On Fri, Mar 14, 2008 at 11:23:43PM -0500, Eric Sandeen wrote:
> Josef 'Jeff' Sipek wrote:
> > On Fri, Mar 14, 2008 at 10:24:49PM -0500, Eric Sandeen wrote:
> >> This should fix the longstanding issues with xfs and old ABI
> >> arm boxes, which lead to various asserts and xfs shutdowns,
> >> and for which an (incorrect) patch has been floating around
> >> for years. (Said patch made ARM internally consistent, but
> >> altered the normal xfs on-disk format such that it looked
> >> corrupted on other architectures):
> >> http://lists.arm.linux.org.uk/lurker/message/20040311.002034.5ecf21a2.html
> > ...
> >> Signed-off-by: Eric Sandeen <sandeen@xxxxxxxxxxx>
> >>
> >> ---
> >>
> >> Index: linux-2.6.24/fs/xfs/linux-2.6/xfs_linux.h
> >> ===================================================================
> >> --- linux-2.6.24.orig/fs/xfs/linux-2.6/xfs_linux.h
> >> +++ linux-2.6.24/fs/xfs/linux-2.6/xfs_linux.h
> >> @@ -300,4 +300,11 @@ static inline __uint64_t howmany_64(__ui
> >> return x;
> >> }
> >>
> >> +/* ARM old ABI has some weird alignment/padding */
> >> +#if defined(__arm__) && !defined(__ARM_EABI__)
> >> +#define __arch_pack __attribute__((packed))
> >> +#else
> >> +#define __arch_pack
> >> +#endif
> >
> > Shouldn't this be unconditional? Just because it ends up being ok on x86
> > doesn't mean that it won't break some time later on...(do we want another
> > bad_features2 incident?)
>
> I think that packing structures when they don't need to be can actually
> be harmful, efficiency-wise. I read a nice explanation of this....
> which I can't find now.
Agreed. For in-memory only structures it makes sense to let the compiler do
whatever is the best, but for structures that are on-disk, you really have
no choice, you have to have the same layout in memory - which frequently
means packed. Unless I missed it, the structs you modified are on-disk, and
therefore _must_ be the way the docs say - which happens to be packed.
Josef 'Jeff' Sipek.
--
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like
that.
- Linus Torvalds
|