[PATCH for-next 3/4] io_uring: grow a field in struct io_uring_cmd

[Sagi Grimberg]

>>> Use the leftover space to carve 'next' field that enables linking of
>>> io_uring_cmd structs. Also introduce a list head and few helpers.
>>>
>>> This is in preparation to support nvme-mulitpath, allowing multiple
>>> uring passthrough commands to be queued.
>>
>> It's not clear to me why we need linking at that level?
> 
> I think the attempt is to allow something like blk_steal_bios that
> nvme leverages for io_uring_cmd(s).

I'll rephrase because now that I read it, I think my phrasing is
confusing.

I think the attempt is to allow something like blk_steal_bios that
nvme leverages, but for io_uring_cmd(s). Essentially allow io_uring_cmd
to be linked in a requeue_list.

> 
> nvme failover steals all the bios from requests that fail (and should
> failover) and puts them on a requeue list, and then schedules
> a work that takes these bios one-by-one and submits them on a different
> bottom namespace (see nvme_failover_req/nvme_requeue_work).

Maybe if io_kiocb could exposed to nvme, and it had some generic space
that nvme could use, that would work as well...