Re: System freezes after OOM

[Mikulas Patocka <mpatocka@redhat.com>]

On Fri, 15 Jul 2016, David Rientjes wrote:

> On Fri, 15 Jul 2016, Mikulas Patocka wrote:
> 
> > > There is no guarantee that _anything_ can return memory to the mempool,
> > 
> > You misunderstand mempools if you make such claims.
> > 
> > There is in fact guarantee that objects will be returned to mempool. In 
> > the past I reviewed device mapper thoroughly to make sure that it can make 
> > forward progress even if there is no available memory.
> > 
> > I don't know what should I tell you if you keep on repeating the same 
> > false claim over and over again. Should I explain mempool oprerations to 
> > you in detail? Or will you find it on your own?
> > 
> 
> If you are talking about patches you're proposing for 4.8 or any guarantee 
> of memory freeing that the oom killer/reaper will provide in 4.8, that's 
> fine.  However, the state of the 4.7 kernel is the same as it was when I 
> fixed this issue that timed out hundreds of our machines and is 
> contradicted by that evidence.  Our machines time out after two hours with 
> the oom victim looping forever in mempool_alloc(), so if there was a 

And what about the oom reaper? It should have freed all victim's pages 
even if the victim is looping in mempool_alloc. Why the oom reaper didn't 
free up memory?

> guarantee that elements would be returned in a completely livelocked 
> kernel in 4.7 or earlier kernels, that would not have been the case.  I 

And what kind of targets do you use in device mapper in the configuration 
that livelocked? Do you use some custom google-developed drivers?

Please describe the whole stack of block I/O devices when this livelock 
happened.

Most device mapper drivers can really make forward progress when they are 
out of memory, so I'm interested what kind of configuration do you have.

> frankly don't care about your patch reviewing of dm mempool usage when 
> dm_request() livelocked our kernel.

If it livelocked, it is a bug in some underlying block driver, not a bug 
in mempool_alloc.

> Feel free to formally propose patches either for 4.7 or 4.8.

Mikulas