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

[Boaz Harrosh <bharrosh@panasas.com>]

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