On 1/13/23 3:12 AM, Ming Lei wrote: > Hello, > > On Thu, Jan 12, 2023 at 08:35:36AM +0100, Stefan Metzmacher wrote: >> Am 12.01.23 um 04:40 schrieb Jens Axboe: >>> On 1/11/23 8:27?PM, Ming Lei wrote: >>>> Hi Stefan and Jens, >>>> >>>> Thanks for the help. >>>> >>>> BTW, the issue is observed when I write ublk-nbd: >>>> >>>> https://github.com/ming1/ubdsrv/commits/nbd >>>> >>>> and it isn't completed yet(multiple send sqe chains not serialized >>>> yet), the issue is triggered when writing big chunk data to ublk-nbd. >>> >>> Gotcha >>> >>>> On Wed, Jan 11, 2023 at 05:32:00PM +0100, Stefan Metzmacher wrote: >>>>> Hi Ming, >>>>> >>>>>> Per my understanding, a short send on SOCK_STREAM should terminate the >>>>>> remainder of the SQE chain built by IOSQE_IO_LINK. >>>>>> >>>>>> But from my observation, this point isn't true when using io_sendmsg or >>>>>> io_sendmsg_zc on TCP socket, and the other remainder of the chain still >>>>>> can be completed after one short send is found. MSG_WAITALL is off. >>>>> >>>>> This is due to legacy reasons, you need pass MSG_WAITALL explicitly >>>>> in order to a retry or an error on a short write... >>>>> It should work for send, sendmsg, sendmsg_zc, recv and recvmsg. >>>> >>>> Turns out there is another application bug in which recv sqe may cut in the >>>> send sqe chain. >>>> >>>> After the issue is fixed, if MSG_WAITALL is set, short send can't be >>>> observed any more. But if MSG_WAITALL isn't set, short send can be >>>> observed and the send io chain still won't be terminated. >>> >>> Right, if MSG_WAITALL is set, then the whole thing will be written. If >>> we get a short send, it's retried appropriately. Unless an error occurs, >>> it should send the whole thing. >>> >>>> So if MSG_WAITALL is set, will io_uring be responsible for retry in case >>>> of short send, and application needn't to take care of it? >> >> With new kernels yes, but the application should be prepared to have retry >> logic in order to be compatible with older kernels. > > Now ublk-nbd can be played, mkfs/mount and fio starts to work. > > But short send still can be observed sometimes when sending nbd write > request, which is done by sendmsg(), and the message includes two vectors, > (the 1st is the nbd_request, another one is the data to be written to disk). > > Short send is reported by cqe in which cqe->res is always 28, which is > size of 'struct nbd_request', also the length of the 1st io vec. And not > see send cqe failure message. > > And MSG_WAITALL is set for all ublk-nbd io actually. > > Follows the steps: > > 1) install liburing 2.0+ > > 2) build ublk & reproduce the issue: > > - git clone https://github.com/ming1/ubdsrv.git -b nbd > > - cd ubdsrv > > - vim build_with_liburing_src && set LIBURING_DIR to your liburing dir > > - ./build_with_liburing_src&& make -j4 > > 3) run the nbd test > - cd ubdsrv > - make test T=nbd > > Sometimes the test hangs, and the following log can be observed > in syslog: > > nbd_send_req_done: short send/receive tag 2 op 1 8000000000800002, len 524316 written 28 cqe flags 0 > ... > I can reproduce this, but it's a SEND that ends up being triggered, not a SENDMSG. Should the payload carrying op not be a SENDMSG? I'm assuming two vecs for that one. -- Jens Axboe
