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

Waiman Long
2016-04-08T16:16:49Z

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@hpe.com>
---
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