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

[Sagi Grimberg <sagi@grimberg.me>]

>>>>> 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.
> 
> I see. I wonder if there's some other way we can accomplish that, so we
> don't have to shrink the current space. io_kiocb already support
> linking, so seems like that should be workable.
> 
>>> 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...
> 
> It will be more exposed in 5.20, but passthrough is already using the
> per-op allotted space in the io_kiocb. But as mentioned above, there's
> already linking support between io_kiocbs, and that is likely what
> should be used here too.

Agree. I don't think there is an issue with passing uring_cmd() an
io_kiocb instead of a io_uring_cmd which is less space strict.