Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934339Ab3JPNJE (ORCPT ); Wed, 16 Oct 2013 09:09:04 -0400 Received: from mail-wg0-f49.google.com ([74.125.82.49]:55973 "EHLO mail-wg0-f49.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934006Ab3JPNJC (ORCPT ); Wed, 16 Oct 2013 09:09:02 -0400 Date: Wed, 16 Oct 2013 15:08:57 +0200 From: Frederic Weisbecker To: Steven Rostedt Cc: LKML , Ingo Molnar , Thomas Gleixner , "H. Peter Anvin" , Andrew Morton , "paulmck@linux.vnet.ibm.com" , Peter Zijlstra , "x86@kernel.org" , "Wang, Xiaoming" , "Li, Zhuangzhi" , "Liu, Chuansheng" Subject: Re: [PATCH] x86: Remove WARN_ON(in_nmi()) from vmalloc_fault Message-ID: <20131016130856.GE12773@localhost.localdomain> References: <20131015163906.342d8ffa@gandalf.local.home> <20131016114036.GB12773@localhost.localdomain> <20131016084518.44eaf61a@gandalf.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20131016084518.44eaf61a@gandalf.local.home> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2712 Lines: 64 On Wed, Oct 16, 2013 at 08:45:18AM -0400, Steven Rostedt wrote: > On Wed, 16 Oct 2013 13:40:37 +0200 > Frederic Weisbecker wrote: > > > On Tue, Oct 15, 2013 at 04:39:06PM -0400, Steven Rostedt wrote: > > > Since the NMI iretq nesting has been fixed, there's no reason that > > > an NMI handler can not take a page fault for vmalloc'd code. No locks > > > are taken in that code path, and the software now handles nested NMIs > > > when the fault re-enables NMIs on iretq. > > > > > > Not only that, if the vmalloc_fault() WARN_ON_ONCE() is hit, and that > > > warn on triggers a vmalloc fault for some reason, then we can go into > > > an infinite loop (the WARN_ON_ONCE() does the WARN() before updating > > > the variable to make it happen "once"). > > > > > > Reported-by: "Liu, Chuansheng" > > > Signed-off-by: Steven Rostedt > > > > Thanks! For now we probably indeed want this patch. But I hope it's only > > for the short term. > > Why? > > > > > I still think that allowing faults in NMIs is very nasty, as we expect NMIs to never > > be disturbed. > > We do faults (well, breakpoints really) in NMI to enable tracing. > > > I'm not even sure if that interacts correctly with the rcu_nmi_enter() > > and preempt_count & NMI_MASK things. Not sure how perf is ready for that either (now > > hardware events can be interrupted by fault trace events). > > I'm a bit confused. What doesn't interact correctly with > rcu_nmi_enter()? 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. > > > > > 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. 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? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/