this is a second version of the patches to fix data corruption in mmapped
data when blocksize < pagesize as tested by xfstests generic/030 test.
The patchset fixes XFS and ext4. I've checked and btrfs doesn't need fixing
because it doesn't support blocksize < pagesize. If that's ever going
to change btrfs will likely need a similar treatment. ocfs2, ext2, ext3 are
OK since they happily allocate blocks during writeback. For other filesystems
like gfs2, ubifs, nilfs, ceph,... I'm not sure whether they support blocksize <
pagesize at all. Interesting is also NFS which may care but I don't understand
its ->page_mkwrite() handler good enough to judge.
Changes since v1:
- changed helper function name and moved it to mm/truncate.c - I originally
thought we can make the helper function update i_size to simplify the
interface but it's actually impossible due to generic_write_end() lock
- used round_up() instead of ALIGN()
- taught truncate_setsize() to use the helper function