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

[Geert Uytterhoeven <geert@linux-m68k.org>]

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@linux-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