If we have an inode with a large number of hard links, some of which may
be extrefs, turn a regular ref into an extref, fsync the inode and then
replay the fsync log (after a crash/reboot), we can endup with an fsync
log that makes the replay code always fail with -EOVERFLOW when processing
the inode's references.

This is easy to reproduce with the test case I made for xfstests. Its steps
are the following:

   _scratch_mkfs "-O extref" >> $seqres.full 2>&1
   _init_flakey
   _mount_flakey

   # Create a test file with 3001 hard links. This number is large enough to
   # make btrfs start using extrefs at some point even if the fs has the maximum
   # possible leaf/node size (64Kb).
   echo "hello world" > $SCRATCH_MNT/foo
   for i in `seq 1 3000`; do
       ln $SCRATCH_MNT/foo $SCRATCH_MNT/foo_link_`printf "%04d" $i`
   done

   # Make sure all metadata and data are durably persisted.
   sync

   # Now remove one link, add a new one with a new name, add another new one with
   # the same name as the one we just removed and fsync the inode.
   rm -f $SCRATCH_MNT/foo_link_0001
   ln $SCRATCH_MNT/foo $SCRATCH_MNT/foo_link_3001
   ln $SCRATCH_MNT/foo $SCRATCH_MNT/foo_link_0001
   rm -f $SCRATCH_MNT/foo_link_0002
   ln $SCRATCH_MNT/foo $SCRATCH_MNT/foo_link_3002
   ln $SCRATCH_MNT/foo $SCRATCH_MNT/foo_link_3003
   $XFS_IO_PROG -c "fsync" $SCRATCH_MNT/foo

   # Simulate a crash/power loss. This makes sure the next mount
   # will see an fsync log and will replay that log.

   _load_flakey_table $FLAKEY_DROP_WRITES
   _unmount_flakey

   _load_flakey_table $FLAKEY_ALLOW_WRITES
   _mount_flakey

   # Check that the number of hard links is correct, we are able to remove all
   # the hard links and read the file's data. This is just to verify we don't
   # get stale file handle errors (due to dangling directory index entries that
   # point to inodes that no longer exist).
   echo "Link count: $(stat --format=%h $SCRATCH_MNT/foo)"
   [ -f $SCRATCH_MNT/foo ] || echo "Link foo is missing"
   for ((i = 1; i <= 3003; i++)); do
       name=foo_link_`printf "%04d" $i`
       if [ $i -eq 2 ]; then
           [ -f $SCRATCH_MNT/$name ] && echo "Link $name found"
       else
           [ -f $SCRATCH_MNT/$name ] || echo "Link $name is missing"
       fi
   done
   rm -f $SCRATCH_MNT/foo_link_*
   cat $SCRATCH_MNT/foo
   rm -f $SCRATCH_MNT/foo

   status=0
   exit

The fix is simply to correct the overflow condition when overwriting a
reference item because it was wrong, trying to increase the item in the
fs/subvol tree by an impossible amount. Also ensure that we don't insert
one normal ref and one ext ref for the same dentry - this happened because
processing a dir index entry from the parent in the log happened when
the normal ref item was full, which made the logic insert an extref and
later when the normal ref had enough room, it would be inserted again
when processing the ref item from the child inode in the log.

This issue has been present since the introduction of the extrefs feature
(2012).

A test case for xfstests follows soon. This test only passes if the previous
patch titled "Btrfs: fix fsync when extend references are added to an inode"
is applied too.
Signed-off-by: NFilipe Manana <fdmanana@suse.com>
Signed-off-by: NChris Mason <clm@fb.com>