The purpose of the i_dio_count is to protect against truncation while the I/O operation is in progress. As dax_do_io() only does synchronous I/O, the locking performed by the caller or within dax_do_io() for read should be enough to protect it against truncation. There is no need to touch the i_dio_count.
Eliminating two atomic operations can sometimes give a noticeable improvement in I/O performance as NVDIMM is much faster than other disk devices. Suggested-by: Christoph Hellwig <[email protected]> Signed-off-by: Waiman Long <[email protected]> --- fs/dax.c | 14 ++++++-------- 1 files changed, 6 insertions(+), 8 deletions(-) diff --git a/fs/dax.c b/fs/dax.c index 90322eb..1b4b500 100644 --- a/fs/dax.c +++ b/fs/dax.c @@ -253,8 +253,12 @@ static ssize_t dax_io(struct inode *inode, struct iov_iter *iter, * 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 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. + * + * The do_blockdev_direct_IO() function increment i_dio_count while the I/O + * is in progress. However, the dax_do_io() always does synchronous I/O. The + * locking done by the caller or within dax_do_io() for read (DIO_LOCKING) + * should be enough to protect against concurrent truncation. We don't really + * need to touch i_dio_count here. */ ssize_t dax_do_io(struct kiocb *iocb, struct inode *inode, struct iov_iter *iter, loff_t pos, get_block_t get_block, @@ -277,10 +281,6 @@ ssize_t dax_do_io(struct kiocb *iocb, struct inode *inode, } } - /* Protects against truncate */ - if (!(flags & DIO_SKIP_DIO_COUNT)) - inode_dio_begin(inode); - retval = dax_io(inode, iter, pos, end, get_block, &bh); if ((flags & DIO_LOCKING) && iov_iter_rw(iter) == READ) @@ -294,8 +294,6 @@ ssize_t dax_do_io(struct kiocb *iocb, struct inode *inode, retval = err; } - if (!(flags & DIO_SKIP_DIO_COUNT)) - inode_dio_end(inode); out: return retval; } -- 1.7.1
