On Sat, Mar 29, 2008 at 08:30:00PM -0500, Eric Sandeen wrote:
> David Chinner wrote:
> > There is a bug in mkfs.xfs that can result in writing the features2
> > field in the superblock to the wrong location. This only occurs
> > on some architectures, typically those with 32 bit userspace and
> > 64 bit kernels.
> >
> > This patch detects the defect at mount time, logs a warning
> > such as:
> >
> > XFS: correcting sb_features alignment problem
> >
> > in dmesg and corrects the problem so that everything is OK.
> > it also blacklists the bad field in the superblock so it does
> > not get used for something else later on.
>
> ...
>
> > /*
> > + * Check for a bad features2 field alignment. This happened on
> > + * some platforms due to xfs_sb_t not being 64bit size aligned
> > + * when sb_features was added and hence the compiler put it in
> > + * the wrong place.
> > + *
> > + * If we detect a bad field, we or the set bits into the existing
> > + * features2 field in case it has already been modified and we
> > + * don't want to lose any features. Zero the bad one and mark
> > + * the two fields as needing updates once the transaction subsystem
> > + * is online.
> > + */
> > + if (xfs_sb_has_bad_features2(sbp)) {
> > + cmn_err(CE_WARN,
> > + "XFS: correcting sb_features alignment problem");
> > + sbp->sb_features2 |= sbp->sb_bad_features2;
> > + sbp->sb_bad_features2 = 0;
> > + update_flags |= XFS_SB_FEATURES2 | XFS_SB_BAD_FEATURES2;
> > + }
>
> Hm, the other problem here may be that if we zero bad_features2, then
> any older kernel will mount up as attr2... and run into the corruption
> problem I found on F8...
>
> Should we make features2 and bad_features2 match rather than zeroing
> bad_features2?
I thought that was discussed here (or was it on IRC?), and the conclusion
was the best way is to always have features2 == bad_features2. It is the
safest way to handle things - the filesystem is guaranteed to work
everywhere properly (old & new kernels). Both the userspace (xfs_repair)
and kernel have to of course do the same thing (or bad_features2 with
features2, and save the result in both locations).
At least that's what I seem to remember.
Josef 'Jeff' Sipek.
--
Once you have their hardware. Never give it back.
(The First Rule of Hardware Acquisition)
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