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

[Jens Axboe <axboe@kernel.dk>]

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.

>> 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.

-- 
Jens Axboe