Re: [RFC PATCH 3/7] io_uring: support to register bvec buffer to specified io_uring

[Ming Lei <ming.lei@redhat.com>]

On Thu, May 01, 2025 at 06:31:03PM -0700, Caleb Sander Mateos wrote:
> On Wed, Apr 30, 2025 at 8:34 AM Ming Lei <ming.lei@redhat.com> wrote:
> >
> > On Mon, Apr 28, 2025 at 05:43:12PM -0700, Caleb Sander Mateos wrote:
> > > On Mon, Apr 28, 2025 at 2:44 AM Ming Lei <ming.lei@redhat.com> wrote:
> > > >
> > > > Extend io_buffer_register_bvec() and io_buffer_unregister_bvec() for
> > > > supporting to register/unregister bvec buffer to specified io_uring,
> > > > which FD is usually passed from userspace.
> > > >
> > > > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > > > ---
> > > >  include/linux/io_uring/cmd.h |  4 ++
> > > >  io_uring/rsrc.c              | 83 +++++++++++++++++++++++++++---------
> > > >  2 files changed, 67 insertions(+), 20 deletions(-)
> > > >
> > > > diff --git a/include/linux/io_uring/cmd.h b/include/linux/io_uring/cmd.h
> > > > index 78fa336a284b..7516fe5cd606 100644
> > > > --- a/include/linux/io_uring/cmd.h
> > > > +++ b/include/linux/io_uring/cmd.h
> > > > @@ -25,6 +25,10 @@ struct io_uring_cmd_data {
> > > >
> > > >  struct io_buf_data {
> > > >         unsigned short index;
> > > > +       bool has_fd;
> > > > +       bool registered_fd;
> > > > +
> > > > +       int ring_fd;
> > > >         struct request *rq;
> > > >         void (*release)(void *);
> > > >  };
> > > > diff --git a/io_uring/rsrc.c b/io_uring/rsrc.c
> > > > index 5f8ab130a573..701dd33fecf7 100644
> > > > --- a/io_uring/rsrc.c
> > > > +++ b/io_uring/rsrc.c
> > > > @@ -969,21 +969,6 @@ static int __io_buffer_register_bvec(struct io_ring_ctx *ctx,
> > > >         return 0;
> > > >  }
> > > >
> > > > -int io_buffer_register_bvec(struct io_uring_cmd *cmd,
> > > > -                           struct io_buf_data *buf,
> > > > -                           unsigned int issue_flags)
> > > > -{
> > > > -       struct io_ring_ctx *ctx = cmd_to_io_kiocb(cmd)->ctx;
> > > > -       int ret;
> > > > -
> > > > -       io_ring_submit_lock(ctx, issue_flags);
> > > > -       ret = __io_buffer_register_bvec(ctx, buf);
> > > > -       io_ring_submit_unlock(ctx, issue_flags);
> > > > -
> > > > -       return ret;
> > > > -}
> > > > -EXPORT_SYMBOL_GPL(io_buffer_register_bvec);
> > > > -
> > > >  static int __io_buffer_unregister_bvec(struct io_ring_ctx *ctx,
> > > >                                        struct io_buf_data *buf)
> > > >  {
> > > > @@ -1006,19 +991,77 @@ static int __io_buffer_unregister_bvec(struct io_ring_ctx *ctx,
> > > >         return 0;
> > > >  }
> > > >
> > > > -int io_buffer_unregister_bvec(struct io_uring_cmd *cmd,
> > > > -                             struct io_buf_data *buf,
> > > > -                             unsigned int issue_flags)
> > > > +static inline int do_reg_unreg_bvec(struct io_ring_ctx *ctx,
> > > > +                                   struct io_buf_data *buf,
> > > > +                                   unsigned int issue_flags,
> > > > +                                   bool reg)
> > > >  {
> > > > -       struct io_ring_ctx *ctx = cmd_to_io_kiocb(cmd)->ctx;
> > > >         int ret;
> > > >
> > > >         io_ring_submit_lock(ctx, issue_flags);
> > > > -       ret = __io_buffer_unregister_bvec(ctx, buf);
> > > > +       if (reg)
> > > > +               ret = __io_buffer_register_bvec(ctx, buf);
> > > > +       else
> > > > +               ret = __io_buffer_unregister_bvec(ctx, buf);
> > >
> > > It feels like unifying __io_buffer_register_bvec() and
> > > __io_buffer_unregister_bvec() would belong better in the prior patch
> > > that changes their signatures.
> >
> > Can you share how to do above in previous patch?
> 
> I was thinking you could define do_reg_unreg_bvec() in the previous
> patch. It's a logical step once you've extracted out all the
> differences between io_buffer_register_bvec() and
> io_buffer_unregister_bvec() into the helpers
> __io_buffer_register_bvec() and __io_buffer_unregister_bvec(). But
> either way is fine.

'has_fd' and 'ring_fd' fields isn't added yet, the defined do_reg_unreg_bvec()
could be quite simple, looks no big difference, I can do that...

> 
> >
> > >
> > > >         io_ring_submit_unlock(ctx, issue_flags);
> > > >
> > > >         return ret;
> > > >  }
> > > > +
> > > > +static int io_buffer_reg_unreg_bvec(struct io_ring_ctx *ctx,
> > > > +                                   struct io_buf_data *buf,
> > > > +                                   unsigned int issue_flags,
> > > > +                                   bool reg)
> > > > +{
> > > > +       struct io_ring_ctx *remote_ctx = ctx;
> > > > +       struct file *file = NULL;
> > > > +       int ret;
> > > > +
> > > > +       if (buf->has_fd) {
> > > > +               file = io_uring_register_get_file(buf->ring_fd, buf->registered_fd);
> > > > +               if (IS_ERR(file))
> > > > +                       return PTR_ERR(file);
> > >
> > > It would be good to avoid the overhead of this lookup and
> > > reference-counting in the I/O path. Would it be possible to move this
> > > lookup to when UBLK_IO_FETCH_REQ (and UBLK_IO_COMMIT_AND_FETCH_REQ, if
> > > it specifies a different ring_fd) is submitted? I guess that might
> > > require storing an extra io_ring_ctx pointer in struct ublk_io.
> >
> > Let's start from the flexible way & simple implementation.
> >
> > Any optimization & improvement can be done as follow-up.
> 
> Sure, we can start with this as-is. But I suspect the extra
> reference-counting here will significantly decrease the benefit of the
> auto-register register feature.

The reference-counting should only be needed for registering buffer to
external ring, which may have been slow because of the cross-ring thing...

Maybe we can start automatic buffer register for ubq_daemon context only,
meantime allow to register buffer from external io_uring by adding per-io
spin_lock, which may help the per-io task Uday is working on too.

And the interface still allow to support automatic buffer register to
external io_uring since `ublk_auto_buf_reg` includes 'flags' field, we can
enable it in future when efficient implementation is figured out.

What do you think of this approach?

> 
> >
> > Each command may register buffer to different io_uring context,
> > it can't be done in UBLK_IO_FETCH_REQ stage, because new IO with same
> > tag may register buffer to new io_uring context.
> 
> Right, if UBLK_IO_COMMIT_AND_FETCH_REQ specifies a different io_uring
> fd, then we'd have to look it up anew. But it seems likely that all
> UBLK_IO_COMMIT_AND_FETCH_REQs for a single tag will specify the same
> io_uring (certainly that's how our ublk server works). And in that
> case, the I/O could just reuse the io_uring context that was looked up
> for the prior UBLK_IO_(COMMIT_AND_)FETCH_REQ.

It is a special case, and one follow-up feature flag can help to
optimize this case only.


Thanks, 
Ming