIE)
> > -		     || fatal_signal_pending(current)))
> > +		     || fatal_signal_pending(current))
> > +		     || current->flags & PF_EXITING)
> >  		goto bypass;
> >  
> >  	if (unlikely(task_in_memcg_oom(current)))
> 
> This would become problematic if significant amount of memory is charged 
> in the exit() path. 

But this would hurt also for fatal_signal_pending tasks, wouldn't it?
Besides that I do not see any source of allocation after exit_signals.

> I don't know of an egregious amount of memory being 
> allocated and charged after PF_EXITING is set, but if it happens in the 
> future then this could potentially cause system oom conditions even in 
> memcg configurations 

Even if that happens then the global OOM killer would give the exiting
task access to memory reserves and wouldn't kill anything else.

So I am not sure what problem do you see exactly.

Besides that allocating egregious amount of memory after exit_signals
sounds fundamentally broken to me.

> that are designed such as the one Tejun suggested to 
> be able to handle such conditions in userspace:
> 
> 		     ___root___
> 		    /	       \
> 		user		oom
> 		/  \		/ \
> 		A  B		C D
> 
> where the limit of user is equal to the amount of system memory minus 
> whatever amount of memory is needed by the system oom handler attached as 
> a descendant of oom and still allows the limits of A + B to exceed the 
> limit of user.
> 
> So how do we ensure that memory allocations in the exit() path don't cause 
> system oom conditions whereas the above configuration no longer provides 
> any strict guarantee?
> 
> Thanks.

-- 
Michal Hocko
SUSE Labs
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