When performing direct I/O on DAX, the current ext4 code does not pass
in the DIO_SKIP_DIO_COUNT flag to dax_do_io() when inode_dio_begin()
has, in fact, been called. This causes dax_do_io() to invoke the
inode_dio_begin()/inode_dio_end() pair internally. This doubling of
inode_dio_begin()/inode_dio_end() calls are wasteful.
For __blockdev_direct_IO(), however, the inode_dio_end() call can be
deferred for AIO. So setting DIO_SKIP_DIO_COUNT may not be appropriate.
This patch removes the extra internal inode_dio_begin()/inode_dio_end()
calls for DAX 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 | 9 ++++++++-
fs/ext4/inode.c | 11 ++++++++---
2 files changed, 16 insertions(+), 4 deletions(-)
diff --git a/fs/ext4/indirect.c b/fs/ext4/indirect.c
index 3027fa6..1dfc280 100644
--- a/fs/ext4/indirect.c
+++ b/fs/ext4/indirect.c
@@ -706,9 +706,16 @@ retry:
inode_dio_end(inode);
goto locked;
}
+ /*
+ * Need to pass in DIO_SKIP_DIO_COUNT to dax_do_io() to prevent
+ * duplicated inode_dio_begin/inode_dio_end pair. The flag
+ * isn't used in __blockdev_direct_IO() as the inode_dio_end()
+ * call can be deferred for AIO.
+ */
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,
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index dab84a2..b18ee2f 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -3358,9 +3358,14 @@ 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.
+ *
+ * The dax_do_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 = IS_DAX(inode) ? DIO_SKIP_DIO_COUNT : 0;
inode_dio_begin(inode);
+ }
/* If we do a overwrite dio, i_mutex locking can be released */
overwrite = *((int *)iocb->private);
@@ -3393,10 +3398,10 @@ 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
--
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