Re: [PATCH v2 1/1] io_uring: allocate memory for overflowed CQEs

[Jens Axboe <axboe@kernel.dk>]

On 2/23/21 8:06 PM, Hao Xu wrote:
> 在 2021/2/23 下午8:40, Pavel Begunkov 写道:
>> Instead of using a request itself for overflowed CQE stashing, allocate
>> a separate entry. The disadvantage is that the allocation may fail and
>> it will be accounted as lost (see rings->cq_overflow), so we lose
>> reliability in case of memory pressure. However, it opens a way for for
>> multiple CQEs per an SQE and even generating SQE-less CQEs >
>> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
>> ---
> Hi Pavel,
> Allow me to ask a stupid question, why do we need to support multiple 
> CQEs per SQE or even SQE-less CQEs in the future?

Not a stupid question at all, since it's not something we've done
before. There's been discussion about this in the past, in the presence
of the zero copy IO where we ideally want to post two CQEs for an SQE.
Most recently I've been playing with multishot poll support, where a
POLL_ADD will stay active after triggering. Hence you could be posting
many CQEs for that SQE, over the life time of the request.

-- 
Jens Axboe