Re: [PATCH v9 10/17] fuse: Add io-uring sqe commit and fetch support

[Joanne Koong <joannelkoong@gmail.com>]

On Tue, Jan 21, 2025 at 4:18 PM Bernd Schubert <bernd@bsbernd.com> wrote:
>
...
> >>
> >>>
> >>>> +
> >>>> +       err = fuse_ring_ent_set_commit(ring_ent);
> >>>> +       if (err != 0) {
> >>>> +               pr_info_ratelimited("qid=%d commit_id %llu state %d",
> >>>> +                                   queue->qid, commit_id, ring_ent->state);
> >>>> +               spin_unlock(&queue->lock);
> >>>> +               return err;
> >>>> +       }
> >>>> +
> >>>> +       ring_ent->cmd = cmd;
> >>>> +       spin_unlock(&queue->lock);
> >>>> +
> >>>> +       /* without the queue lock, as other locks are taken */
> >>>> +       fuse_uring_commit(ring_ent, issue_flags);
> >>>> +
> >>>> +       /*
> >>>> +        * Fetching the next request is absolutely required as queued
> >>>> +        * fuse requests would otherwise not get processed - committing
> >>>> +        * and fetching is done in one step vs legacy fuse, which has separated
> >>>> +        * read (fetch request) and write (commit result).
> >>>> +        */
> >>>> +       fuse_uring_next_fuse_req(ring_ent, queue, issue_flags);
> >>>
> >>> If there's no request ready to read next, then no request will be
> >>> fetched and this will return. However, as I understand it, once the
> >>> uring is registered, userspace should only be interacting with the
> >>> uring via FUSE_IO_URING_CMD_COMMIT_AND_FETCH. However for the case
> >>> where no request was ready to read, it seems like userspace would have
> >>> nothing to commit when it wants to fetch the next request?
> >>
> >> We have
> >>
> >> FUSE_IO_URING_CMD_REGISTER
> >> FUSE_IO_URING_CMD_COMMIT_AND_FETCH
> >>
> >>
> >> After _CMD_REGISTER the corresponding ring-entry is ready to get fuse
> >> requests and waiting. After it gets a request assigned and handles it
> >> by fuse server the _COMMIT_AND_FETCH scheme applies. Did you possibly
> >> miss that _CMD_REGISTER will already have it waiting?
> >>
> >
> > Sorry for the late reply. After _CMD_REGISTER and _COMMIT_AND_FETCH,
> > it seems possible that there is no fuse request waiting until a later
> > time? This is the scenario I'm envisioning:
> > a) uring registers successfully and fetches request through _CMD_REGISTER
> > b) server replies to request and fetches new request through _COMMIT_AND_FETCH
> > c) server replies to request, tries to fetch new request but no
> > request is ready, through _COMMIT_AND_FETCH
> >
> > maybe I'm missing something in my reading of the code, but how will
> > the server then fetch the next request once the request is ready? It
> > will have to commit something in order to fetch it since there's only
> > _COMMIT_AND_FETCH which requires a commit, no?
> >
>
> The right name would be '_COMMIT_AND_FETCH_OR_WAIT'. Please see
> fuse_uring_next_fuse_req().
>
> retry:
>         spin_lock(&queue->lock);
>         fuse_uring_ent_avail(ent, queue);           --> entry available
>         has_next = fuse_uring_ent_assign_req(ent);
>         spin_unlock(&queue->lock);
>
>         if (has_next) {
>                 err = fuse_uring_send_next_to_ring(ent, issue_flags);
>                 if (err)
>                         goto retry;
>         }
>
>
> If there is no available request, the io-uring cmd stored in ent->cmd is
> just queued/available.

Could you point me to where the wait happens?  I think that's the part
I'm missing. In my reading of the code, if there's no available
request (eg queue->fuse_req_queue is empty), then I see that has_next
will return false and fuse_uring_next_fuse_req() /
fuse_uring_commit_fetch() returns without having fetched anything.
Where does the "if there is no available request, the io-uring cmd is
just queued/available" happen?

>
> Btw, Miklos added it to linux-next.
>

Awesome, can't wait to use it.

Thanks,
Joanne
>
> Cheers,
> Bernd
>