On Mon, May 19, 2025 at 9:55 PM Ming Lei <ming.lei@xxxxxxxxxx> wrote:
>
> UBLK_F_SUPPORT_ZERO_COPY requires ublk server to issue explicit buffer
> register/unregister uring_cmd for each IO, this way is not only inefficient,
> but also introduce dependency between buffer consumer and buffer register/
> unregister uring_cmd, please see tools/testing/selftests/ublk/stripe.c
> in which backing file IO has to be issued one by one by IOSQE_IO_LINK.
>
> Prepare for adding feature UBLK_F_AUTO_BUF_REG for addressing the existing
> zero copy limitation:
>
> - register request buffer automatically to ublk uring_cmd's io_uring
> context before delivering io command to ublk server
>
> - unregister request buffer automatically from the ublk uring_cmd's
> io_uring context when completing the request
>
> - io_uring will unregister the buffer automatically when uring is
> exiting, so we needn't worry about accident exit
>
> For using this feature, ublk server has to create one sparse buffer table
>
> Reviewed-by: Caleb Sander Mateos <csander@xxxxxxxxxxxxxxx>
> Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
> ---
> drivers/block/ublk_drv.c | 70 ++++++++++++++++++++++++++++++++++++----
> 1 file changed, 64 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
> index ae2f47dc8224..3e56a9d267fb 100644
> --- a/drivers/block/ublk_drv.c
> +++ b/drivers/block/ublk_drv.c
> @@ -133,6 +133,14 @@ struct ublk_uring_cmd_pdu {
> */
> #define UBLK_IO_FLAG_NEED_GET_DATA 0x08
>
> +/*
> + * request buffer is registered automatically, so we have to unregister it
> + * before completing this request.
> + *
> + * io_uring will unregister buffer automatically for us during exiting.
> + */
> +#define UBLK_IO_FLAG_AUTO_BUF_REG 0x10
> +
> /* atomic RW with ubq->cancel_lock */
> #define UBLK_IO_FLAG_CANCELED 0x80000000
>
> @@ -205,6 +213,7 @@ struct ublk_params_header {
> __u32 types;
> };
>
> +static void ublk_io_release(void *priv);
> static void ublk_stop_dev_unlocked(struct ublk_device *ub);
> static void ublk_abort_queue(struct ublk_device *ub, struct ublk_queue *ubq);
> static inline struct request *__ublk_check_and_get_req(struct ublk_device *ub,
> @@ -619,6 +628,11 @@ static inline bool ublk_support_zero_copy(const struct ublk_queue *ubq)
> return ubq->flags & UBLK_F_SUPPORT_ZERO_COPY;
> }
>
> +static inline bool ublk_support_auto_buf_reg(const struct ublk_queue *ubq)
> +{
> + return false;
> +}
> +
> static inline bool ublk_support_user_copy(const struct ublk_queue *ubq)
> {
> return ubq->flags & UBLK_F_USER_COPY;
> @@ -626,7 +640,8 @@ static inline bool ublk_support_user_copy(const struct ublk_queue *ubq)
>
> static inline bool ublk_need_map_io(const struct ublk_queue *ubq)
> {
> - return !ublk_support_user_copy(ubq) && !ublk_support_zero_copy(ubq);
> + return !ublk_support_user_copy(ubq) && !ublk_support_zero_copy(ubq) &&
> + !ublk_support_auto_buf_reg(ubq);
> }
>
> static inline bool ublk_need_req_ref(const struct ublk_queue *ubq)
> @@ -637,8 +652,13 @@ static inline bool ublk_need_req_ref(const struct ublk_queue *ubq)
> *
> * for zero copy, request buffer need to be registered to io_uring
> * buffer table, so reference is needed
> + *
> + * For auto buffer register, ublk server still may issue
> + * UBLK_IO_COMMIT_AND_FETCH_REQ before one registered buffer is used up,
> + * so reference is required too.
