Re: [RFC PATCH v3 1/3] io_uring: add helper for uring_cmd completion in submitter-task

[Kanchan Joshi <joshiiitr@gmail.com>]

On Thu, Mar 18, 2021 at 7:31 AM Jens Axboe <axboe@kernel.dk> wrote:
>
> On 3/16/21 8:01 AM, Kanchan Joshi wrote:
> > Completion of a uring_cmd ioctl may involve referencing certain
> > ioctl-specific fields, requiring original subitter context.
> > Introduce 'uring_cmd_complete_in_task' that driver can use for this
> > purpose. The API facilitates task-work infra, while driver gets to
> > implement cmd-specific handling in a callback.
> >
> > Signed-off-by: Kanchan Joshi <joshi.k@samsung.com>
> > ---
> >  fs/io_uring.c            | 36 ++++++++++++++++++++++++++++++++----
> >  include/linux/io_uring.h |  8 ++++++++
> >  2 files changed, 40 insertions(+), 4 deletions(-)
> >
> > diff --git a/fs/io_uring.c b/fs/io_uring.c
> > index 583f8fd735d8..ca459ea9cb83 100644
> > --- a/fs/io_uring.c
> > +++ b/fs/io_uring.c
> > @@ -772,9 +772,12 @@ struct io_kiocb {
> >               /* use only after cleaning per-op data, see io_clean_op() */
> >               struct io_completion    compl;
> >       };
> > -
> > -     /* opcode allocated if it needs to store data for async defer */
> > -     void                            *async_data;
> > +     union {
> > +             /* opcode allocated if it needs to store data for async defer */
> > +             void                            *async_data;
> > +             /* used for uring-cmd, when driver needs to update in task */
> > +             void (*driver_cb)(struct io_uring_cmd *cmd);
> > +     };
>
> I don't like this at all, it's very possible that we'd need async
> data for passthrough commands as well in certain cases. And what it
> gets to that point, we'll have no other recourse than to un-unionize
> this and pay the cost. It also means we end up with:
>
> > @@ -1716,7 +1719,7 @@ static void io_dismantle_req(struct io_kiocb *req)
> >  {
> >       io_clean_op(req);
> >
> > -     if (req->async_data)
> > +     if (io_op_defs[req->opcode].async_size && req->async_data)
> >               kfree(req->async_data);
> >       if (req->file)
> >               io_put_file(req, req->file, (req->flags & REQ_F_FIXED_FILE));
>
> which are also very fragile.

I did not want to have it this way....but faced troubles with the more
natural way of doing this. Please see below.

> We already have the task work, just have the driver init and/or call a
> helper to get it run from task context with the callback it desires?
>
> If you look at this:
>
> > @@ -2032,6 +2035,31 @@ static void io_req_task_submit(struct callback_head *cb)
> >       __io_req_task_submit(req);
> >  }
> >
> > +static void uring_cmd_work(struct callback_head *cb)
> > +{
> > +     struct io_kiocb *req = container_of(cb, struct io_kiocb, task_work);
> > +     struct io_uring_cmd *cmd = &req->uring_cmd;
> > +
> > +     req->driver_cb(cmd);
> > +}
> > +int uring_cmd_complete_in_task(struct io_uring_cmd *ioucmd,
> > +                     void (*driver_cb)(struct io_uring_cmd *))
> > +{
> > +     int ret;
> > +     struct io_kiocb *req = container_of(ioucmd, struct io_kiocb, uring_cmd);
> > +
> > +     req->driver_cb = driver_cb;
> > +     req->task_work.func = uring_cmd_work;
> > +     ret = io_req_task_work_add(req);
> > +     if (unlikely(ret)) {
> > +             req->result = -ECANCELED;
> > +             percpu_ref_get(&req->ctx->refs);
> > +             io_req_task_work_add_fallback(req, io_req_task_cancel);
> > +     }
> > +     return ret;
> > +}
> > +EXPORT_SYMBOL(uring_cmd_complete_in_task);
>
> Then you're basically jumping through hoops to get that callback.
> Why don't you just have:
>
> io_uring_schedule_task(struct io_uring_cmd *cmd, task_work_func_t cb)
> {
>         struct io_kiocb *req = container_of(cmd, struct io_kiocb, uring_cmd);
>         int ret;
>
>         req->task_work.func = cb;
>         ret = io_req_task_work_add(req);
>         if (unlikely(ret)) {
>                 req->result = -ECANCELED;
>                 io_req_task_work_add_fallback(req, io_req_task_cancel);
>         }
>         return ret;
> }
>
> ?
I started with that, but the problem was implementing the driver callback .
The callbacks receive only one argument which is "struct callback_head
*", and the driver needs to extract "io_uring_cmd *" out of it.
This part -
+static void uring_cmd_work(struct callback_head *cb)
+{
+     struct io_kiocb *req = container_of(cb, struct io_kiocb, task_work);
+     struct io_uring_cmd *cmd = &req->uring_cmd;

If the callback has to move to the driver (nvme), the driver needs
visibility to "struct io_kiocb" which is uring-local.
Do you see a better way to handle this?
I also thought about keeping the driver_cb inside the unused part of
uring_cmd (instead of union with req->async_data), but it had two
problems - 1. uring needs to peek inside driver-part of uring_cmd to
invoke this callback
2. losing precious space  (I am using that space to avoid per-command
dynamic-allocation in driver)

> Also, please export any symbol with _GPL. I don't want non-GPL drivers
> using this infrastructure.

Got it.


-- 
Kanchan