Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753500Ab3EPSpI (ORCPT ); Thu, 16 May 2013 14:45:08 -0400 Received: from 173-166-109-252-newengland.hfc.comcastbusiness.net ([173.166.109.252]:60793 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752494Ab3EPSpE (ORCPT ); Thu, 16 May 2013 14:45:04 -0400 Date: Thu, 16 May 2013 20:40:41 +0200 From: Peter Zijlstra To: "Michael S. Tsirkin" Cc: linux-kernel@vger.kernel.org, Catalin Marinas , Will Deacon , David Howells , Hirokazu Takata , Michal Simek , Koichi Yasutake , Benjamin Herrenschmidt , Paul Mackerras , Chris Metcalf , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , x86@kernel.org, Arnd Bergmann , linux-arm-kernel@lists.infradead.org, linux-m32r@ml.linux-m32r.org, linux-m32r-ja@ml.linux-m32r.org, microblaze-uclinux@itee.uq.edu.au, linux-am33-list@redhat.com, linuxppc-dev@lists.ozlabs.org, linux-arch@vger.kernel.org, linux-mm@kvack.org, kvm@vger.kernel.org, rostedt@goodmis.org Subject: Re: [PATCH v2 10/10] kernel: might_fault does not imply might_sleep Message-ID: <20130516184041.GP19669@dyad.programming.kicks-ass.net> References: <1f85dc8e6a0149677563a2dfb4cef9a9c7eaa391.1368702323.git.mst@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1f85dc8e6a0149677563a2dfb4cef9a9c7eaa391.1368702323.git.mst@redhat.com> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3071 Lines: 87 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. > 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? > } > #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. > */ > - 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 -- 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/