[PATCH] block_write_full_page: switch synchronous writes to use WRITE_SYNC_PLUG

[Theodore Tso <tytso@mit.edu>]

Now that we have a distinction between WRITE_SYNC and WRITE_SYNC_PLUG,
use WRITE_SYNC_PLUG in __block_write_full_page() to avoid unplugging
the block device I/O queue between each page that gets flushed out.

The upstream callers of block_write_full_page() which wait for the
writes to finish call wait_on_buffer(), wait_on_writeback_range()
(which ultimately calls sync_page(), which calls
blk_run_backing_dev(), which will unplug the device queue), and so on.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
---

We should get this applied to avoid any performance regressions
resulting from commit a64c8610.

 fs/buffer.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index 977e12a..95b5390 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -1646,7 +1646,8 @@ static int __block_write_full_page(struct inode *inode, struct page *page,
 	struct buffer_head *bh, *head;
 	const unsigned blocksize = 1 << inode->i_blkbits;
 	int nr_underway = 0;
-	int write_op = (wbc->sync_mode == WB_SYNC_ALL ? WRITE_SYNC : WRITE);
+	int write_op = (wbc->sync_mode == WB_SYNC_ALL ?
+			WRITE_SYNC_PLUG : WRITE);
 
 	BUG_ON(!PageLocked(page));
 
-- 
1.5.6.3