msync improvements

These two patches are improvements on the implementation of the msync
system call.  To give some context, I found them while working on
the implementation of a persistent dirty bitmap.

Paolo Bonzini (2):
  msync: support syncing a small part of the file
  msync: start async writeout when MS_ASYNC

 include/linux/fs.h |    3 +-
 mm/fadvise.c       |    2 +-
 mm/filemap.c       |   11 +++++---
 mm/msync.c         |   63 +++++++++++++++++++++++++++++++++-------------------
 4 files changed, 50 insertions(+), 29 deletions(-)