linux-kernel - Re: System freezes after OOM

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Date:	Mon, 18 Jul 2016 09:22:02 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Mikulas Patocka <mpatocka@...hat.com>
Cc:	Ondrej Kozina <okozina@...hat.com>,
	Jerome Marchand <jmarchan@...hat.com>,
	Stanislav Kozina <skozina@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, dm-devel@...hat.com
Subject: Re: System freezes after OOM

On Fri 15-07-16 13:02:17, Mikulas Patocka wrote:
> 
> 
> On Fri, 15 Jul 2016, Michal Hocko wrote:
> 
> > On Fri 15-07-16 08:11:22, Mikulas Patocka wrote:
> > > 
> > > The stacktraces showed that the kcryptd process was throttled when it 
> > > tried to do mempool allocation. Mempool adds the __GFP_NORETRY flag to the 
> > > allocation, but unfortunatelly, this flag doesn't prevent the allocator 
> > > from throttling.
> > 
> > Yes and in fact it shouldn't prevent any throttling. The flag merely
> > says that the allocation should give up rather than retry
> > reclaim/compaction again and again.
> > 
> > > I say that the process doing mempool allocation shouldn't ever be 
> > > throttled. Maybe add __GFP_NOTHROTTLE?
> > 
> > A specific gfp flag would be an option but we are slowly running out of
> > bit space there and I am not yet convinced PF_LESS_THROTTLE is
> > unsuitable.
> 
> PF_LESS_THROTTLE will make it throttle less, but it doesn't eliminate 
> throttling entirely. So, maybe add PF_NO_THROTTLE? But PF_* flags are also 
> almost exhausted.

I am not really sure we can make anybody so special to not throttle at all.
Seeing a congested backig device sounds like a reasonable compromise.
Besides that it seems that we do not really need to eliminate
wait_iff_congested for dm to work properly again AFAIU. I plan to repost
both patch today after some more internal review. If we need to do more
changes I would suggest making them in separet patches.
-- 
Michal Hocko
SUSE Labs