From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754141AbZGJJaj (ORCPT ); Fri, 10 Jul 2009 05:30:39 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751401AbZGJJac (ORCPT ); Fri, 10 Jul 2009 05:30:32 -0400 Received: from cn.fujitsu.com ([222.73.24.84]:59150 "EHLO song.cn.fujitsu.com" rhost-flags-OK-FAIL-OK-OK) by vger.kernel.org with ESMTP id S1751376AbZGJJab (ORCPT ); Fri, 10 Jul 2009 05:30:31 -0400 Message-ID: <4A570A34.5090002@cn.fujitsu.com> Date: Fri, 10 Jul 2009 17:30:28 +0800 From: Li Zefan User-Agent: Thunderbird 2.0.0.9 (X11/20071115) MIME-Version: 1.0 To: Jens Axboe CC: Linux Kernel , Alan.Brunelle@hp.com, Ingo Molnar , Steven Rostedt , Frederic Weisbecker Subject: Re: [PATCH] Fix blktrace unaligned memory access References: <20090710075339.GX23611@kernel.dk> <4A5704EA.5000807@cn.fujitsu.com> <20090710091337.GD23611@kernel.dk> In-Reply-To: <20090710091337.GD23611@kernel.dk> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Jens Axboe wrote: > On Fri, Jul 10 2009, Li Zefan wrote: >> Jens Axboe wrote: >>> Hi, >>> >>> It seems that relay_reserve() (or the ring_buffer_event_data(), that one >>> still needs some love) can return unaligned memory, there's no way >>> around that when you have pdu lengths that aren't a nice size. This can >>> cause unaligned access warnings on platforms that care about alignment. >>> >> Seems relay_reserve() does nothing for alignment..On the other hand, >> ring_buffer_event_data() returns a ptr which is 32bit-aligned, but >> this still means it can cause unaligned accesses on 64bits arch, while >> I think it's fixable in ring buffer, it's certainly not an easy job. > > Right, it's a bit nasty... > Lai Jiangshan noticed this issue long ago and had some ideas in mind how to fix ring buffer, but never try it out for it will probably be frustrating.. >>> This is an RFC, perhaps we can fix this in some other way. This one takes >>> the simple approach, use an on-stack copy and memcpy() that to the >>> destination. >>> >> or get_unaligned() ? > > put_unaligned(), you mean? The big question is then which is faster, using > put_unaligned() or doing the memcpy() of the structure... > Ah, I meant put_unaligned(). I think the patch you posted can be a workaround at least for now, and can be improved by detecting HAVE_EFFICIENT_UNALIGNED_ACCESS