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

[Christoph Hellwig <hch@infradead.org>]

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.  Can't we make
the verifieds deal with this situation instead of creating extra
overhead?  If we can't please at least limit it to file systems that do
have parent pointers enabled.