> */
> - return ublk_support_user_copy(ubq) || ublk_support_zero_copy(ubq);
> + return ublk_support_user_copy(ubq) || ublk_support_zero_copy(ubq) ||
> + ublk_support_auto_buf_reg(ubq);
> }
>
> static inline void ublk_init_req_ref(const struct ublk_queue *ubq,
> @@ -1155,6 +1175,35 @@ static inline void __ublk_abort_rq(struct ublk_queue *ubq,
> blk_mq_end_request(rq, BLK_STS_IOERR);
> }
>
> +static bool ublk_auto_buf_reg(struct request *req, struct ublk_io *io,
> + unsigned int issue_flags)
> +{
> + struct ublk_rq_data *data = blk_mq_rq_to_pdu(req);
> + int ret;
> +
> + ret = io_buffer_register_bvec(io->cmd, req, ublk_io_release, 0,
> + issue_flags);
> + if (ret) {
> + blk_mq_end_request(req, BLK_STS_IOERR);
> + return false;
> + }
> + /* one extra reference is dropped by ublk_io_release */
> + refcount_set(&data->ref, 2);
> + io->flags |= UBLK_IO_FLAG_AUTO_BUF_REG;
> + return true;
> +}
> +
> +static bool ublk_prep_auto_buf_reg(struct ublk_queue *ubq,
> + struct request *req, struct ublk_io *io,
> + unsigned int issue_flags)
> +{
> + if (ublk_support_auto_buf_reg(ubq) && ublk_rq_has_data(req))
> + return ublk_auto_buf_reg(req, io, issue_flags);
> +
> + ublk_init_req_ref(ubq, req);
> + return true;
> +}
> +
> static bool ublk_start_io(const struct ublk_queue *ubq, struct request *req,
> struct ublk_io *io)
> {
> @@ -1180,7 +1229,6 @@ static bool ublk_start_io(const struct ublk_queue *ubq, struct request *req,
> mapped_bytes >> 9;
> }
>
> - ublk_init_req_ref(ubq, req);
> return true;
> }
>
> @@ -1226,7 +1274,8 @@ static void ublk_dispatch_req(struct ublk_queue *ubq,
> if (!ublk_start_io(ubq, req, io))
> return;
>
> - ublk_complete_io_cmd(io, req, UBLK_IO_RES_OK, issue_flags);
> + if (ublk_prep_auto_buf_reg(ubq, req, io, issue_flags))
> + ublk_complete_io_cmd(io, req, UBLK_IO_RES_OK, issue_flags);
> }
>
> static void ublk_cmd_tw_cb(struct io_uring_cmd *cmd,
> @@ -1994,7 +2043,8 @@ static int ublk_fetch(struct io_uring_cmd *cmd, struct ublk_queue *ubq,
>
> static int ublk_commit_and_fetch(const struct ublk_queue *ubq,
> struct ublk_io *io, struct io_uring_cmd *cmd,
> - const struct ublksrv_io_cmd *ub_cmd)
> + const struct ublksrv_io_cmd *ub_cmd,
> + unsigned int issue_flags)
> {
> struct request *req = io->req;
>
> @@ -2014,6 +2064,14 @@ static int ublk_commit_and_fetch(const struct ublk_queue *ubq,
> return -EINVAL;
> }
>
> + if (ublk_support_auto_buf_reg(ubq)) {
> + if (io->flags & UBLK_IO_FLAG_AUTO_BUF_REG) {
> + WARN_ON_ONCE(io_buffer_unregister_bvec(cmd, 0,
> + issue_flags));
Since the io_ring_ctx is determined from the io_uring_cmd, this only
works if the UBLK_IO_COMMIT_AND_FETCH_REQ is submitted to the same
io_uring as the previous UBLK_IO_(COMMIT_AND_)FETCH_REQ for the ublk
I/O. It would be good to document that. And I would probably drop the
WARN_ON_ONCE() here, since it can be triggered from userspace.
Otherwise,
Reviewed-by: Caleb Sander Mateos <csander@xxxxxxxxxxxxxxx>
> + io->flags &= ~UBLK_IO_FLAG_AUTO_BUF_REG;
> + }
> + }
> +
> ublk_fill_io_cmd(io, cmd, ub_cmd->addr);
>
> /* now this cmd slot is owned by ublk driver */
> @@ -2110,7 +2168,7 @@ static int __ublk_ch_uring_cmd(struct io_uring_cmd *cmd,
> goto out;
> break;
> case UBLK_IO_COMMIT_AND_FETCH_REQ:
> - ret = ublk_commit_and_fetch(ubq, io, cmd, ub_cmd);
> + ret = ublk_commit_and_fetch(ubq, io, cmd, ub_cmd, issue_flags);
> if (ret)
> goto out;
> break;
> --
> 2.47.0
>
