[PATCH v2 3/4] ext4: Pass in DIO_SKIP_DIO_COUNT flag if inode_dio_begin() called

[Waiman Long]

When performing direct I/O, the current ext4 code does
not pass in the DIO_SKIP_DIO_COUNT flag to dax_do_io() or
__blockdev_direct_IO() when inode_dio_begin() has, in fact, been
called. This causes dax_do_io()/__blockdev_direct_IO() to invoke
inode_dio_begin()/inode_dio_end() internally.  This doubling of
inode_dio_begin()/inode_dio_end() calls are wasteful.

This patch removes the extra internal inode_dio_begin()/inode_dio_end()
calls when those calls are being issued by the caller directly. For
really fast storage systems like NVDIMM, the removal of the extra
inode_dio_begin()/inode_dio_end() can give a meaningful boost to
I/O performance.

On a 4-socket Haswell-EX system (72 cores) running 4.6-rc1 kernel,
fio with 38 threads doing parallel I/O on two shared files on an
NVDIMM with DAX gave the following aggregrate bandwidth with and
without the patch:

  Test          W/O patch       With patch      % change
  ----          ---------       ----------      --------
  Read-only      8688MB/s       10173MB/s        +17.1%
  Read-write     2687MB/s        2830MB/s         +5.3%

Signed-off-by: Waiman Long <Waiman.Long@xxxxxxx>
---
 fs/ext4/indirect.c |   10 ++++++++--
 fs/ext4/inode.c    |   12 +++++++++---
 2 files changed, 17 insertions(+), 5 deletions(-)

diff --git a/fs/ext4/indirect.c b/fs/ext4/indirect.c
index 3027fa6..4304be6 100644
--- a/fs/ext4/indirect.c
+++ b/fs/ext4/indirect.c
@@ -706,14 +706,20 @@ retry:
 			inode_dio_end(inode);
 			goto locked;
 		}
+		/*
+		 * Need to pass in DIO_SKIP_DIO_COUNT to prevent
+		 * duplicated inode_dio_begin/inode_dio_end sequence.
+		 */
 		if (IS_DAX(inode))
 			ret = dax_do_io(iocb, inode, iter, offset,
-					ext4_dio_get_block, NULL, 0);
+					ext4_dio_get_block, NULL,
+					DIO_SKIP_DIO_COUNT);
 		else
 			ret = __blockdev_direct_IO(iocb, inode,
 						   inode->i_sb->s_bdev, iter,
 						   offset, ext4_dio_get_block,
-						   NULL, NULL, 0);
+						   NULL, NULL,
+						   DIO_SKIP_DIO_COUNT);
 		inode_dio_end(inode);
 	} else {
 locked:
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index dab84a2..779aa33 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -3358,9 +3358,15 @@ static ssize_t ext4_ext_direct_IO(struct kiocb *iocb, struct iov_iter *iter,
 	 * Make all waiters for direct IO properly wait also for extent
 	 * conversion. This also disallows race between truncate() and
 	 * overwrite DIO as i_dio_count needs to be incremented under i_mutex.
+	 *
+	 * Both dax_do_io() and __blockdev_direct_IO() will unnecessarily
+	 * call inode_dio_begin()/inode_dio_end() again if the
+	 * DIO_SKIP_DIO_COUNT flag is not set.
 	 */
-	if (iov_iter_rw(iter) == WRITE)
+	if (iov_iter_rw(iter) == WRITE) {
+		dio_flags = DIO_SKIP_DIO_COUNT;
 		inode_dio_begin(inode);
+	}
 
 	/* If we do a overwrite dio, i_mutex locking can be released */
 	overwrite = *((int *)iocb->private);
@@ -3393,10 +3399,10 @@ static ssize_t ext4_ext_direct_IO(struct kiocb *iocb, struct iov_iter *iter,
 		get_block_func = ext4_dio_get_block_overwrite;
 	else if (is_sync_kiocb(iocb)) {
 		get_block_func = ext4_dio_get_block_unwritten_sync;
-		dio_flags = DIO_LOCKING;
+		dio_flags |= DIO_LOCKING;
 	} else {
 		get_block_func = ext4_dio_get_block_unwritten_async;
-		dio_flags = DIO_LOCKING;
+		dio_flags |= DIO_LOCKING;
 	}
 #ifdef CONFIG_EXT4_FS_ENCRYPTION
 	BUG_ON(ext4_encrypted_inode(inode) && S_ISREG(inode->i_mode));
-- 
1.7.1