Re: [PATCH 2/3] xfs: fix implicit padding in directory and attr CRC formats

Dave Chinner
2013-06-13T02:27:36Z

On Wed, Jun 12, 2013 at 07:58:19PM -0500, Ben Myers wrote:
> On Wed, Jun 12, 2013 at 12:19:07PM +1000, Dave Chinner wrote:
> > From: Dave Chinner <dchinner@redhat.com>
> >
> > Michael L. Semon has been testing CRC patches ona 32 bit system and
> on a
>
> > been seeing assert failures in the directory code from xfs/080.
> > Thanks to Michael's heroic efforts with printk debugging, we found
> > that the problem was that the last free space being left in the
> > directory structure was too small to fit a unused tag structure and
> > it was being corrupted and attempting to log a region out of bounds.
> > Hence the assert failure looked something like:
> >
> > .....
> > #5 calling xfs_dir2_data_log_unused() 36 32
> > #1 4092 4095 4096
> > #2 8182 8183 4096
> first?
> last?
> bp->b_length?

Yup.

> >
> > Note that this only affects 32 bit systems with CRCs enabled.
> > Everything else is just fine. Note that filesystems created before
> CRC enabled filesystems
>
> I suggest this be added to head off any confusion.

Sure. Do I need to resubmit this, or are you going to just modify
the commit message yourself before applying it?

> > index 995f1f5..7826782 100644
> > --- a/fs/xfs/xfs_dir2_format.h
> > +++ b/fs/xfs/xfs_dir2_format.h
> > @@ -266,6 +266,7 @@ struct xfs_dir3_blk_hdr {
> > struct xfs_dir3_data_hdr {
> > struct xfs_dir3_blk_hdr hdr;
> > xfs_dir2_data_free_t best_free[XFS_DIR2_DATA_FD_COUNT];
> > + __be32 pad; /* 64 bit alignment */
>
> I counted these up and it looks fine. Nice work gents.

pahole is a much better way of determining structure size - it tells
you exactly what the compiler did, rather than having to assume what
the compiler is going to do...

$ pahole fs/xfs/xfs_dir2_data.o |less
.....
struct xfs_dir3_blk_hdr {
__be32 magic; /* 0 4 */
__be32 crc; /* 4 4 */
__be64 blkno; /* 8 8 */
__be64 lsn; /* 16 8 */
uuid_t uuid; /* 24 16 */
__be64 owner; /* 40 8 */

/* size: 48, cachelines: 1, members: 6 */
/* last cacheline: 48 bytes */
};
struct xfs_dir3_data_hdr {
struct xfs_dir3_blk_hdr hdr; /* 0 48 */
xfs_dir2_data_free_t best_free[3]; /* 48 12 */
__be32 pad; /* 60 4 */
/* --- cacheline 1 boundary (64 bytes) --- */

/* size: 64, cachelines: 1, members: 3 */
};
....

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com