I tried to get multipathing failover running and failed. It seems to be
that there are two bugs in multipath.c :

- If a path/request fails mp_bh->path doesn't get updated to the new
  path.  Thus multipath_end_request will access a NULL pointer if a
  redirected request returns.

- After fixing this I experienced several other errors.  First of all in
  bio->bi_flags the BIO_UPTODATE flag is not set when redirecting a
  request.

  Causing the md driver to think that every redirected request failed
  when it returns.  Since that alone didn't fix the problems, I simply
  copied the master_bio again and it looks like everything seems to work.

Cc: <neilb@cse.unsw.edu.au>
Cc: Jens Axboe <axboe@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>