Re: [PATCH V4 16/27] ublk: add new feature UBLK_F_BATCH_IO

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

On Tue, Dec 02, 2025 at 08:05:17AM -0800, Caleb Sander Mateos wrote:
> On Mon, Dec 1, 2025 at 5:44 PM Ming Lei <ming.lei@redhat.com> wrote:
> >
> > On Mon, Dec 01, 2025 at 01:16:04PM -0800, Caleb Sander Mateos wrote:
> > > On Thu, Nov 20, 2025 at 6:00 PM Ming Lei <ming.lei@redhat.com> wrote:
> > > >
> > > > Add new feature UBLK_F_BATCH_IO which replaces the following two
> > > > per-io commands:
> > > >
> > > >         - UBLK_U_IO_FETCH_REQ
> > > >
> > > >         - UBLK_U_IO_COMMIT_AND_FETCH_REQ
> > > >
> > > > with three per-queue batch io uring_cmd:
> > > >
> > > >         - UBLK_U_IO_PREP_IO_CMDS
> > > >
> > > >         - UBLK_U_IO_COMMIT_IO_CMDS
> > > >
> > > >         - UBLK_U_IO_FETCH_IO_CMDS
> > > >
> > > > Then ublk can deliver batch io commands to ublk server in single
> > > > multishort uring_cmd, also allows to prepare & commit multiple
> > > > commands in batch style via single uring_cmd, communication cost is
> > > > reduced a lot.
> > > >
> > > > This feature also doesn't limit task context any more for all supported
> > > > commands, so any allowed uring_cmd can be issued in any task context.
> > > > ublk server implementation becomes much easier.
> > > >
> > > > Meantime load balance becomes much easier to support with this feature.
> > > > The command `UBLK_U_IO_FETCH_IO_CMDS` can be issued from multiple task
> > > > contexts, so each task can adjust this command's buffer length or number
> > > > of inflight commands for controlling how much load is handled by current
> > > > task.
> > > >
> > > > Later, priority parameter will be added to command `UBLK_U_IO_FETCH_IO_CMDS`
> > > > for improving load balance support.
> > > >
> > > > UBLK_U_IO_GET_DATA isn't supported in batch io yet, but it may be
> > >
> > > UBLK_U_IO_NEED_GET_DATA?
> >
> > Yeah.
> >
> > >
> > > > enabled in future via its batch pair.
> > > >
> > > > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > > > ---
> > > >  drivers/block/ublk_drv.c      | 58 ++++++++++++++++++++++++++++++++---
> > > >  include/uapi/linux/ublk_cmd.h | 16 ++++++++++
> > > >  2 files changed, 69 insertions(+), 5 deletions(-)
> > > >
> > > > diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
> > > > index 849199771f86..90cd1863bc83 100644
> > > > --- a/drivers/block/ublk_drv.c
> > > > +++ b/drivers/block/ublk_drv.c
> > > > @@ -74,7 +74,8 @@
> > > >                 | UBLK_F_AUTO_BUF_REG \
> > > >                 | UBLK_F_QUIESCE \
> > > >                 | UBLK_F_PER_IO_DAEMON \
> > > > -               | UBLK_F_BUF_REG_OFF_DAEMON)
> > > > +               | UBLK_F_BUF_REG_OFF_DAEMON \
> > > > +               | UBLK_F_BATCH_IO)
> > > >
> > > >  #define UBLK_F_ALL_RECOVERY_FLAGS (UBLK_F_USER_RECOVERY \
> > > >                 | UBLK_F_USER_RECOVERY_REISSUE \
> > > > @@ -320,12 +321,12 @@ static void ublk_batch_dispatch(struct ublk_queue *ubq,
> > > >
> > > >  static inline bool ublk_dev_support_batch_io(const struct ublk_device *ub)
> > > >  {
> > > > -       return false;
> > > > +       return ub->dev_info.flags & UBLK_F_BATCH_IO;
> > > >  }
> > > >
> > > >  static inline bool ublk_support_batch_io(const struct ublk_queue *ubq)
