linux-kernel - Re: System freezes after OOM

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Date:	Fri, 15 Jul 2016 07:25:59 -0400 (EDT)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	David Rientjes <rientjes@...gle.com>
cc:	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	mhocko@...nel.org, okozina@...hat.com, jmarchan@...hat.com,
	skozina@...hat.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: System freezes after OOM



On Thu, 14 Jul 2016, David Rientjes wrote:

> On Thu, 14 Jul 2016, Tetsuo Handa wrote:
> 
> > David Rientjes wrote:
> > > On Wed, 13 Jul 2016, Mikulas Patocka wrote:
> > > 
> > > > What are the real problems that f9054c70d28bc214b2857cf8db8269f4f45a5e23 
> > > > tries to fix?
> > > > 
> > > 
> > > It prevents the whole system from livelocking due to an oom killed process 
> > > stalling forever waiting for mempool_alloc() to return.  No other threads 
> > > may be oom killed while waiting for it to exit.
> > 
> > Is that concern still valid? We have the OOM reaper for CONFIG_MMU=y case.
> > 
> 
> Umm, show me an explicit guarantee where the oom reaper will free memory 
> such that other threads may return memory to this process's mempool so it 
> can make forward progress in mempool_alloc() without the need of utilizing 
> memory reserves.  First, it might be helpful to show that the oom reaper 
> is ever guaranteed to free any memory for a selected oom victim.

The function mempool_alloc sleeps with "io_schedule_timeout(5*HZ);"

So, if the oom reaper frees some memory into the page allocator, the 
process that is stuck in mempoo_alloc will sleep for up to 5 seconds, then 
it will retry the allocation with "element = pool->alloc(gfp_temp, 
pool->pool_data)" (that will allocate from the page allocator) and succed.

Mikulas