io_uring: retry in case of short read on block device

In case of buffered reading from block device, when short read happens,
we should retry to read more, otherwise the IO will be completed
partially, for example, the following fio expects to read 2MB, but it
can only read 1M or less bytes:

    fio --name=onessd --filename=/dev/nvme0n1 --filesize=2M \
	--rw=randread --bs=2M --direct=0 --overwrite=0 --numjobs=1 \
	--iodepth=1 --time_based=0 --runtime=2 --ioengine=io_uring \
	--registerfiles --fixedbufs --gtod_reduce=1 --group_reporting

Fix the issue by allowing short read retry for block device, which sets
FMODE_BUF_RASYNC really.

Fixes: 9a173346 ("io_uring: fix short read retries for non-reg files")
Cc: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/20210821150751.1290434-1-ming.lei@redhat.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -3382,6 +3382,12 @@ static inline int io_iter_do_read(struct io_kiocb *req, struct iov_iter *iter)
 		return -EINVAL;
 }
 
+static bool need_read_all(struct io_kiocb *req)
+{
+	return req->flags & REQ_F_ISREG ||
+		S_ISBLK(file_inode(req->file)->i_mode);
+}
+
 static int io_read(struct io_kiocb *req, unsigned int issue_flags)
 {
 	struct iovec inline_vecs[UIO_FASTIOV], *iovec = inline_vecs;
@@ -3436,7 +3442,7 @@ static int io_read(struct io_kiocb *req, unsigned int issue_flags)
 	} else if (ret == -EIOCBQUEUED) {
 		goto out_free;
 	} else if (ret <= 0 || ret == io_size || !force_nonblock ||
-		   (req->flags & REQ_F_NOWAIT) || !(req->flags & REQ_F_ISREG)) {
+		   (req->flags & REQ_F_NOWAIT) || !need_read_all(req)) {
 		/* read all, failed, already did sync or don't want to retry */
 		goto done;
 	}