On Sat, Apr 26, 2025 at 6:37 PM Ming Lei <ming.lei@xxxxxxxxxx> wrote:
>
> On Sat, Apr 26, 2025 at 01:38:14PM -0700, Caleb Sander Mateos wrote:
> > On Sat, Apr 26, 2025 at 2:41 AM Ming Lei <ming.lei@xxxxxxxxxx> wrote:
> > >
> > > The simple check of UBLK_IO_FLAG_OWNED_BY_SRV can avoid incorrect
> > > register/unregister io buffer easily, so check it before calling
> > > starting to register/un-register io buffer.
> > >
> > > Also only allow io buffer register/unregister uring_cmd in case of
> > > UBLK_F_SUPPORT_ZERO_COPY.
> >
> > Indeed, both these checks make sense. (Hopefully there aren't any
> > applications depending on the ability to use ublk zero-copy without
> > setting the flag.) I too was thinking of adding the
> > UBLK_IO_FLAG_OWNED_BY_SRV check because it could allow the
> > kref_get_unless_zero() to be replaced with the cheaper kref_get(). I
> > think the checks could be split into 2 separate commits, but up to
> > you.
>
> Let's do it in single patch for making everyone easier.
>
> >
> > >
> > > Fixes: 1f6540e2aabb ("ublk: zc register/unregister bvec")
> > > Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
> > > ---
> > > drivers/block/ublk_drv.c | 23 ++++++++++++++++++++++-
> > > 1 file changed, 22 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
> > > index 40f971a66d3e..347790b3a633 100644
> > > --- a/drivers/block/ublk_drv.c
> > > +++ b/drivers/block/ublk_drv.c
> > > @@ -609,6 +609,11 @@ 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);
> > > @@ -1950,9 +1955,16 @@ static int ublk_register_io_buf(struct io_uring_cmd *cmd,
> > > unsigned int index, unsigned int issue_flags)
> > > {
> > > struct ublk_device *ub = cmd->file->private_data;
> > > + struct ublk_io *io = &ubq->ios[tag];
> >
> > I thought you had mentioned in
> > https://lore.kernel.org/linux-block/aAmYJxaV1-yWEMRo@fedora/ wanting
> > to the ability to offload the ublk zero-copy buffer registration to a
> > thread other than ubq_daemon. Are you still planning to do that, or
> > does the "auto-register" feature supplant the need for that?
>
> The auto-register idea is actually thought of when I was working on ublk
> selftest offload function.
>
> If this auto-register feature is supported, it becomes less important to
> relax the ubq_daemon limit for register_io_buffer command, then I jump
> on this feature & post put the patch.
>
> But I will continue to work on the offload test code and finally relax
> the limit for register/unregister io buffer command, hope it can be
> done in next week.
>
> > Accessing
> > the ublk_io here only seems safe when on the ubq_daemon thread.
>
> Both ublk_register_io_buf()/ublk_unregister_io_buf() just reads ublk_io or
> the request buffer only, so it is just fine for the two to run from other
> contexts.
Isn't it racy to check io->flags when it could be concurrently
modified by another thread (the ubq_daemon)?
Best,
Caleb
>
> >
> > > struct request *req;
> > > int ret;
> > >
> > > + if (!ublk_support_zero_copy(ubq))
> > > + return -EINVAL;
> > > +
> > > + if (!(io->flags & UBLK_IO_FLAG_OWNED_BY_SRV))
> > > + return -EINVAL;
> >
> > Every opcode except UBLK_IO_FETCH_REQ now checks io->flags &
> > UBLK_IO_FLAG_OWNED_BY_SRV. Maybe it would make sense to lift the check
> > up to __ublk_ch_uring_cmd() to avoid duplicating it?
>
> Good point.
>
>
> Thanks,
> Ming
>
