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

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Message-Id: <200706151724.17094.arnd@arndb.de>
Date:	Fri, 15 Jun 2007 17:24:16 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	vtaras@...nvz.org
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 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 <><
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