direct-io: fix boundary block handling

When we read/write a file sequentially, we will read/write not only the
data blocks but also the indirect blocks that may not be physically
adjacent to the data blocks.  So filesystems set the BH_Boundary flag to
submit the previous I/O before reading/writing an indirect block.

However the generic direct IO code mishandles buffer_boundary(), setting
sdio->boundary before each submit_page_section() call which results in
sending only one page bios as underlying code thinks this page is the last
in the contiguous extent.  So fix the problem by setting sdio->boundary
only if the current page is really the last one in the mapped extent.

With this patch and "direct-io: submit bio after boundary buffer is added
to it" I've measured about 10% throughput improvement of direct IO reads
on ext3 with SATA harddrive (from 90 MB/s to 100 MB/s).  With ramdisk, the
improvement was about 3-fold (from 350 MB/s to 1.2 GB/s).  For other
filesystems (such as ext4), the improvements won't be as visible because
the frequency of BH_Boundary flag being set is much smaller.

Signed-off-by: Jan Kara <jack@suse.cz>
Reported-by: Kazuya Mio <k-mio@sx.jp.nec.com>
Tested-by: Kazuya Mio <k-mio@sx.jp.nec.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/direct-io.c

@@ -969,7 +969,8 @@ static int do_direct_IO(struct dio *dio, struct dio_submit *sdio,
 			this_chunk_bytes = this_chunk_blocks << blkbits;
 			BUG_ON(this_chunk_bytes == 0);
 
-			sdio->boundary = buffer_boundary(map_bh);
+			if (this_chunk_blocks == sdio->blocks_available)
+				sdio->boundary = buffer_boundary(map_bh);
 			ret = submit_page_section(dio, sdio, page,
 						  offset_in_page,
 						  this_chunk_bytes,