Presently we invalidate all of a file's pages when writing to any part of
that file with direct-IO.

After a direct IO write only invalidate the pages that the write intersected.
invalidate_inode_pages2_range(mapping, pgoff start, pgoff end) is added and
called from generic_file_direct_IO().

While we're in there, invalidate_inode_pages2() was calling
unmap_mapping_range() with the wrong convention in the single page case.
It was providing the byte offset of the final page rather than the length
of the hole being unmapped.  This is also fixed.

This was lightly tested with a 10k op fsx run with O_DIRECT on a 16MB file
in ext3 on a junky old IDE drive.  Totaling vmstat columns of blocks read
and written during the runs shows that read traffic drops significantly.
The run time seems to have gone down a little.

Two runs before the patch gave the following user/real/sys times and total
blocks in and out:

0m28.029s 0m20.093s 0m3.166s 16673 125107 
0m27.949s 0m20.068s 0m3.227s 18426 126094

and after the patch:

0m26.775s 0m19.996s 0m3.060s 3505 124982
0m26.856s 0m19.935s 0m3.052s 3505 125279

akpm:

- Don't look up more pages than we're going to use

- Don't test page->index until we've locked the page

- Check for the cursor wrapping at the end of the mapping.
Signed-off-by: Zach Brown <zach.brown@oracle.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>