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

To: Boaz Harrosh <bharrosh@xxxxxxxxxxx>
Subject: Re: [PATCH] fs: Add new pre-allocation ioctls to vfs for compatibility with legacy xfs ioctls
From: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
Date: Sun, 1 Feb 2009 11:59:04 +0100 (CET)
Cc: Arnd Bergmann <arnd@xxxxxxxx>, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>, Ankit Jain <me@xxxxxxxxxxxxx>, viro@xxxxxxxxxxxxxxxxxx, hch@xxxxxxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx, mfasheh@xxxxxxxx, joel.becker@xxxxxxxxxx, ocfs2-devel@xxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, xfs-masters@xxxxxxxxxxx, xfs@xxxxxxxxxxx
In-reply-to: <49857BEB.30404@xxxxxxxxxxx>
References: <4980C71F.1010804@xxxxxxxxxxxxx> <200901310138.34164.arnd@xxxxxxxx> <20090130171423.f99c88d0.akpm@xxxxxxxxxxxxxxxxxxxx> <200901310248.42820.arnd@xxxxxxxx> <49856FE6.8020601@xxxxxxxxxxx> <Pine.LNX.4.64.0902011104320.20875@anakin> <49857BEB.30404@xxxxxxxxxxx>
Sender: geert@xxxxxxxxxxxxxx
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@xxxxxxxxxxxxxx

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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