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

[Jens Axboe <axboe@kernel.dk>]

On 7/30/20 10:08 AM, Pavel Begunkov wrote:
> 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

10us? :-)

> take to drag through task_work?

A 64-bit value copy is really not a lot of overhead... But yes, we'd
need to push the completion through task_work at that point, as we can't
do it from the completion side. That's not a lot of overhead, and most
notably, it's overhead that only affects this particular type.

That's not a bad starting point, and something that can always be
optimized later if need be. But I seriously doubt it'd be anything to
worry about.

-- 
Jens Axboe