On Fri 29-04-16 12:27:55, Waiman Long wrote: > 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 <hch@xxxxxxxxxxxxx> > Signed-off-by: Waiman Long <Waiman.Long@xxxxxxx> We cannot easily do this currently - the reason is that in several places we wait for i_dio_count to drop to 0 (look for inode_dio_wait()) while holding i_mutex to wait for all outstanding DIO / DAX IO. You'd break this logic with this patch. If we indeed put all writes under i_mutex, this problem would go away but as Dave explains in his email, we consciously do as much IO as we can without i_mutex to allow reasonable scalability of multiple writers into the same file. The downside of that is that overwrites and writes vs reads are not atomic wrt each other as POSIX requires. It has been that way for direct IO in XFS case for a long time, with DAX this non-conforming behavior is proliferating more. I agree that's not ideal but serializing all writes on a file is rather harsh for persistent memory as well... Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
