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

[Jens Axboe <axboe@kernel.dk>]

On 7/11/22 12:22 PM, Sagi Grimberg wrote:
> 
>>>> 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.

-- 
Jens Axboe