From: Eric Sandeen <sandeen@sandeen.net>
Subject: Re: [xfs-masters] [PATCH] fs: Add new pre-allocation ioctls to vfs
 for compatibility with legacy xfs ioctls
Date: Sun, 01 Feb 2009 09:37:20 -0600
Message-ID: <4985C1B0.8060905@sandeen.net>
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>	<Pine.LNX.4.64.0902011104320.20875@anakin>	<49857BEB.30404@panasas.com>	<Pine.LNX.4.64.0902011151180.20875@anakin> <4985966D.8040402@panasas.com>
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Cc: Geert Uytterhoeven <geert@linux-m68k.org>,
	Arnd Bergmann <arnd@arndb.de>, 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 <me@ankitjain.org>,
	linux-fsdevel@vger.kernel.org,
	Andrew Morton <akpm@linux-foundation.org>, xfs@oss.sgi.com,
	ocfs2-devel@oss.oracle.com
To: Boaz Harrosh <bharrosh@panasas.com>
Return-path: <linux-kernel-owner+glk-linux-kernel-3=40m.gmane.org-S1753601AbZBAPlt@vger.kernel.org>
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Sender: linux-kernel-owner@vger.kernel.org
List-Id: linux-fsdevel.vger.kernel.org

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.

-Eric