Re: [PATCH v20 02/12] Add infrastructure for copy offload in block and request layer.

[Bart Van Assche <bvanassche@acm.org>]

On 5/30/24 00:16, Nitesh Shetty wrote:
> +static inline bool blk_copy_offload_attempt_combine(struct request_queue *q,
> +                         struct bio *bio)
> +{
> +    struct blk_plug *plug = current->plug;
> +    struct request *rq;
> +
> +    if (!plug || rq_list_empty(plug->mq_list))
> +        return false;
> +
> +    rq_list_for_each(&plug->mq_list, rq) {
> +        if (rq->q == q) {
> +            if (!blk_copy_offload_combine(rq, bio))
> +                return true;
> +            break;
> +        }
> +
> +        /*
> +         * Only keep iterating plug list for combines if we have multiple
> +         * queues
> +         */
> +        if (!plug->multiple_queues)
> +            break;
> +    }
> +    return false;
> +}

This new approach has the following two disadvantages:
* Without plug, REQ_OP_COPY_SRC and REQ_OP_COPY_DST are not combined. These two
   operation types are the only operation types for which not using a plug causes
   an I/O failure.
* A loop is required to combine the REQ_OP_COPY_SRC and REQ_OP_COPY_DST operations.

Please switch to the approach Hannes suggested, namely bio chaining. Chaining
REQ_OP_COPY_SRC and REQ_OP_COPY_DST bios before these are submitted eliminates the
two disadvantages mentioned above.

Thanks,

Bart.