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

Caleb Sander Mateos
2025-04-29T01:39:00Z

On Mon, Apr 28, 2025 at 6:36 PM Ming Lei <ming.lei@redhat.com> wrote:
>
> On Tue, Apr 29, 2025 at 08:55:48AM +0800, Ming Lei wrote:
> > On Mon, Apr 28, 2025 at 09:01:04AM -0700, Caleb Sander Mateos wrote:
> > > 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()?
> >
> > Yeah, it can be removed.
>
> Actually the removal can only be done after the 3rd patch is applied with
> zero copy check is added.

Right, I just meant it can be removed in this series.

Thanks,
Caleb