Re: [Devel] [PATCH hci-8.1 3/7] drivers/md/dm-qcow2: prepare process_one_qio() for fastpath

[Alexander Ivanov <alexander.ivanov@virtuozzo.com>]

On 10/29/26 15:03, Andrey Zhadchenko wrote:
> dm-qcow2 heavily relies on main kwork do_qcow2_work() being
> sequential. All metadata change happens there and cannot
> run in parallel. Unfortunately that caps dm-qcow2 performance.
> But if the request does not require metadata changes we should
> be able to immediately dispatch it.
> process_one_qio() and handle_metadata() already know what to
> do: just add avoid_metadata_changes() flag and return codes.

Looks lake a typo, avoid_metadata_changes is not a function.

> While at it, just return in handle_metadata() instead of
> setting ret to 0 and jumping to if (ret < 0) label.
>
> Also exclude compressed clusters as there are some complications
> with wpc_readers.
>
> https://virtuozzo.atlassian.net/browse/VSTOR-95494
> Feature: dm-qcow2: block device over QCOW2 files driver
> Signed-off-by: Andrey Zhadchenko <andrey.zhadchenko@virtuozzo.com>
> ---
>   drivers/md/dm-qcow2-map.c | 35 +++++++++++++++++++++++++----------
>   1 file changed, 25 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/md/dm-qcow2-map.c b/drivers/md/dm-qcow2-map.c
> index 1023224484f5b..c5d68b6b21f12 100644
> --- a/drivers/md/dm-qcow2-map.c
> +++ b/drivers/md/dm-qcow2-map.c
> @@ -3534,7 +3534,7 @@ static int prepare_cluster_discard(struct qcow2 *qcow2, struct qio **qio,
>   }
>   
>   static int handle_metadata(struct qcow2 *qcow2, struct qio **qio,
> -			   struct qcow2_map *map)
> +			   struct qcow2_map *map, bool avoid_metadata_changes)
>   {
>   	bool write = op_is_write((*qio)->bi_op);
>   	int ret;
> @@ -3543,15 +3543,20 @@ static int handle_metadata(struct qcow2 *qcow2, struct qio **qio,
>   	if (ret < 0 || !*qio) /* Error or postponed */
>   		goto check_err;
>   
> +	if (avoid_metadata_changes && map->compressed)
> +		return 2;
Looks like a magic constant. Maybe enum/define?
> +
>   	ret = 1;
>   	if (unlikely(qcow2->backward_merge_in_process)) {
> +		if (avoid_metadata_changes)
> +			return 2;
>   		/* Keep in mind the below may replace *qio */
>   		ret = prepare_backward_merge(qcow2, qio, map, write);
>   	} else if (unlikely(fake_l1cow_qio(*qio)) &&
>   		(!map->clu_is_cow || (map->level & L2_LEVEL))) {
>   		/* Nothing to COW or L1 is mapped exactly once */
>   		qio_endio(*qio);
> -		ret = 0;
> +		return 0;
>   	} else if (unlikely(op_is_discard((*qio)->bi_op))) {
>   		/*
>   		 * Discard carries no data and is advisory. If it does not
> @@ -3559,13 +3564,17 @@ static int handle_metadata(struct qcow2 *qcow2, struct qio **qio,
>   		 * is not present, end early.
>   		 */
>   		if (qio_discard_updates_metadata(qcow2, *qio, map)) {
> +			if (avoid_metadata_changes)
> +				return 2;
>   			ret = prepare_cluster_discard(qcow2, qio, map);
>   		} else if (!map->data_clu_alloced || map->clu_is_cow) {
>   			qio_endio(*qio);
> -			ret = 0;
> +			return 0;
>   		}
>   	} else if (write &&
>   		   (!qio_is_fully_alloced(qcow2, *qio, map) || map->clu_is_cow)) {
> +		if (avoid_metadata_changes)
> +			return 2;
>   		if (map->clu_is_cow) {
>   			/* COW to compressed or shared with snapshot cluster */
>   			ret = prepare_l1l2_replace(qcow2, *qio, map);
> @@ -3677,13 +3686,17 @@ static void process_read_qio(struct qcow2 *qcow2, struct qio *qio,
>   	qio_endio(qio);
>   }
>   
> -static void process_one_qio(struct qcow2 *qcow2, struct qio *qio)
> +static int process_one_qio(struct qcow2 *qcow2, struct qio *qio, bool avoid_metadata_changes)
>   {
>   	struct qcow2_map map = { .qcow2 = qcow2, };
>   	bool write;
> +	int ret;
>   
> -	if (!handle_metadata(qcow2, &qio, &map))
> -		return;
> +	ret = handle_metadata(qcow2, &qio, &map, avoid_metadata_changes);
> +	if (!ret)
> +		return 0;
> +	else if (ret == 2)
> +		return 1;
>   
>   	/*
>   	 * Merge machinery makes out of order merges for present
> @@ -3695,7 +3708,7 @@ static void process_one_qio(struct qcow2 *qcow2, struct qio *qio)
>   	if (unlikely(qcow2->backward_merge_in_process) &&
>   	    (fake_merge_qio(qio) || qcow2_file_is_writable(qcow2))) {
>   		submit_top_delta_read(&map, qio);
> -		return;
> +		return 0;
>   	}
>   
>   	write = op_is_write(qio->bi_op);
> @@ -3703,13 +3716,13 @@ static void process_one_qio(struct qcow2 *qcow2, struct qio *qio)
>   	/* Process discard early. Don't do COW just to discard */
>   	if (unlikely(op_is_discard(qio->bi_op))) {
>   		issue_discard(&map, qio);
> -		return;
> +		return 0;
>   	}
>   
>   	if (unlikely(map.compressed)) {
>   		/* Compressed qio never uses sub-clus */
>   		submit_read_compressed(&map, qio, write);
> -		return;
> +		return 0;
>   	}
>   
>   	if (!write) {
> @@ -3722,6 +3735,8 @@ static void process_one_qio(struct qcow2 *qcow2, struct qio *qio)
>   		else
>   			perform_rw_mapped(&map, qio);
>   	}
> +
> +	return 0;
>   }
>   
>   static struct bio_vec *create_bvec_from_rq(struct request *rq)
> @@ -3824,7 +3839,7 @@ static void process_deferred_qios(struct qcow2 *qcow2, struct list_head *qios)
>   			continue;
>   		}
>   
> -		process_one_qio(qcow2, qio);
> +		process_one_qio(qcow2, qio, false);
>   	}
>   }
>   
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