linux-kernel - Re: [PATCH v5 5/5] block: loop: support DIO & AIO

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Date:	Wed, 10 Jun 2015 00:46:11 -0700
From:	Christoph Hellwig <hch@...radead.org>
To:	Ming Lei <ming.lei@...onical.com>
Cc:	linux-kernel@...r.kernel.org,
	Dave Kleikamp <dave.kleikamp@...cle.com>,
	Jens Axboe <axboe@...nel.dk>, Zach Brown <zab@...bo.net>,
	Christoph Hellwig <hch@...radead.org>,
	Maxim Patlasov <mpatlasov@...allels.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Tejun Heo <tj@...nel.org>, Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH v5 5/5] block: loop: support DIO & AIO

> +	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.

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