Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754809AbaFRTdc (ORCPT ); Wed, 18 Jun 2014 15:33:32 -0400 Received: from cantor2.suse.de ([195.135.220.15]:50629 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754699AbaFRTdb (ORCPT ); Wed, 18 Jun 2014 15:33:31 -0400 Date: Wed, 18 Jun 2014 21:33:29 +0200 From: "Luis R. Rodriguez" To: Stephen Warren Cc: "Luis R. Rodriguez" , hpa@linux.intel.com, akpm@linux-foundation.org, linux-kernel@vger.kernel.org, Andrew Lunn , Michal Hocko , Petr Mladek , Joe Perches , Arun KS , Kees Cook , Davidlohr Bueso , Chris Metcalf Subject: Re: [RFT 1/2] printk: make dynamic kernel ring buffer alignemnt explicit Message-ID: <20140618193329.GO4841@wotan.suse.de> References: <1403090065-13879-1-git-send-email-mcgrof@do-not-panic.com> <53A1B693.2000609@wwwdotorg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <53A1B693.2000609@wwwdotorg.org> User-Agent: Mutt/1.5.17 (2007-11-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jun 18, 2014 at 09:56:03AM -0600, Stephen Warren wrote: > On 06/18/2014 05:14 AM, Luis R. Rodriguez wrote: > > From: "Luis R. Rodriguez" > > > > We have to consider alignment for the ring buffer both for the > > default static size, and then also for when an dynamic allocation > > is made when the log_buf_len=n kernel parameter is passed to set > > the size specifically to a size larger than the default size set > > by the architecture through CONFIG_LOG_BUF_SHIFT. > > > > The default static kernel ring buffer can be aligned properly if > > architectures set CONFIG_LOG_BUF_SHIFT properly, we provide ranges > > for the size though so even if CONFIG_LOG_BUF_SHIFT has a sensible > > aligned value it can be reduced to a non aligned value. Commit > > 6ebb017de9 by Andrew ensures the static buffer is always aligned > > and the decision of alignment is done by the compiler by using > > __alignof__(struct log) (curious what value caused the crash?). > > IIRC the issue was that __log_buf's type is char[] so without the > __aligned it could have any alignment at all, e.g. 1 or 2. However, > struct printk_log is stored in the buffer rather than just char*, and so > if __log_buf isn't aligned to the required alignment for that structure, > that can caused unaligned accesses to fields in the structure, which > isn't supported on ARM in at least some cases. > > As such, I think the change to setup_log_buf() in this patch makes sense > (although I suppose in practice memblock_virt_alloc() probably has some > minimum internal alignment that dwards LOG_ALIGN, but that's an > implementation detail we shouldn't rely on). Thanks for the feedback. memblock_virt_alloc() will by default align to L1 cache, so if that satisfies the architecture alignment it should be safe, but perhaps not optimal for saving a few bytes. Still curious if without this patch a crash can be triggered somehow with some log_buf_len=n, if so this can go to stable. Luis -- 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/