[PATCH 1/2 V1] fs/block-dev.c:fix performance regression in O_DIRECT writes to md block devices

[majianpeng <majianpeng@gmail.com>]

For regular file, write operaion used blk_plug function.But for block
file,write operation did not use blk_plug.
This patch is also for write-cache mode for block-device.
 
Signed-off-by: Jianpeng Ma <majianpeng@gmail.com>
---
 fs/block_dev.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
 
diff --git a/fs/block_dev.c b/fs/block_dev.c
index c2bbe1f..cf10778 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -1579,9 +1579,11 @@ ssize_t blkdev_aio_write(struct kiocb *iocb, const struct iovec *iov,
 {
  struct file *file = iocb->ki_filp;
  ssize_t ret;
+ struct blk_plug plug;
 
  BUG_ON(iocb->ki_pos != pos);
 
+ blk_start_plug(&plug);
  ret = __generic_file_aio_write(iocb, iov, nr_segs, &iocb->ki_pos);
  if (ret > 0 || ret == -EIOCBQUEUED) {
  ssize_t err;
@@ -1590,6 +1592,7 @@ ssize_t blkdev_aio_write(struct kiocb *iocb, const struct iovec *iov,
  if (err < 0 && ret > 0)
  ret = err;
  }
+ blk_finish_plug(&plug);
  return ret;
 }
 EXPORT_SYMBOL_GPL(blkdev_aio_write);
-- 
1.7.5.4