Re: [patch -mm] mm, oom: add global access to memory reserves on livelock

[Michal Hocko]

On Mon 24-08-15 14:04:28, David Rientjes wrote:
> On Fri, 21 Aug 2015, Michal Hocko wrote:
> 
> > There might be many threads waiting for the allocation and this can lead
> > to quick oom reserves depletion without releasing resources which are
> > holding back the oom victim. As Tetsuo has shown, such a load can be
> > generated from the userspace without root privileges so it is much
> > easier to make the system _completely_ unusable with this patch. Not that
> > having an OOM deadlock would be great but you still have emergency tools
> > like sysrq triggered OOM killer to attempt to sort the situation out.
> > Once your are out of reserves nothing will help you, though. So I think it
> > is a bad idea to give access to reserves without any throttling.
> > 
> 
> I don't believe a solution that requires admin intervention is 
> maintainable.

Why?

> It would be better to reboot when memory reserves are fully depleted.

The question is when are the reserves depleted without any way to
replenish them. While playing with GFP_NOFS patch set which gives
__GFP_NOFAIL allocations access to memory reserves
(http://marc.info/?l=linux-mm&m=143876830916540&w=2) I could see the
warning hit while the system still resurrected from the memory pressure.

> > Johannes' idea to give a partial access to memory reserves to the task
> > which has invoked the OOM killer was much better IMO.
> 
> That's what this patch does, just without the "partial."  Processes are 
> required to reclaim and then invoke the oom killler every time an 
> allocation is made using memory reserves with this approach after the 
> expiration has lapsed.
> 
> We can discuss only allowing partial access to memory reserves equal to 
> ALLOC_HARD | ALLOC_HARDER, or defining a new watermark, but I'm concerned 
> about what happens when that threshold is reached and the oom killer is 
> still livelocked.  It would seem better to attempt recovery at whatever 
> cost and then panic if fully depleted.

I think an OOM reserve/watermark makes more sense. It will not solve the
livelock but neithere granting the full access to reserves will. But the
partial access has a potential to leave some others means to intervene.

-- 
Michal Hocko
SUSE Labs
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