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

To: Boaz Harrosh <bharrosh@xxxxxxxxxxx>
Subject: Re: [xfs-masters] [PATCH] fs: Add new pre-allocation ioctls to vfs for compatibility with legacy xfs ioctls
From: Eric Sandeen <sandeen@xxxxxxxxxxx>
Date: Sun, 01 Feb 2009 09:37:20 -0600
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: <4985966D.8040402@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>
User-agent: Thunderbird (Macintosh/20081209)
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.


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