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Re: [xfs-masters] [PATCH] fs: Add new pre-allocation ioctls to vfs for c

To: Eric Sandeen <sandeen@xxxxxxxxxxx>
Subject: Re: [xfs-masters] [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 18:25:30 +0200
Cc: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>, Arnd Bergmann <arnd@xxxxxxxx>, mfasheh@xxxxxxxx, joel.becker@xxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, hch@xxxxxxxxxxxxx, xfs-masters@xxxxxxxxxxx, viro@xxxxxxxxxxxxxxxxxx, Ankit Jain <me@xxxxxxxxxxxxx>, linux-fsdevel@xxxxxxxxxxxxxxx, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>, xfs@xxxxxxxxxxx, ocfs2-devel@xxxxxxxxxxxxxx
In-reply-to: <4985C1B0.8060905@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> <Pine.LNX.4.64.0902011151180.20875@anakin> <4985966D.8040402@xxxxxxxxxxx> <4985C1B0.8060905@xxxxxxxxxxx>
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Eric Sandeen wrote:
> Boaz Harrosh wrote:
> 
> ...
> 
>> 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
> 
> While I'm no gcc guru, I can confirm that gratuitous use of the packed
> attribute is suboptimal; adding "packed" to every ondisk structure made
> obdump -d xfs.ko | wc -l explode by about 15,000 lines on ia64.

Yes! but are the structures the same? that is sizeof(foo_packed) == sizeof(foo) 
?
If not then clearly above is expected.

In anyway, if __attribute__((packed)) makes some brain-dead gcc do the wrong 
thing
putting a _Padding member where you expect an alignment hole, and a 
BUILD_BUG_ON(sizeof() != ())
statement somewhere in code is a must, specifically for the brain-dead.

> 
> -Eric

There are to many places in Kernel where these things are left to chance that 
give
me an headache, not talking about cross platform mounts.

Boaz

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