On Mon, Apr 03, 2023 at 04:38:30PM +0800, Ziyang Zhang wrote: > On 2023/3/29 18:52, Ming Lei wrote: > > On Wed, Mar 29, 2023 at 06:01:16PM +0800, Ziyang Zhang wrote: > >> On 2023/3/29 17:00, Ming Lei wrote: > >>> On Wed, Mar 29, 2023 at 10:57:53AM +0800, Ziyang Zhang wrote: > >>>> On 2023/3/28 23:09, Ming Lei wrote: > >>>>> Apply io_uring fused command for supporting zero copy: > >>>>> > >>>> > >>>> [...] > >>>> > >>>>> > >>>>> @@ -1374,7 +1533,12 @@ static int ublk_ch_uring_cmd(struct io_uring_cmd *cmd, unsigned int issue_flags) > >>>>> if (!ubq || ub_cmd->q_id != ubq->q_id) > >>>>> goto out; > >>>>> > >>>>> - if (ubq->ubq_daemon && ubq->ubq_daemon != current) > >>>>> + /* > >>>>> + * The fused command reads the io buffer data structure only, so it > >>>>> + * is fine to be issued from other context. > >>>>> + */ > >>>>> + if ((ubq->ubq_daemon && ubq->ubq_daemon != current) && > >>>>> + (cmd_op != UBLK_IO_FUSED_SUBMIT_IO)) > >>>>> goto out; > >>>>> > >>>> > >>>> Hi Ming, > >>>> > >>>> What is your use case that fused io_uring cmd is issued from another thread? > >>>> I think it is good practice to operate one io_uring instance in one thread > >>>> only. > >>> > >>> So far we limit io command has to be issued from the queue context, > >>> which is still not friendly from userspace viewpoint, the reason is > >>> that we can't get io_uring exit notification and ublk's use case is > >>> very special since the queued io command may not be completed forever, > >> > >> OK, so UBLK_IO_FUSED_SUBMIT_IO is guaranteed to be completed because it is > >> not queued. FETCH_REQ and COMMIT_AMD_FETCH are queued io commands and could > >> not be completed forever so they have to be issued from ubq_daemon. Right? > > > > Yeah, any io command should be issued from ubq daemon context. > > > >> > >> BTW, maybe NEED_GET_DATA can be issued from other context... > > > > So far it won't be supported. > > > > As I mentioned in the link, if io_uring can provide io_uring exit > > callback, we may relax this limit. > > > > Hi, Ming > > Sorry, I do not understand... I think UBLK_IO_NEED_GET_DATA is normal IO just like > UBLK_IO_FUSED_SUBMIT_IO. It is issued from one pthread(ubq_daemon for now) and > is completed just in time(not queued). So I think we can allow UBLK_IO_NEED_GET_DATA > to be issued from other context. No, it isn't. UBLK_IO_FUSED_SUBMIT_IO is actually for handling target IO, and this command just reads/provides IO buffer meta to io_uring in read-only approach, and io buffer meta won't be changed, and any io state won't be changed, so it is fine to call concurrently. UBLK_IO_NEED_GET_DATA is still part of io commands, in which io->addr needs to be set, and io->flags is touched, and it can't be done safely concurrently. Also after zero-copy is supported, UBLK_IO_NEED_GET_DATA may become legacy code path, because ublk server can read/write io data directly in userspace via read()/write(), and there isn't buffer allocation issue any more. Thanks, Ming
