Re: [PATCH for 3.2] memcg: do not trap chargers with full callstack on OOM

[Michal Hocko <mhocko@suse.cz>]

On Tue 09-07-13 15:00:17, Michal Hocko wrote:
> On Mon 24-06-13 16:13:45, Johannes Weiner wrote:
> > Hi guys,
> > 
> > On Sat, Jun 22, 2013 at 10:09:58PM +0200, azurIt wrote:
> > > >> But i'm sure of one thing - when problem occurs, nothing is able to
> > > >> access hard drives (every process which tries it is freezed until
> > > >> problem is resolved or server is rebooted).
> > > >
> > > >I would be really interesting to see what those tasks are blocked on.
> > > 
> > > I'm trying to get it, stay tuned :)
> > > 
> > > Today i noticed one bug, not 100% sure it is related to 'your' patch
> > > but i didn't seen this before. I noticed that i have lots of cgroups
> > > which cannot be removed - if i do 'rmdir <cgroup_directory>', it
> > > just hangs and never complete. Even more, it's not possible to
> > > access the whole cgroup filesystem until i kill that rmdir
> > > (anything, which tries it, just hangs). All unremoveable cgroups has
> > > this in 'memory.oom_control': oom_kill_disable 0 under_oom 1
> > 
> > Somebody acquires the OOM wait reference to the memcg and marks it
> > under oom but then does not call into mem_cgroup_oom_synchronize() to
> > clean up.  That's why under_oom is set and the rmdir waits for
> > outstanding references.
> > 
> > > And, yes, 'tasks' file is empty.
> > 
> > It's not a kernel thread that does it because all kernel-context
> > handle_mm_fault() are annotated properly, which means the task must be
> > userspace and, since tasks is empty, have exited before synchronizing.
> 
> Yes, well spotted. I have missed that while reviewing your patch.
> The follow up fix looks correct.

Hmm, I guess you wanted to remove !(fault & VM_FAULT_ERROR) test as well
otherwise the else BUG() path would be unreachable and we wouldn't know
that something fishy is going on.

> > Can you try with the following patch on top?
> > 
> > diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> > index 5db0490..9a0b152 100644
> > --- a/arch/x86/mm/fault.c
> > +++ b/arch/x86/mm/fault.c
> > @@ -846,17 +846,6 @@ static noinline int
> >  mm_fault_error(struct pt_regs *regs, unsigned long error_code,
> >  	       unsigned long address, unsigned int fault)
> >  {
> > -	/*
> > -	 * Pagefault was interrupted by SIGKILL. We have no reason to
> > -	 * continue pagefault.
> > -	 */
> > -	if (fatal_signal_pending(current)) {
> > -		if (!(fault & VM_FAULT_RETRY))
> > -			up_read(&current->mm->mmap_sem);
> > -		if (!(error_code & PF_USER))
> > -			no_context(regs, error_code, address);
> > -		return 1;
> > -	}
> >  	if (!(fault & VM_FAULT_ERROR))
> >  		return 0;
> >  
> 
> -- 
> Michal Hocko
> SUSE Labs
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-- 
Michal Hocko
SUSE Labs