Re: [PATCH 05/13] xfs: add unlink list pointers to xfs_inode

[Dave Chinner <david@fromorbit.com>]

On Tue, Aug 18, 2020 at 05:02:51PM -0700, Darrick J. Wong wrote:
> On Wed, Aug 12, 2020 at 07:25:48PM +1000, Dave Chinner wrote:
> > From: Dave Chinner <dchinner@redhat.com>
> > 
> > To move away from using the on disk inode buffers to track and log
> > unlinked inodes, we need pointers to track them in memory. Because
> > we have arbitrary remove order from the list, it needs to be a
> > double linked list.
> > 
> > We start by noting that inodes are always in memory when they are
> > active on the unlinked list, and hence we can track these inodes
> > without needing to take references to the inodes or store them in
> > the list. We cannot, however, use inode locks to protect the inodes
> > on the list - the list needs an external lock to serialise all
> > inserts and removals. We can use the existing AGI buffer lock for
> > this right now as that already serialises all unlinked list
> > traversals and modifications.
> > 
> > Hence we can convert the in-memory unlinked list to a simple
> > list_head list in the perag. We can use list_empty() to detect an
> > empty unlinked list, likewise we can detect the end of the list when
> > the inode next pointer points back to the perag list_head. This
> > makes insert, remove and traversal.
> > 
> > The only complication here is log recovery of old filesystems that
> > have multiple lists. These always remove from the head of the list,
> > so we can easily construct just enough of the unlinked list for
> > recovery from any list to work correctly.
> > 
> > Signed-off-by: Dave Chinner <dchinner@redhat.com>
> 
> Hm.  This is orthogonal to this patch, but should we get meaner about
> failing the mount if the AGI read fails or the unlinked walk fails?

I don't think it matters. We can leak the unlinked lists and still
have the filesystem operate correctly, so I don't think failing the
mount is necessary or desirable (i.e. how many existing filesystems
will now suddenly refuse to mount?)

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com