[Report] requests are submitted to hardware in reverse order from nvme/virtio-blk queue_rqs()

[Christoph Hellwig]

On Mon, Oct 07, 2024 at 03:39:42PM -0700, Bart Van Assche wrote:
> For my patch series that supports pipelining for zoned writes, I need
> the submission order to be preserved. Jens mentioned two possible
> solutions:
> - Either keep the approach that requests on plug->mq_list are in reverse
>   order and reverse the request order just before submitting requests.
> - Or change plug->mq_list into a doubly linked list.
> 
> The second approach seems the most interesting to me. I'm concerned that
> with the first approach it will be difficult to preserve the request
> order if a subset of the requests on plug->mq_list are submitted, e.g.
> because a queue full condition is encountered by
> blk_mq_dispatch_plug_list().

Note that you don't really need a full doubly linked, you just need a
tail pointer in the plug, i.g. the same scheme as struct bio_list.