From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx149.postini.com [74.125.245.149]) by kanga.kvack.org (Postfix) with SMTP id 4C8426B0002 for ; Tue, 21 May 2013 08:39:01 -0400 (EDT) Date: Tue, 21 May 2013 13:21:26 +0200 From: Peter Zijlstra Subject: Re: [PATCH v2 10/10] kernel: might_fault does not imply might_sleep Message-ID: <20130521112126.GJ26912@twins.programming.kicks-ass.net> References: <1f85dc8e6a0149677563a2dfb4cef9a9c7eaa391.1368702323.git.mst@redhat.com> <20130516184041.GP19669@dyad.programming.kicks-ass.net> <20130519093526.GD19883@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130519093526.GD19883@redhat.com> Sender: owner-linux-mm@kvack.org List-ID: 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 On Sun, May 19, 2013 at 12:35:26PM +0300, Michael S. Tsirkin wrote: > 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? Aside from the no_resched() thing which Steven already explained and my previous email asking why you need the preempt_disable() at all, that should indeed work. The reason I was asking was that I wasn't sure you weren't doing: spin_lock(&my_lock); error = copy_to_user(); spin_unlock(&my_lock); and expecting the copy_to_user() to always take the exception table route. This works on mainline (since spin_lock implies a preempt disable and preempt_disable is the same as pagefault_disable). However as should be clear by now, it doesn't quite work that way for -rt. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org