From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755241AbZBAQgu (ORCPT ); Sun, 1 Feb 2009 11:36:50 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752771AbZBAQgi (ORCPT ); Sun, 1 Feb 2009 11:36:38 -0500 Received: from mx2.redhat.com ([66.187.237.31]:42446 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752204AbZBAQgh (ORCPT ); Sun, 1 Feb 2009 11:36:37 -0500 Message-ID: <4985CF66.6090409@sandeen.net> Date: Sun, 01 Feb 2009 10:35:50 -0600 From: Eric Sandeen User-Agent: Thunderbird 2.0.0.19 (Macintosh/20081209) MIME-Version: 1.0 To: Boaz Harrosh CC: Geert Uytterhoeven , Arnd Bergmann , mfasheh@suse.com, joel.becker@oracle.com, linux-kernel@vger.kernel.org, hch@infradead.org, xfs-masters@oss.sgi.com, viro@zeniv.linux.org.uk, Ankit Jain , linux-fsdevel@vger.kernel.org, Andrew Morton , xfs@oss.sgi.com, ocfs2-devel@oss.oracle.com Subject: Re: [xfs-masters] [PATCH] fs: Add new pre-allocation ioctls to vfs for compatibility with legacy xfs ioctls References: <4980C71F.1010804@ankitjain.org> <200901310138.34164.arnd@arndb.de> <20090130171423.f99c88d0.akpm@linux-foundation.org> <200901310248.42820.arnd@arndb.de> <49856FE6.8020601@panasas.com> <49857BEB.30404@panasas.com> <4985966D.8040402@panasas.com> <4985C1B0.8060905@sandeen.net> <4985CCFA.4070008@panasas.com> In-Reply-To: <4985CCFA.4070008@panasas.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Boaz Harrosh wrote: > Eric Sandeen wrote: >> Boaz Harrosh wrote: >> >> ... >> >>> I don't understand >>> >>> if you have a structure like >>> struct foo { >>> u32 one; >>> u32 two; >>> }; >>> vs >>> struct foo_packed { >>> u32 one; >>> u32 two; >>> } __packed; >>> >>> Just adding an __attribute__((packed)) to it clearly does not change >>> the layout of the structure. Are you saying the __attribute__((packed)) >>> is an hint to the compiler that foo_packed might be used unaligned. This >>> is just brain-dead, because I can use an unaligned pointer to foo just as >>> I can to foo_packed. Otherwise there is no difference what-so-ever between >>> the two. I have to see it to believe. It is totally the wrong hint in the >>> wrong place taking away valuable meaning of saying "please don't use padding >>> holes in this structure" >>> >>> Sorry for been so slow, I just don't get it. >>> Boaz >> While I'm no gcc guru, I can confirm that gratuitous use of the packed >> attribute is suboptimal; adding "packed" to every ondisk structure made >> obdump -d xfs.ko | wc -l explode by about 15,000 lines on ia64. > > Yes! but are the structures the same? that is sizeof(foo_packed) == sizeof(foo) ? > If not then clearly above is expected. Yes, they are the same. They're disk structure definitions after all; ia64 doesn't *need* the packing, but adding the packed attribute changes the code that gcc generates. See also, perhaps, http://digitalvampire.org/blog/index.php/2006/07/31/why-you-shouldnt-use-__attribute__packed/ For an interface like this maybe it's fine, but sprnkling it around like pixie dust may not be a good plan. :) -Eric