Re: [PATCH v4 6/6] io_uring: add support for zone-append

[Pavel Begunkov <asml.silence@gmail.com>]

On 27/07/2020 23:34, Jens Axboe wrote:
> On 7/27/20 1:16 PM, Kanchan Joshi wrote:
>> On Fri, Jul 24, 2020 at 10:00 PM Jens Axboe <axboe@kernel.dk> wrote:
>>>
>>> On 7/24/20 9:49 AM, Kanchan Joshi wrote:
>>>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>>>> index 7809ab2..6510cf5 100644
>>>> --- a/fs/io_uring.c
>>>> +++ b/fs/io_uring.c
>>>> @@ -1284,8 +1301,15 @@ static void __io_cqring_fill_event(struct io_kiocb *req, long res, long cflags)
>>>>       cqe = io_get_cqring(ctx);
>>>>       if (likely(cqe)) {
>>>>               WRITE_ONCE(cqe->user_data, req->user_data);
>>>> -             WRITE_ONCE(cqe->res, res);
>>>> -             WRITE_ONCE(cqe->flags, cflags);
>>>> +             if (unlikely(req->flags & REQ_F_ZONE_APPEND)) {
>>>> +                     if (likely(res > 0))
>>>> +                             WRITE_ONCE(cqe->res64, req->rw.append_offset);
>>>> +                     else
>>>> +                             WRITE_ONCE(cqe->res64, res);
>>>> +             } else {
>>>> +                     WRITE_ONCE(cqe->res, res);
>>>> +                     WRITE_ONCE(cqe->flags, cflags);
>>>> +             }
>>>
>>> This would be nice to keep out of the fast path, if possible.
>>
>> I was thinking of keeping a function-pointer (in io_kiocb) during
>> submission. That would have avoided this check......but argument count
>> differs, so it did not add up.
> 
> But that'd grow the io_kiocb just for this use case, which is arguably
> even worse. Unless you can keep it in the per-request private data,
> but there's no more room there for the regular read/write side.
> 
>>>> diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
>>>> index 92c2269..2580d93 100644
>>>> --- a/include/uapi/linux/io_uring.h
>>>> +++ b/include/uapi/linux/io_uring.h
>>>> @@ -156,8 +156,13 @@ enum {
>>>>   */
>>>>  struct io_uring_cqe {
>>>>       __u64   user_data;      /* sqe->data submission passed back */
>>>> -     __s32   res;            /* result code for this event */
>>>> -     __u32   flags;
>>>> +     union {
>>>> +             struct {
>>>> +                     __s32   res;    /* result code for this event */
>>>> +                     __u32   flags;
>>>> +             };
>>>> +             __s64   res64;  /* appending offset for zone append */
>>>> +     };
>>>>  };
>>>
>>> Is this a compatible change, both for now but also going forward? You
>>> could randomly have IORING_CQE_F_BUFFER set, or any other future flags.
>>
>> Sorry, I didn't quite understand the concern. CQE_F_BUFFER is not
>> used/set for write currently, so it looked compatible at this point.
> 
> Not worried about that, since we won't ever use that for writes. But it
> is a potential headache down the line for other flags, if they apply to
> normal writes.
> 
>> Yes, no room for future flags for this operation.
>> Do you see any other way to enable this support in io-uring?
> 
> Honestly I think the only viable option is as we discussed previously,
> pass in a pointer to a 64-bit type where we can copy the additional
> completion information to.

TBH, I hate the idea of such overhead/latency at times when SSDs can
serve writes in less than 10ms. Any chance you measured how long does it
take to drag through task_work?

> 
>>> Layout would also be different between big and little endian, so not
>>> even that easy to set aside a flag for this. But even if that was done,
>>> we'd still have this weird API where liburing or the app would need to
>>> distinguish this cqe from all others based on... the user_data? Hence
>>> liburing can't do it, only the app would be able to.
>>>
>>> Just seems like a hack to me.
>>
>> Yes, only user_data to distinguish. Do liburing helpers need to look
>> at cqe->res (and decide something) before returning the cqe to
>> application?
> 
> They generally don't, outside of the internal timeout. But it's an issue
> for the API, as it forces applications to handle the CQEs a certain way.
> Normally there's flexibility. This makes the append writes behave
> differently than everything else, which is never a good idea.
> 

-- 
Pavel Begunkov