Re: [PATCH 6/6] io_uring: enable multishot mode for accept

[Hao Xu <haoxu@linux.alibaba.com>]

在 2021/9/6 上午3:44, Jens Axboe 写道:
> On 9/4/21 4:46 PM, Pavel Begunkov wrote:
>> On 9/4/21 7:40 PM, Jens Axboe wrote:
>>> On 9/4/21 9:34 AM, Hao Xu wrote:
>>>> 在 2021/9/4 上午12:29, Jens Axboe 写道:
>>>>> On 9/3/21 5:00 AM, Hao Xu wrote:
>>>>>> Update io_accept_prep() to enable multishot mode for accept operation.
>>>>>>
>>>>>> Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
>>>>>> ---
>>>>>>    fs/io_uring.c | 14 ++++++++++++--
>>>>>>    1 file changed, 12 insertions(+), 2 deletions(-)
>>>>>>
>>>>>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>>>>>> index eb81d37dce78..34612646ae3c 100644
>>>>>> --- a/fs/io_uring.c
>>>>>> +++ b/fs/io_uring.c
>>>>>> @@ -4861,6 +4861,7 @@ static int io_recv(struct io_kiocb *req, unsigned int issue_flags)
>>>>>>    static int io_accept_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
>>>>>>    {
>>>>>>    	struct io_accept *accept = &req->accept;
>>>>>> +	bool is_multishot;
>>>>>>    
>>>>>>    	if (unlikely(req->ctx->flags & IORING_SETUP_IOPOLL))
>>>>>>    		return -EINVAL;
>>>>>> @@ -4872,14 +4873,23 @@ static int io_accept_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
>>>>>>    	accept->flags = READ_ONCE(sqe->accept_flags);
>>>>>>    	accept->nofile = rlimit(RLIMIT_NOFILE);
>>>>>>    
>>>>>> +	is_multishot = accept->flags & IORING_ACCEPT_MULTISHOT;
>>>>>> +	if (is_multishot && (req->flags & REQ_F_FORCE_ASYNC))
>>>>>> +		return -EINVAL;
>>>>>
>>>>> I like the idea itself as I think it makes a lot of sense to just have
>>>>> an accept sitting there and generating multiple CQEs, but I'm a bit
>>>>> puzzled by how you pass it in. accept->flags is the accept4(2) flags,
>>>>> which can currently be:
>>>>>
>>>>> SOCK_NONBLOCK
>>>>> SOCK_CLOEXEC
>>>>>
>>>>> While there's not any overlap here, that is mostly by chance I think. A
>>>>> cleaner separation is needed here, what happens if some other accept4(2)
>>>>> flag is enabled and it just happens to be the same as
>>>>> IORING_ACCEPT_MULTISHOT?
>>>> Make sense, how about a new IOSQE flag, I saw not many
>>>> entries left there.
>>>
>>> Not quite sure what the best approach would be... The mshot flag only
>>> makes sense for a few request types, so a bit of a shame to have to
>>> waste an IOSQE flag on it. Especially when the flags otherwise passed in
>>> are so sparse, there's plenty of bits there.
>>>
>>> Hence while it may not be the prettiest, perhaps using accept->flags is
>>> ok and we just need some careful code to ensure that we never have any
>>> overlap.
>>
>> Or we can alias with some of the almost-never-used fields like
>> ->ioprio or ->buf_index.
> 
> It's not a bad idea, as long as we can safely use flags from eg ioprio
> for cases where ioprio would never be used. In that sense it's probably
> safer than using buf_index.
> 
> The alternative is, as has been brougt up before, adding a flags2 and
> reserving the last flag in ->flags to say "there are flags in flags2".
May I ask when do we have to use a bit in ->flags to do this? My
understanding is just leverage a __u8 in pad2[2] in io_uring_sqe to
deliver ext_flags.
> Not exactly super pretty either, but we'll need to extend them at some
> point.
>