Re: [PATCH 3/7] xfs: fix a bug in the online fsck directory leaf1 bestcount check

["Darrick J. Wong" <djwong@kernel.org>]

On Thu, Dec 16, 2021 at 04:05:37PM +1100, Dave Chinner wrote:
> On Wed, Dec 15, 2021 at 05:09:32PM -0800, Darrick J. Wong wrote:
> > From: Darrick J. Wong <djwong@kernel.org>
> > 
> > When xfs_scrub encounters a directory with a leaf1 block, it tries to
> > validate that the leaf1 block's bestcount (aka the best free count of
> > each directory data block) is the correct size.  Previously, this author
> > believed that comparing bestcount to the directory isize (since
> > directory data blocks are under isize, and leaf/bestfree blocks are
> > above it) was sufficient.
> > 
> > Unfortunately during testing of online repair, it was discovered that it
> > is possible to create a directory with a hole between the last directory
> > block and isize.
> 
> We have xfs_da3_swap_lastblock() that can leave an -empty- da block
> between the last referenced block and isize, but that's not a "hole"
> in the file. If you don't mean xfs_da3_swap_lastblock(), then can
> you clarify what you mean by a "hole" here and explain to me how the
> situation it occurs in comes about?

I don't actually know how it comes about.  I wrote a test that sets up
fsstress to expand and contract directories and races xfs_scrub -n, and
noticed that I'd periodically get complaints about directories (usually
$SCRATCH_MNT/p$CPU) where the last block(s) before i_size were actually
holes.

I began reading the dir2 code to try to figure out how this came about
(clearly we're not updating i_size somewhere) but then took the shortcut
of seeing if xfs_repair or xfs_check complained about this situation.
Neither of them did, and I found a couple more directories in a similar
situation on my crash test dummy machine, and concluded "Wellllp, I
guess this is part of the ondisk format!" and committed the patch.

Also, I thought xfs_da3_swap_lastblock only operates on leaf and da
btree blocks, not the blocks containing directory entries?  I /think/
the actual explanation is that something goes wrong in
xfs_dir2_shrink_inode (maybe?) such that the mapping goes away but
i_disk_size doesn't get updated?  Not sure how /that/ can happen,
though...

--D

> 
> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@fromorbit.com