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Message-ID: <aS5EgbJQFa2fm6GR@fedora>
Date: Tue, 2 Dec 2025 09:44:33 +0800
From: Ming Lei <ming.lei@...hat.com>
To: Caleb Sander Mateos <csander@...estorage.com>
Cc: Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
Uday Shankar <ushankar@...estorage.com>,
Stefani Seibold <stefani@...bold.net>,
Andrew Morton <akpm@...ux-foundation.org>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH V4 16/27] ublk: add new feature UBLK_F_BATCH_IO
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@...hat.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@...hat.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.
>
> > + break;
> > }
> > out:
> > return ret;
> > @@ -4163,9 +4200,13 @@ static int ublk_ctrl_add_dev(const struct ublksrv_ctrl_cmd *header)
> >
> > ub->dev_info.flags |= UBLK_F_CMD_IOCTL_ENCODE |
> > UBLK_F_URING_CMD_COMP_IN_TASK |
> > - UBLK_F_PER_IO_DAEMON |
> > + (ublk_dev_support_batch_io(ub) ? 0 : UBLK_F_PER_IO_DAEMON) |
>
> Seems redundant with the logic below to clear UBLK_F_PER_IO_DAEMON if
> (ublk_dev_support_batch_io(ub))?
Good catch.
>
> > UBLK_F_BUF_REG_OFF_DAEMON;
> >
> > + /* So far, UBLK_F_PER_IO_DAEMON won't be exposed for BATCH_IO */
> > + if (ublk_dev_support_batch_io(ub))
> > + ub->dev_info.flags &= ~UBLK_F_PER_IO_DAEMON;
> > +
> > /* GET_DATA isn't needed any more with USER_COPY or ZERO COPY */
> > if (ub->dev_info.flags & (UBLK_F_USER_COPY | UBLK_F_SUPPORT_ZERO_COPY |
> > UBLK_F_AUTO_BUF_REG))
> > @@ -4518,6 +4559,13 @@ static int ublk_wait_for_idle_io(struct ublk_device *ub,
> > unsigned int elapsed = 0;
> > int ret;
> >
> > + /*
> > + * For UBLK_F_BATCH_IO ublk server can get notified with existing
> > + * or new fetch command, so needn't wait any more
> > + */
> > + if (ublk_dev_support_batch_io(ub))
> > + return 0;
> > +
> > while (elapsed < timeout_ms && !signal_pending(current)) {
> > unsigned int queues_cancelable = 0;
> > int i;
> > diff --git a/include/uapi/linux/ublk_cmd.h b/include/uapi/linux/ublk_cmd.h
> > index cd894c1d188e..5e8b1211b7f4 100644
> > --- a/include/uapi/linux/ublk_cmd.h
> > +++ b/include/uapi/linux/ublk_cmd.h
> > @@ -335,6 +335,22 @@
> > */
> > #define UBLK_F_BUF_REG_OFF_DAEMON (1ULL << 14)
> >
> > +
> > +/*
> > + * Support the following commands for delivering & committing io command
> > + * in batch.
> > + *
> > + * - UBLK_U_IO_PREP_IO_CMDS
> > + * - UBLK_U_IO_COMMIT_IO_CMDS
> > + * - UBLK_U_IO_FETCH_IO_CMDS
> > + * - UBLK_U_IO_REGISTER_IO_BUF
> > + * - UBLK_U_IO_UNREGISTER_IO_BUF
>
> Seems like it might make sense to provided batched versions of
> UBLK_U_IO_REGISTER_IO_BUF and UBLK_U_IO_UNREGISTER_IO_BUF. That could
> be done in the future, I guess, but it might simplify
> ublk_ch_batch_io_uring_cmd() to only have to handle struct
> ublk_batch_io.
Agree, and it can be added in future.
Thanks,
Ming
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