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    Subject[PATCH 3.17 010/146] Btrfs: fix build_backref_tree issue with multiple shared blocks
    Date
    3.17-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Josef Bacik <jbacik@fb.com>

    commit bbe9051441effce51c9a533d2c56440df64db2d7 upstream.

    Marc Merlin sent me a broken fs image months ago where it would blow up in the
    upper->checked BUG_ON() in build_backref_tree. This is because we had a
    scenario like this

    block a -- level 4 (not shared)
    |
    block b -- level 3 (reloc block, shared)
    |
    block c -- level 2 (not shared)
    |
    block d -- level 1 (shared)
    |
    block e -- level 0 (shared)

    We go to build a backref tree for block e, we notice block d is shared and add
    it to the list of blocks to lookup it's backrefs for. Now when we loop around
    we will check edges for the block, so we will see we looked up block c last
    time. So we lookup block d and then see that the block that points to it is
    block c and we can just skip that edge since we've already been up this path.
    The problem is because we clear need_check when we see block d (as it is shared)
    we never add block b as needing to be checked. And because block c is in our
    path already we bail out before we walk up to block b and add it to the backref
    check list.

    To fix this we need to reset need_check if we trip over a block that doesn't
    need to be checked. This will make sure that any subsequent blocks in the path
    as we're walking up afterwards are added to the list to be processed. With this
    patch I can now mount Marc's fs image and it'll complete the balance without
    panicing. Thanks,

    Reported-by: Marc MERLIN <marc@merlins.org>
    Signed-off-by: Josef Bacik <jbacik@fb.com>
    Signed-off-by: Chris Mason <clm@fb.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    fs/btrfs/relocation.c | 5 ++++-
    1 file changed, 4 insertions(+), 1 deletion(-)

    --- a/fs/btrfs/relocation.c
    +++ b/fs/btrfs/relocation.c
    @@ -978,8 +978,11 @@ again:
    need_check = false;
    list_add_tail(&edge->list[UPPER],
    &list);
    - } else
    + } else {
    + if (upper->checked)
    + need_check = true;
    INIT_LIST_HEAD(&edge->list[UPPER]);
    + }
    } else {
    upper = rb_entry(rb_node, struct backref_node,
    rb_node);



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