linux-kernel - Re: [PATCH] diskquota: 32bit quota tools on 64bit architectures

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Date:	Mon, 18 Jun 2007 11:41:12 +0400
From:	Vasily Tarasov <vtaras@...nvz.org>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Mikael Pettersson <mikpe@...uu.se>, akpm@...ux-foundation.org,
	dev@...ru, devel@...nvz.org, jack@...e.cz,
	linux-kernel@...r.kernel.org, nataliep@...gle.com, vvs@...ru
Subject: Re: [PATCH] diskquota: 32bit quota tools on 64bit architectures

On Fri, 2007-06-15 at 17:24 +0200, Arnd Bergmann wrote:
> On Friday 15 June 2007, Vasily Tarasov wrote:
> > I just noticed that we can not avoid the addition of packed attribute.
> > Look, for example:
> > 
> > struct if_dqblk {
> >         __u64 dqb_bhardlimit; 
> >         __u64 dqb_bsoftlimit;
> >         __u64 dqb_curspace;
> >         __u64 dqb_ihardlimit;
> >         __u64 dqb_isoftlimit;
> >         __u64 dqb_curinodes; 
> >         __u64 dqb_btime;
> >         __u64 dqb_itime;
> >         __u32 dqb_valid;
> > };
> > 
> > sizeof(if_dqblk) = 0x48
> > On 32 bit: 0x44
> > 
> > If I replace __u64/__u32 with compat equivalents - it will not help!
> > alligned attribute can _only_ _increase_ the size of structure, but not
> > decrease it.
> 
> No, the gcc documentation isn't quite clear there, see the discussion about
> compat_u64 and compat_s64 types. It actually does the right thing when
> you use 'typedef __u64 __attribute__((aligned(64))) compat_64', as my
> patch does.
> 
> 	Arnd <><
> 

Wow!... Thank you for the explanation.
I'll resend the patch soon.

Vasily

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