Re: [PATCH] xfs: reset child dir '..' entry when unlinking child

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

On Mon, Jul 05, 2021 at 09:41:03AM +0100, Christoph Hellwig wrote:
> On Fri, Jul 02, 2021 at 08:02:33PM -0700, Darrick J. Wong wrote:
> > From: Darrick J. Wong <djwong@kernel.org>
> > 
> > While running xfs/168, I noticed a second source of post-shrink
> > corruption errors causing shutdowns.
> > 
> > Let's say that directory B has a low inode number and is a child of
> > directory A, which has a high number.  If B is empty but open, and
> > unlinked from A, B's dotdot link continues to point to A.  If A is then
> > unlinked and the filesystem shrunk so that A is no longer a valid inode,
> > a subsequent AIL push of B will trip the inode verifiers because the
> > dotdot entry points outside of the filesystem.
> > 
> > To avoid this problem, reset B's dotdot entry to the root directory when
> > unlinking directories, since the root directory cannot be removed.
> 
> Uggh.  This causes extra overhead for every remove.

Not that much overhead.  A child directory can only be unlinked if it's
empty; empty directories by definition contain only a dotdot entry,
which meanns they're in shortform format; and we already have to log the
inode to reflect the i_nlinks change.

> Can't we make
> the verifieds deal with this situation instead of creating extra
> overhead?

I'll address that in the other thread suggesting I "just fix the
verifiers".

> If we can't please at least limit it to file systems that do
> have parent pointers enabled.

Parent pointers haven't been merged yet; this is the '..' entry that
has been stored in every directory since the beginning.

--D