From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx1.redhat.com (mx1.redhat.com [209.132.183.28]) by ozlabs.org (Postfix) with ESMTP id 995582C0090 for ; Sun, 19 May 2013 19:36:52 +1000 (EST) Date: Sun, 19 May 2013 12:35:26 +0300 From: "Michael S. Tsirkin" To: Peter Zijlstra Subject: Re: [PATCH v2 10/10] kernel: might_fault does not imply might_sleep Message-ID: <20130519093526.GD19883@redhat.com> References: <1f85dc8e6a0149677563a2dfb4cef9a9c7eaa391.1368702323.git.mst@redhat.com> <20130516184041.GP19669@dyad.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <20130516184041.GP19669@dyad.programming.kicks-ass.net> Cc: linux-m32r-ja@ml.linux-m32r.org, kvm@vger.kernel.org, Catalin Marinas , Will Deacon , David Howells , linux-mm@kvack.org, Paul Mackerras , "H. Peter Anvin" , linux-arch@vger.kernel.org, linux-am33-list@redhat.com, Hirokazu Takata , x86@kernel.org, Ingo Molnar , Arnd Bergmann , microblaze-uclinux@itee.uq.edu.au, Chris Metcalf , rostedt@goodmis.org, Thomas Gleixner , linux-arm-kernel@lists.infradead.org, Michal Simek , linux-m32r@ml.linux-m32r.org, linux-kernel@vger.kernel.org, Koichi Yasutake , linuxppc-dev@lists.ozlabs.org List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Thu, May 16, 2013 at 08:40:41PM +0200, Peter Zijlstra wrote: > On Thu, May 16, 2013 at 02:16:10PM +0300, Michael S. Tsirkin wrote: > > There are several ways to make sure might_fault > > calling function does not sleep. > > One is to use it on kernel or otherwise locked memory - apparently > > nfs/sunrpc does this. As noted by Ingo, this is handled by the > > migh_fault() implementation in mm/memory.c but not the one in > > linux/kernel.h so in the current code might_fault() schedules > > differently depending on CONFIG_PROVE_LOCKING, which is an undesired > > semantical side effect. > > > > Another is to call pagefault_disable: in this case the page fault > > handler will go to fixups processing and we get an error instead of > > sleeping, so the might_sleep annotation is a false positive. > > vhost driver wants to do this now in order to reuse socket ops > > under a spinlock (and fall back on slower thread handler > > on error). > > Are you using the assumption that spin_lock() implies preempt_disable() implies > pagefault_disable()? Note that this assumption isn't valid for -rt where the > spinlock becomes preemptible but we'll not disable pagefaults. No, I was not assuming that. What I'm trying to say is that a caller that does something like this under a spinlock: preempt_disable pagefault_disable error = copy_to_user pagefault_enable preempt_enable_no_resched is not doing anything wrong and should not get a warning, as long as error is handled correctly later. Right? > > Address both issues by: > > - dropping the unconditional call to might_sleep > > from the fast might_fault code in linux/kernel.h > > - checking for pagefault_disable() in the > > CONFIG_PROVE_LOCKING implementation > > > > Signed-off-by: Michael S. Tsirkin > > --- > > include/linux/kernel.h | 1 - > > mm/memory.c | 14 +++++++++----- > > 2 files changed, 9 insertions(+), 6 deletions(-) > > > > diff --git a/include/linux/kernel.h b/include/linux/kernel.h > > index e96329c..322b065 100644 > > --- a/include/linux/kernel.h > > +++ b/include/linux/kernel.h > > @@ -198,7 +198,6 @@ void might_fault(void); > > #else > > static inline void might_fault(void) > > { > > - might_sleep(); > > This removes potential resched points for PREEMPT_VOLUNTARY -- was that > intentional? No it's a bug. Thanks for pointing this out. OK so I guess it should be might_sleep_if(!in_atomic()) and this means might_fault would have to move from linux/kernel.h to linux/uaccess.h, since in_atomic() is in linux/hardirq.h Makes sense? > > } > > #endif > > > > diff --git a/mm/memory.c b/mm/memory.c > > index 6dc1882..1b8327b 100644 > > --- a/mm/memory.c > > +++ b/mm/memory.c > > @@ -4222,13 +4222,17 @@ void might_fault(void) > > if (segment_eq(get_fs(), KERNEL_DS)) > > return; > > > > - might_sleep(); > > /* > > - * it would be nicer only to annotate paths which are not under > > - * pagefault_disable, however that requires a larger audit and > > - * providing helpers like get_user_atomic. > > + * It would be nicer to annotate paths which are under preempt_disable > > + * but not under pagefault_disable, however that requires a new flag > > + * for differentiating between the two. > > -rt has this, pagefault_disable() doesn't change the preempt count but pokes > at task_struct::pagefault_disable. Good to know. So maybe we can import this at least for CONFIG_PROVE_LOCKING? To make the patch smaller I'd prefer doing both for now, this way this patchset does not have to poke in too many mm internals. I can try doing that - unless someone else has plans to merge this part soon anyway? > > */ > > - if (!in_atomic() && current->mm) > > + if (in_atomic()) > > + return; > > + > > + might_sleep(); > > + > > + if (current->mm) > > might_lock_read(¤t->mm->mmap_sem); > > } > > EXPORT_SYMBOL(might_fault); > > -- > > MST