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From: Christoph Hellwig <hch@lst.de>
To: Ming Lei <ming.lei@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>, Mike Snitzer <snitzer@redhat.com>,
	linux-block@vger.kernel.org, dm-devel@redhat.com,
	Jeffle Xu <jefflexu@linux.alibaba.com>,
	Christoph Hellwig <hch@lst.de>
Subject: Re: [dm-devel] [RFC PATCH V2 3/3] dm: support bio polling
Date: Mon, 21 Jun 2021 09:36:56 +0200	[thread overview]
Message-ID: <20210621073656.GB6896@lst.de> (raw)
In-Reply-To: <20210617103549.930311-4-ming.lei@redhat.com>

On Thu, Jun 17, 2021 at 06:35:49PM +0800, Ming Lei wrote:
> +	/*
> +	 * Only support bio polling for normal IO, and the target io is
> +	 * exactly inside the dm io instance
> +	 */
> +	ci->io->submit_as_polled = !!(ci->bio->bi_opf & REQ_POLLED);

Nit: the !! is not needed.

> @@ -1608,6 +1625,22 @@ static void init_clone_info(struct clone_info *ci, struct mapped_device *md,
>  	ci->map = map;
>  	ci->io = alloc_io(md, bio);
>  	ci->sector = bio->bi_iter.bi_sector;
> +
> +	if (bio->bi_opf & REQ_POLLED) {
> +		INIT_HLIST_NODE(&ci->io->node);
> +
> +		/*
> +		 * Save .bi_end_io into dm_io, so that we can reuse .bi_end_io
> +		 * for storing dm_io list
> +		 */
> +		if (bio->bi_opf & REQ_SAVED_END_IO) {
> +			ci->io->saved_bio_end_io = NULL;

So if it already was saved the list gets cleared here?  Can you explain
this logic a little more?

> +		} else {
> +			ci->io->saved_bio_end_io = bio->bi_end_io;
> +			INIT_HLIST_HEAD((struct hlist_head *)&bio->bi_end_io);

I think you want to hide these casts in helpers that clearly document
why this is safe rather than sprinkling the casts all over the code.
I also wonder if there is any better way to structur this.

> +static int dm_poll_bio(struct bio *bio, unsigned int flags)
> +{
> +	struct dm_io *io;
> +	void *saved_bi_end_io = NULL;
> +	struct hlist_head tmp = HLIST_HEAD_INIT;
> +	struct hlist_head *head = (struct hlist_head *)&bio->bi_end_io;
> +	struct hlist_node *next;
> +
> +	/*
> +	 * This bio can be submitted from FS as POLLED so that FS may keep
> +	 * polling even though the flag is cleared by bio splitting or
> +	 * requeue, so return immediately.
> +	 */
> +	if (!(bio->bi_opf & REQ_POLLED))
> +		return 0;

I can't really parse the comment, can you explain this a little more?
But if we need this check, shouldn't it move to bio_poll()?

> +	hlist_for_each_entry(io, &tmp, node) {
> +		if (io->saved_bio_end_io && !saved_bi_end_io) {
> +			saved_bi_end_io = io->saved_bio_end_io;
> +			break;
> +		}
> +	}

So it seems like you don't use bi_cookie at all.  Why not turn
bi_cookie into a union to stash the hlist_head and use that?

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  parent reply	other threads:[~2021-06-21  7:40 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-17 10:35 [dm-devel] [RFC PATCH V2 0/3] block/dm: support bio polling Ming Lei
2021-06-17 10:35 ` [dm-devel] [RFC PATCH V2 1/3] block: add helper of blk_queue_poll Ming Lei
2021-06-21  7:20   ` Christoph Hellwig
2021-06-21  8:38     ` Ming Lei
2021-06-17 10:35 ` [dm-devel] [RFC PATCH V2 2/3] block: add ->poll_bio to block_device_operations Ming Lei
2021-06-21  7:25   ` Christoph Hellwig
2021-06-21  8:41     ` Ming Lei
2021-06-17 10:35 ` [dm-devel] [RFC PATCH V2 3/3] dm: support bio polling Ming Lei
2021-06-17 23:08   ` Ming Lei
2021-06-18  8:19   ` JeffleXu
2021-06-18 13:29     ` Ming Lei
2021-06-18 14:39     ` Ming Lei
2021-06-18 20:56       ` Mike Snitzer
2021-06-19  0:27         ` Ming Lei
2021-06-21  1:32         ` JeffleXu
2021-06-21 11:33       ` JeffleXu
2021-06-21 14:04         ` Ming Lei
2021-06-22  2:26           ` JeffleXu
2021-06-22  2:45             ` Ming Lei
2021-06-22  7:45               ` JeffleXu
2021-06-30  8:30         ` Ming Lei
2021-06-21  7:36   ` Christoph Hellwig [this message]
2021-06-21  9:09     ` Ming Lei

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