Re: [PATCH] xfs: di_flushiter considered harmful

Markus Trippelsdorf
2013-07-22T11:07:37Z

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;
}

--
Markus