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

[Pavel Begunkov <asml.silence@gmail.com>]

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.

> Probably best to solve that issue in include/linux/net.h, ala:
> 
> /* Flags for socket, socketpair, accept4 */
> #define SOCK_CLOEXEC	O_CLOEXEC
> #ifndef SOCK_NONBLOCK
> #define SOCK_NONBLOCK	O_NONBLOCK
> #endif
> 
> /*
>  * Only used for io_uring accept4, and deliberately chosen to overlap
>  * with the O_* file bits for read/write mode so we won't risk overlap
>  * other flags added for socket/socketpair/accept4 use in the future.
>  */
> #define SOCK_URING_MULTISHOT	00000001
> 
> which should be OK, as these overlap with the O_* filespace and the
> read/write bits are at the start of that space.
> 
> Should be done as a prep patch and sent out to netdev as well, so we can
> get their sign-off on this "hack". If we can get that done, then we have
> our flag and we can just stuff it in accept->flags as long as we clear
> it before calling into accept from io_uring.
> 

-- 
Pavel Begunkov