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Date: Wed, 10 Jun 2015 00:46:11 -0700
From: Christoph Hellwig <hch@infradead.org>
To: Ming Lei <ming.lei@canonical.com>
Cc: linux-kernel@vger.kernel.org, Dave Kleikamp <dave.kleikamp@oracle.com>,
        Jens Axboe <axboe@kernel.dk>, Zach Brown <zab@zabbo.net>,
        Christoph Hellwig <hch@infradead.org>,
        Maxim Patlasov <mpatlasov@parallels.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>, Tejun Heo <tj@kernel.org>,
        Dave Chinner <david@fromorbit.com>
Subject: Re: [PATCH v5 5/5] block: loop: support DIO & AIO
Message-ID: <20150610074611.GD6964@infradead.org>
References: <1433857766-6153-1-git-send-email-ming.lei@canonical.com>
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> +	int ret;
> +
> +	/* nomerge for loop request queue */
> +	WARN_ON(cmd->rq->bio != cmd->rq->biotail);
> +
> +	bvec = __bvec_iter_bvec(bio->bi_io_vec, bio->bi_iter);
> +	iov_iter_bvec(&iter, ITER_BVEC | rw, bvec,
> +		      bio_segments(bio), blk_rq_bytes(cmd->rq));
> +
> +	cmd->iocb.ki_pos = pos;
> +	cmd->iocb.ki_filp = file;
> +	cmd->iocb.ki_complete = lo_rw_aio_complete;
> +	cmd->iocb.ki_flags = IOCB_DIRECT;
> +
> +	if (rw == WRITE)
> +		ret = file->f_op->write_iter(&cmd->iocb, &iter);
> +	else
> +		ret = file->f_op->read_iter(&cmd->iocb, &iter);

I think we really need a vfs_ wrapper here similar to what I did a while
ago, e.g. vfs_iter_read/write_async.

> +static inline int lo_rw_simple(struct loop_device *lo,
> +		struct request *rq, loff_t pos, bool rw)
> +{
> +	struct loop_cmd *cmd = blk_mq_rq_to_pdu(rq);
> +
> +	if (cmd->use_aio)
> +		return lo_rw_aio(lo, cmd, pos, rw);
> +
> +	if (rw == WRITE)
> +		return lo_write_simple(lo, rq, pos);
> +	else
> +		return lo_read_simple(lo, rq, pos);
> +}

And the io_submit style read/write also works for buffered I/O, so no
need to keep lo_write_simple/lo_read_simple around.

> @@ -1569,7 +1634,8 @@ static void loop_handle_cmd(struct loop_cmd *cmd)
>   failed:
>  	if (ret)
>  		cmd->rq->errors = -EIO;
> -	blk_mq_complete_request(cmd->rq);
> +	if (!cmd->use_aio || ret)
> +		blk_mq_complete_request(cmd->rq);

If you don't complete the request here setting req->error doesn't
make sense.  I'd suggest to move the blk_mq_complete_request for
everything but the trivial error case into the actual I/O handlers
to clean this up a bit, too.