Re: [PATCH v6.15 2/3] ublk: decouple zero copy from user copy

Caleb Sander Mateos
2025-04-28T16:01:16Z

On Sun, Apr 27, 2025 at 6:49 AM Ming Lei <ming.lei@redhat.com> wrote:
>
> UBLK_F_USER_COPY and UBLK_F_SUPPORT_ZERO_COPY are two different
> features, and shouldn't be coupled together.
>
> Commit 1f6540e2aabb ("ublk: zc register/unregister bvec") enables
> user copy automatically in case of UBLK_F_SUPPORT_ZERO_COPY, this way
> isn't correct.
>
> So decouple zero copy from user copy, and use independent helper to
> check each one.

I agree this makes sense.

>
> Fixes: 1f6540e2aabb ("ublk: zc register/unregister bvec")
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
> drivers/block/ublk_drv.c | 35 +++++++++++++++++++++++------------
> 1 file changed, 23 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
> index 40f971a66d3e..0a3a3c64316d 100644
> --- a/drivers/block/ublk_drv.c
> +++ b/drivers/block/ublk_drv.c
> @@ -205,11 +205,6 @@ static inline struct request *__ublk_check_and_get_req(struct ublk_device *ub,
> static inline unsigned int ublk_req_build_flags(struct request *req);
> static inline struct ublksrv_io_desc *ublk_get_iod(struct ublk_queue *ubq,
> int tag);
> -static inline bool ublk_dev_is_user_copy(const struct ublk_device *ub)
> -{
> - return ub->dev_info.flags & (UBLK_F_USER_COPY | UBLK_F_SUPPORT_ZERO_COPY);
> -}
> -
> static inline bool ublk_dev_is_zoned(const struct ublk_device *ub)
> {
> return ub->dev_info.flags & UBLK_F_ZONED;
> @@ -609,14 +604,19 @@ static void ublk_apply_params(struct ublk_device *ub)
> ublk_dev_param_zoned_apply(ub);
> }
>
> +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_user_copy(const struct ublk_queue *ubq)
> {
> - return ubq->flags & (UBLK_F_USER_COPY | UBLK_F_SUPPORT_ZERO_COPY);
> + return ubq->flags & UBLK_F_USER_COPY;
> }
>
> static inline bool ublk_need_map_io(const struct ublk_queue *ubq)
> {
> - return !ublk_support_user_copy(ubq);
> + return !ublk_support_user_copy(ubq) && !ublk_support_zero_copy(ubq);
> }
>
> static inline bool ublk_need_req_ref(const struct ublk_queue *ubq)
> @@ -624,8 +624,11 @@ static inline bool ublk_need_req_ref(const struct ublk_queue *ubq)
> /*
> * read()/write() is involved in user copy, so request reference
> * has to be grabbed
> + *
> + * for zero copy, request buffer need to be registered to io_uring
> + * buffer table, so reference is needed
> */
> - return ublk_support_user_copy(ubq);
> + return ublk_support_user_copy(ubq) || ublk_support_zero_copy(ubq);
> }
>
> static inline void ublk_init_req_ref(const struct ublk_queue *ubq,
> @@ -2245,6 +2248,9 @@ static struct request *ublk_check_and_get_req(struct kiocb *iocb,
> if (!ubq)
> return ERR_PTR(-EINVAL);
>
> + if (!ublk_support_user_copy(ubq))
> + return ERR_PTR(-EACCES);

This partly overlaps with the existing ublk_need_req_ref() check in
__ublk_check_and_get_req() (although that allows
UBLK_F_SUPPORT_ZERO_COPY too). Can that check be removed now that the
callers explicitly check ublk_support_user_copy() or
ublk_support_zero_copy()?

> +
> if (tag >= ubq->q_depth)
> return ERR_PTR(-EINVAL);
>
> @@ -2783,13 +2789,18 @@ 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;
>
> - /* GET_DATA isn't needed any more with USER_COPY */
> - if (ublk_dev_is_user_copy(ub))
> + /* 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))
> ub->dev_info.flags &= ~UBLK_F_NEED_GET_DATA;
>
> - /* Zoned storage support requires user copy feature */
> + /*
> + * Zoned storage support requires reuse `ublksrv_io_cmd->addr` for
> + * returning write_append_lba, which is only allowed in case of
> + * user copy or zero copy

Thanks, this comment is much more helpful.

Best,
Caleb

> + */
> if (ublk_dev_is_zoned(ub) &&
> - (!IS_ENABLED(CONFIG_BLK_DEV_ZONED) || !ublk_dev_is_user_copy(ub))) {
> + (!IS_ENABLED(CONFIG_BLK_DEV_ZONED) || !(ub->dev_info.flags &
> + (UBLK_F_USER_COPY | UBLK_F_SUPPORT_ZERO_COPY)))) {
> ret = -EINVAL;
> goto out_free_dev_number;
> }
> --
> 2.47.0
>