Re: [PATCH 1/1] io_uring/zctx: separate notification user_data

[Jens Axboe <axboe@kernel.dk>]

On 2/16/26 8:48 AM, Pavel Begunkov wrote:
> On 2/16/26 15:10, Jens Axboe wrote:
>> On 2/16/26 4:48 AM, Pavel Begunkov wrote:
>>> People previously asked for the notification CQE to have a different
>>> user_data value from the main request completion. It's useful to
>>> separate buffer and request handling logic and avoid separately
>>> refcounting the request.
>>>
>>> Let the user pass the notification user_data in sqe->addr3. If zero,
>>> it'll inherit sqe->user_data as before. It doesn't change the rules for
>>> when the user can expect a notification CQE, and it should still check
>>> the IORING_CQE_F_MORE flag.
>>
>> This should use and sqe->ioprio flag to manage it, otherwise you're
>> excluding 0. Which may not be important in and of itself, but the
>> flag approach is expected way to do this.
> 
> What's the benefit? It's not unreasonable to exclude zero, it won't
> limit any use cases, and it's not new either (i.e. buffer tags).
> On the other hand, the user will now have to modify two fields
> instead of one, which is cleaner. And you're taking one extra bit
> out of 16bit ->ioprio, which is not critical if it's all going to
> be flags, but it wouldn't be an outrageous idea to take 8 bits
> out of it for some index, for example.

The benefit is that it's weird to exclude a given user_data value, just
so it can get used as both a key and a flag. IMHO much cleaner to have a
flag for it which explicitly says "use the user_data I provide". Also
easier to explain in docs, set this flag and then the value in X will be
the user_data for the completion.

-- 
Jens Axboe