When using bounce buffers for SG_IO commands with unaligned buffers in
  blk_rq_map_user(), we should free the pages from blk_rq_unmap_user() which
  calls bio_uncopy_user() for the non-BIO_USER_MAPPED case.  That function
  failed to free the pages for write requests.

  So we leaked pages and you machine would go OOM.  Rebooting helped ;-)

  This bug was triggered by writing audio CDs (but not on data CDs), as the
  audio frames are not aligned well (2352 bytes), so the user pages don't just
  get mapped.

  Bug was reported by Mathias Homan and debugged by Chris Mason + me.  (Jens
  is away.)

From: Chris Mason <mason@suse.com>

  Fix the leak for real
Signed-off-by: Kurt Garloff <garloff@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>