linux-kernel - Re: [PATCH v2] trace: Set __GFP_NORETRY flag for ring buffer allocating process

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Date:	Thu, 9 Jun 2011 12:42:26 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Steven Rostedt <rostedt@...dmis.org>
cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	vnagarnaik@...gle.com, mingo@...hat.com, fweisbec@...il.com,
	mrubin@...gle.com, dhsharp@...gle.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] trace: Set __GFP_NORETRY flag for ring buffer allocating
 process

On Thu, 9 Jun 2011, Steven Rostedt wrote:

> > Unfortunately, __GFP_NORETRY is racy and don't work as expected.
> > If free memory is not enough, the thread may start to reclaim and
> > another thread can steal the reclaimed memory. And thread0 don't retry.
> > 
> > Then, thread0's alloc page may fail even though system have enough reclaimable
> > memory.
> > 
> >         thread0                                        thread1
> >         ---------------------------------------------------------------
> >         alloc_pages()
> >            get_page_from_freelist() -> fail
> >            try_to_free_pages()
> >                                                     alloc_pages()
> >                                                        get_page_from_freelist() -> success
> >            get_page_from_freelist() -> fail again
> > 
> > I think this is mm issue, and afaik, Minchan and some developers are
> > working on fixing it. but _now_ your patch doesn't work.
> 
> Have you seen this fail in practice?
> 
> I'm not too concern if it only triggers when memory is tight. But if it
> is triggering on normal cases, then that worries me.
> 

It would only happen if there was an antagonist that stole the reclaimed 
pages before your __GFP_NORETRY allocation could allocate them, resulting 
in the system being oom again as it was before reclaim occurred.  Without 
__GFP_NORETRY, we'd automatically retry these allocations in a loop until 
we found the memory since they are order-0, so the only side effect would 
be an increased latency in the allocation.  I think if we still end up oom 
after reclaiming memory that was allocated by another thread that we 
probably don't want to be expanding the ring buffer and, thus, I see no 
problem with just failing.
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