> > > >  {
> > > > -       return false;
> > > > +       return ubq->flags & UBLK_F_BATCH_IO;
> > > >  }
> > > >
> > > >  static inline void ublk_io_lock(struct ublk_io *io)
> > > > @@ -3450,6 +3451,41 @@ static int ublk_validate_batch_fetch_cmd(struct ublk_batch_io_data *data,
> > > >         return 0;
> > > >  }
> > > >
> > > > +static int ublk_handle_non_batch_cmd(struct io_uring_cmd *cmd,
> > > > +                                    unsigned int issue_flags)
> > > > +{
> > > > +       const struct ublksrv_io_cmd *ub_cmd = io_uring_sqe_cmd(cmd->sqe);
> > > > +       struct ublk_device *ub = cmd->file->private_data;
> > > > +       unsigned tag = READ_ONCE(ub_cmd->tag);
> > > > +       unsigned q_id = READ_ONCE(ub_cmd->q_id);
> > > > +       unsigned index = READ_ONCE(ub_cmd->addr);
> > > > +       struct ublk_queue *ubq;
> > > > +       struct ublk_io *io;
> > > > +       int ret = -EINVAL;
> > >
> > > I think it would be clearer to just return -EINVAL instead of adding
> > > this variable, but up to you
> > >
> > > > +
> > > > +       if (!ub)
> > > > +               return ret;
> > >
> > > How is this case possible?
> >
> > Will remove the check.
> >
> > >
> > > > +
> > > > +       if (q_id >= ub->dev_info.nr_hw_queues)
> > > > +               return ret;
> > > > +
> > > > +       ubq = ublk_get_queue(ub, q_id);
> > > > +       if (tag >= ubq->q_depth)
> > >
> > > Can avoid the likely cache miss here by using ub->dev_info.queue_depth
> > > instead, analogous to ublk_ch_uring_cmd_local()
> >
> > OK.
> >
> > >
> > > > +               return ret;
> > > > +
> > > > +       io = &ubq->ios[tag];
> > > > +
> > > > +       switch (cmd->cmd_op) {
> > > > +       case UBLK_U_IO_REGISTER_IO_BUF:
> > > > +               return ublk_register_io_buf(cmd, ub, q_id, tag, io, index,
> > > > +                               issue_flags);
> > > > +       case UBLK_U_IO_UNREGISTER_IO_BUF:
> > > > +               return ublk_unregister_io_buf(cmd, ub, index, issue_flags);
> > > > +       default:
> > > > +               return -EOPNOTSUPP;
> > > > +       }
> > > > +}
> > > > +
> > > >  static int ublk_ch_batch_io_uring_cmd(struct io_uring_cmd *cmd,
> > > >                                        unsigned int issue_flags)
> > > >  {
> > > > @@ -3497,7 +3533,8 @@ static int ublk_ch_batch_io_uring_cmd(struct io_uring_cmd *cmd,
> > > >                 ret = ublk_handle_batch_fetch_cmd(&data);
> > > >                 break;
> > > >         default:
> > > > -               ret = -EOPNOTSUPP;
> > > > +               ret = ublk_handle_non_batch_cmd(cmd, issue_flags);
> > >
> > > We should probably skip the if (data.header.q_id >=
> > > ub->dev_info.nr_hw_queues) check for a non-batch command?
> >
> > It is true only for UBLK_IO_UNREGISTER_IO_BUF.
> 
> My point was that this relies on the q_id field being located at the
> same offset in struct ublksrv_io_cmd and struct ublk_batch_io, which
> seems quite subtle. I think it would make more sense not to read the
> SQE as a struct ublk_batch_io for the non-batch commands.

OK, got it, then the check can be moved to ublk_check_batch_cmd() and
ublk_validate_batch_fetch_cmd(). It can be one delta fix for V5.


Thanks,
Ming