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

To: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
Subject: Re: [PATCH] fs: Add new pre-allocation ioctls to vfs for compatibility with legacy xfs ioctls
From: Boaz Harrosh <bharrosh@xxxxxxxxxxx>
Date: Sun, 01 Feb 2009 14:32:45 +0200
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: <Pine.LNX.4.64.0902011151180.20875@anakin>
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> <Pine.LNX.4.64.0902011151180.20875@anakin>
User-agent: Thunderbird/3.0a2 (X11; 2008072418)
Geert Uytterhoeven wrote:
> 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
> 

I don't understand

if you have a structure like
struct foo {
        u32 one;
        u32 two;
};
vs
struct foo_packed {
        u32 one;
        u32 two;
} __packed;

Just adding an __attribute__((packed)) to it clearly does not change
the layout of the structure. Are you saying the __attribute__((packed))
is an hint to the compiler that foo_packed might be used unaligned. This
is just brain-dead, because I can use an unaligned pointer to foo just as
I can to foo_packed. Otherwise there is no difference what-so-ever between
the two. I have to see it to believe. It is totally the wrong hint in the
wrong place taking away valuable meaning of saying "please don't use padding
holes in this structure"

Sorry for been so slow, I just don't get it.
Boaz

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