On Wed, Dec 27, 2017 at 04:19:58PM +0800, Sean Fu wrote: > generic_file_read_iter has done the count test. > So ext4_file_read_iter don't need to test the count repeatedly. Huh? You do realize that generic_file_read_iter() is not the only variant possible there, right? static ssize_t ext4_dax_read_iter(struct kiocb *iocb, struct iov_iter *to) { struct inode *inode = file_inode(iocb->ki_filp); ssize_t ret; if (!inode_trylock_shared(inode)) { if (iocb->ki_flags & IOCB_NOWAIT) return -EAGAIN; inode_lock_shared(inode); } ... and now IOCB_NOWAIT read with zero count can fail with -EAGAIN.