• Josef Bacik's avatar
    Btrfs: allow delayed refs to be merged · ae1e206b
    Josef Bacik authored
    Daniel Blueman reported a bug with fio+balance on a ramdisk setup.
    Basically what happens is the balance relocates a tree block which will drop
    the implicit refs for all of its children and adds a full backref.  Once the
    block is relocated we have to add the implicit refs back, so when we cow the
    block again we add the implicit refs for its children back.  The problem
    comes when the original drop ref doesn't get run before we add the implicit
    refs back.  The delayed ref stuff will specifically prefer ADD operations
    over DROP to keep us from freeing up an extent that will have references to
    it, so we try to add the implicit ref before it is actually removed and we
    panic.  This worked fine before because the add would have just canceled the
    drop out and we would have been fine.  But the backref walking work needs to
    be able to freeze the delayed ref stuff in time so we have this ever
    increasing sequence number that gets attached to all new delayed ref updates
    which makes us not merge refs and we run into this issue.
    
    So to fix this we need to merge delayed refs.  So everytime we run a
    clustered ref we need to try and merge all of its delayed refs.  The backref
    walking stuff locks the delayed ref head before processing, so if we have it
    locked we are safe to merge any refs inside of the sequence number.  If
    there is no sequence number we can merge all refs.  Doing this not only
    fixes our bug but keeps the delayed ref code from adding and removing
    useless refs and batching together multiple refs into one search instead of
    one search per delayed ref, which will really help our commit times.  I ran
    this with Daniels test and 276 and I haven't seen any problems.  Thanks,
    Reported-by: default avatarDaniel J Blueman <daniel@quora.org>
    Signed-off-by: default avatarJosef Bacik <jbacik@fusionio.com>
    ae1e206b
delayed-ref.c 22.4 KB