linux-kernel - Re: [xfs-masters] [PATCH] fs: Add new pre-allocation ioctls to vfs for compatibility with legacy xfs ioctls

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Date:	Sun, 01 Feb 2009 09:37:20 -0600
From:	Eric Sandeen <sandeen@...deen.net>
To:	Boaz Harrosh <bharrosh@...asas.com>
CC:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Arnd Bergmann <arnd@...db.de>, mfasheh@...e.com,
	joel.becker@...cle.com, linux-kernel@...r.kernel.org,
	hch@...radead.org, xfs-masters@....sgi.com,
	viro@...iv.linux.org.uk, Ankit Jain <me@...itjain.org>,
	linux-fsdevel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>, xfs@....sgi.com,
	ocfs2-devel@....oracle.com
Subject: Re: [xfs-masters] [PATCH] fs: Add new pre-allocation ioctls to vfs
 for compatibility with legacy xfs ioctls

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
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