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

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Date:	Sun, 1 Feb 2009 11:59:04 +0100 (CET)
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Boaz Harrosh <bharrosh@...asas.com>
cc:	Arnd Bergmann <arnd@...db.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ankit Jain <me@...itjain.org>, viro@...iv.linux.org.uk,
	hch@...radead.org, linux-fsdevel@...r.kernel.org, mfasheh@...e.com,
	joel.becker@...cle.com, ocfs2-devel@....oracle.com,
	linux-kernel@...r.kernel.org, xfs-masters@....sgi.com,
	xfs@....sgi.com
Subject: Re: [PATCH] fs: Add new pre-allocation ioctls to vfs for compatibility
 with legacy xfs ioctls

On Sun, 1 Feb 2009, Boaz Harrosh wrote:
> Geert Uytterhoeven wrote:
> > On Sun, 1 Feb 2009, Boaz Harrosh wrote:
> >> Arnd Bergmann wrote:
> >>> +struct space_resv {
> >>> +	__s16		l_type;
> >>> +	__s16		l_whence;
> >>> +	__s64		l_start;
> >>> +	__s64		l_len;		/* len == 0 means until end of file */
> >>> +	__s32		l_sysid;
> >>> +	__u32		l_pid;
> >>> +	__s32		l_pad[4];	/* reserve area			    */
> >>> +};
> >> What about telling the compiler exactly what you said above, just
> >> to be sure we all mean the same thing. (And as documentation for new
> >> comers):
> >>
> >> +struct space_resv_64 {
> >> +	__s16		l_type;
> >> +	__s16		l_whence;
> >> +	__u32		reserved;
> >> +	__s64		l_start;
> >> +	__s64		l_len;		/* len == 0 means until end of file */
> >> +	__s32		l_sysid;
> >> +	__u32		l_pid;
> >> +	__s32		l_pad[4];	/* reserve area			    */
> >> +} __packed;
> > 
> > Because the compiler will assume all fields are always unaligned and will use very
> > suboptimal code to access them?
> 
> This discussion comes up every once in a while. I'm using an old FC7 compiler
> (gcc (GCC) 4.1.2 20070925 (Red Hat 4.1.2-27)) And tests show that when the layout
> of a structure is exactly the same the "__packed" on structure declarations does
> nothing. It only starts to affect when there are real differences in alignment.
> Also tests with gcc 3.4.x showed the same effect.
> 
> On previous discussions no one could come forward and say what compiler version
> breaks when __packed is applied on structure definition. I'm afraid your statement
> above is a myth.

FC7, targeting ia32? Sure, ia32 has no alignment restrictions.
Try e.g. MIPS.

Gr{oetje,eeting}s,

						Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
							    -- Linus Torvalds
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