Re: [PATCH] xfs: di_flushiter considered harmful

[Dave Chinner <david@fromorbit.com>]

On Tue, Jul 23, 2013 at 03:28:27AM +0200, Markus Trippelsdorf wrote:
> On 2013.07.23 at 08:56 +1000, Dave Chinner wrote:
> > On Mon, Jul 22, 2013 at 01:07:32PM +0200, Markus Trippelsdorf wrote:
> > > On 2013.07.22 at 20:18 +1000, Dave Chinner wrote:
> > > > From: Dave Chinner <dchinner@redhat.com>
> > > > 
> > > > When we made all inode updates transactional, we no longer needed
> > > > the log recovery detection for inodes being newer on disk than the
> > > > transaction being replayed - it was redundant as replay of the log
> > > > would always result in the latest version of the inode woul dbe on
> > > > disk. It was redundant, but left in place because it wasn't
> > > > considered to be a problem.
> > > > 
> > > > However, with the new "don't read inodes on create" optimisation,
> > > > flushiter has come back to bite us. Essentially, the optimisation
> > > > made always initialises flushiter to zero in the create transaction,
> > > > and so if we then crash and run recovery and the inode already on
> > > > disk has a non-zero flushiter it will skip recovery of that inode.
> > > > As a result, log recovery does the wrong thing and we end up with a
> > > > corrupt filesystem.
> > > > 
> > > > Because we have to support old kernel to new kernl upgrades, we
> > > > can't just get rid of the flushiter support in log recovery as we
> > > > might be upgrading from a kernel that doesn't have fully transaction
> > > > inode updates.  Unfortunately, for v4 superblocks there is no way to
> > > > guarantee that log recovery knows about this fact.
> > > > 
> > > > We cannot add a new inode format flag to say it's a "special inode
> > > > create" because it won't be understood by older kernels and so
> > > > recovery could do the wrong thing on downgrade. We cannot specially
> > > > detect the combination of zero mode/non-zero flushiter on disk to
> > > > non-zero mode, zero flushiter in the log item during recovery
> > > > because wrapping of the flushiter can result in false detection.
> > > > 
> > > > Hence that makes this "don't use flushiter" optimisation limited to
> > > > a disk format that guarantees that we don't need it. And that means
> > > > the only fix here is to limit the "no read IO on create"
> > > > optimisation to version 5 superblocks....
> > > 
> > > I think your patch misses the following part:
> > > 
> > > @@ -1054,17 +1056,15 @@ xfs_iread(
> > >  
> > >         /* shortcut IO on inode allocation if possible */
> > >         if ((iget_flags & XFS_IGET_CREATE) &&
> > > -           !(mp->m_flags & XFS_MOUNT_IKEEP)) {
> > > +           !(mp->m_flags & XFS_MOUNT_IKEEP) &&
> > > +           xfs_sb_version_hascrc(&mp->m_sb)) {
> > >                 /* initialise the on-disk inode core */
> > >                 memset(&ip->i_d, 0, sizeof(ip->i_d));
> > >                 ip->i_d.di_magic = XFS_DINODE_MAGIC;
> > >                 ip->i_d.di_gen = prandom_u32();
> > > -               if (xfs_sb_version_hascrc(&mp->m_sb)) {
> > > -                       ip->i_d.di_version = 3;
> > > -                       ip->i_d.di_ino = ip->i_ino;
> > > -                       uuid_copy(&ip->i_d.di_uuid, &mp->m_sb.sb_uuid);
> > > -               } else
> > > -                       ip->i_d.di_version = 2;
> > > +               ip->i_d.di_version = 3;
> > > +               ip->i_d.di_ino = ip->i_ino;
> > > +               uuid_copy(&ip->i_d.di_uuid, &mp->m_sb.sb_uuid);
> > >                 return 0;
> > >         }
> > 
> > Sure, it's dead code so doesn't affect the behaviour of the patch.
> > I'll update it, but I need you to reproduce the problem in a simple
> > manner as Mark did with this patch in place so I can find out what
> > the real problem you are seeing is....
> 
> No. It's not dead code. Please look at the patch that you've posted.

I was looking at the code in my tree. It appears that what I sent
out is an incomplete version - the patch in my tree up to date
and has a xfs_sb_version_hascrc() check around this entire set of
code. I guess I missed a 'guilt refresh'...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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