With the change from i_mutex to i_rwsem in 4.7 kernel, the locking
scheme in dax_do_io() can now be changed to take a shared lock for
read so that multiple readers can access the same file concurrently.
With a 38-threads fio I/O test with 2 shared files (on DAX-mount, ext4
formatted NVDIMM) running on a 4-socket Haswell-EX server with 4.7-rc1
kernel, the aggregated bandwidths before and after the patch were:
Test W/O patch With patch % change
---- --------- ---------- --------
Read-only 4711MB/s 16031MB/s +240%
Read-write 1932MB/s 1040MB/s -46%
There was a big increase in parallel read performance. However,
parallel read-write test showed a regression because a mix of readers
and writers will largely disable optimistic spinning.
Signed-off-by: Waiman Long <waiman.l...@hpe.com>
---
fs/dax.c | 9 +++++----
1 files changed, 5 insertions(+), 4 deletions(-)
diff --git a/fs/dax.c b/fs/dax.c
index 761495b..ff57d88 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -247,8 +247,8 @@ static ssize_t dax_io(struct inode *inode, struct iov_iter
*iter,
* @flags: See below
*
* This function uses the same locking scheme as do_blockdev_direct_IO:
- * If @flags has DIO_LOCKING set, we assume that the i_mutex is held by the
- * caller for writes. For reads, we take and release the i_mutex ourselves.
+ * If @flags has DIO_LOCKING set, we assume that the i_rwsem is held by the
+ * caller for writes. For reads, we take and release the i_rwsem ourselves.
* If DIO_LOCKING is not set, the filesystem takes care of its own locking.
* As with do_blockdev_direct_IO(), we increment i_dio_count while the I/O
* is in progress.
@@ -265,8 +265,9 @@ ssize_t dax_do_io(struct kiocb *iocb, struct inode *inode,
memset(&bh, 0, sizeof(bh));
bh.b_bdev = inode->i_sb->s_bdev;
+ /* Take the shared lock for read */
if ((flags & DIO_LOCKING) && iov_iter_rw(iter) == READ)
- inode_lock(inode);
+ inode_lock_shared(inode);
/* Protects against truncate */
if (!(flags & DIO_SKIP_DIO_COUNT))
@@ -275,7 +276,7 @@ ssize_t dax_do_io(struct kiocb *iocb, struct inode *inode,
retval = dax_io(inode, iter, pos, end, get_block, &bh);
if ((flags & DIO_LOCKING) && iov_iter_rw(iter) == READ)
- inode_unlock(inode);
+ inode_unlock_shared(inode);
if (end_io) {
int err;
--
1.7.1
