Date: Wed, 16 Oct 2013 09:14:37 -0400
From: Steven Rostedt <rostedt@goodmis.org>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        "H. Peter Anvin" <hpa@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "paulmck@linux.vnet.ibm.com" <paulmck@linux.vnet.ibm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "x86@kernel.org" <x86@kernel.org>,
        "Wang, Xiaoming" <xiaoming.wang@intel.com>,
        "Li, Zhuangzhi" <zhuangzhi.li@intel.com>,
        "Liu, Chuansheng" <chuansheng.liu@intel.com>
Subject: Re: [PATCH] x86: Remove WARN_ON(in_nmi()) from vmalloc_fault
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On Wed, 16 Oct 2013 15:08:57 +0200
Frederic Weisbecker <fweisbec@gmail.com> wrote:


> Faults can call rcu_user_exit() / rcu_user_enter(). This is not supposed to happen
> between rcu_nmi_enter() and rcu_nmi_exit(). rdtp->dynticks would be incremented in the
> wrong way.
> 
> Ah but we have an in_interrupt() check in context_tracking_user_enter() that protects
> us against that.

I will say that we should probably warn if it's any fault other than a
vmalloc fault. A vmalloc fault should only happen in kernel space, and
should not be happening from user code.

> 
> > 
> > > 
> > > So I hope we can think about something else for the long term.
> > 
> > I still don't understand what's wrong with it. As long as the faulting
> > code does not grab any locks there shouldn't be anything wrong with
> > faulting in NMI. For vmalloc, it is just updating page tables.
> 
> NMI code is written with the idea that it can't be interrupted. May be that
> paranoia (again), you know. And I can't point you any problem in practice.
> I just think that allowing such a thing is asking for troubles.

The WARN_ON() that I removed is from vmalloc fault. I don't see an
issue with NMIs faulting via vmalloc. For any other page fault, sure, I
would be concerned about it. But what's wrong with an NMI running
module code?

> 
> But I'm ok with your patch, it fixes a real bug and as long as we don't have
> a better solution, we should keep that.
> 
> BTW, does faulting in NMIs re-enable NMIs?

Yes, but we now have code to handle that :-)

-- Steve
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