Re: [PATCH 1/4] ublk: cleanup flag handling and fix UBLK_F_NO_AUTO_PART_SCAN

[Ming Lei <ming.lei@redhat.com>]

On Mon, Feb 23, 2026 at 07:59:54AM -0800, Caleb Sander Mateos wrote:
> On Mon, Feb 23, 2026 at 4:17 AM Alexander Atanasov <alex@zazolabs.com> wrote:
> >
> > Currently the code misuse GD_SUPPRESS_PART_SCAN flag
> > as it tries to use it as a switch for the auto partition scan.
> > To fully disable creation of partitions GENHD_FL_NO_PART must
> > be used, switch code to use it instead GD_SUPPRESS_PART_SCAN.
> >
> > Using the new flags rules for partitions become:
> >  - Unprivileged daemons - never scan partitions, they have
> > GENHD_FL_NO_PART always set - they are devices without partitions.
> >
> >  - Trusted daemons - by default have partitions disabled.
> > Partitions can be enabled via UBLK_F_PARTITIONS flag and
> > automatic initial scan can be requested via UBLK_F_AUTO_PART_SCAN flag.
> >
> > Rework the code to work as described above:
> > - remove checks in ublk_partition_scan_work and rely on
> > the caller to schedule the work only if it has to be done.
> > - set GENHD_FL_NO_PART on unprivileged devices
> > - set GENHD_FL_NO_PART depending on UBLK_F_PARTITIONS on trusted
> > devices.
> >
> > Fixes: 8443e2087e70 ("ublk: add UBLK_F_NO_AUTO_PART_SCAN feature flag")
> > Signed-off-by: Alexander Atanasov <alex@zazolabs.com>
> > ---
> >  drivers/block/ublk_drv.c      | 34 +++++++++++++++++++---------------
> >  include/uapi/linux/ublk_cmd.h |  7 +++++--
> >  2 files changed, 24 insertions(+), 17 deletions(-)
> >
> > diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
> > index 3c918db4905c..d0c646eb9435 100644
> > --- a/drivers/block/ublk_drv.c
> > +++ b/drivers/block/ublk_drv.c
> > @@ -81,7 +81,8 @@
> >                 | (IS_ENABLED(CONFIG_BLK_DEV_INTEGRITY) ? UBLK_F_INTEGRITY : 0) \
> >                 | UBLK_F_SAFE_STOP_DEV \
> >                 | UBLK_F_BATCH_IO \
> > -               | UBLK_F_NO_AUTO_PART_SCAN)
> > +               | UBLK_F_PARTITIONS \
> > +               | UBLK_F_AUTO_PART_SCAN)
> >
> >  #define UBLK_F_ALL_RECOVERY_FLAGS (UBLK_F_USER_RECOVERY \
> >                 | UBLK_F_USER_RECOVERY_REISSUE \
> > @@ -2361,14 +2362,9 @@ static void ublk_partition_scan_work(struct work_struct *work)
> >         if (!disk)
> >                 return;
> >
> > -       if (WARN_ON_ONCE(!test_and_clear_bit(GD_SUPPRESS_PART_SCAN,
> > -                                            &disk->state)))
> > -               goto out;
> > -
> >         mutex_lock(&disk->open_mutex);
> >         bdev_disk_changed(disk, false);
> >         mutex_unlock(&disk->open_mutex);
> > -out:
> >         ublk_put_disk(disk);
> >  }
> >
> > @@ -4409,9 +4405,13 @@ static int ublk_ctrl_start_dev(struct ublk_device *ub,
> >          * wait while holding ub->mutex, which can deadlock with other
> >          * operations that need the mutex. Defer partition scan to async
> >          * work.
> > -        * For unprivileged daemons, keep GD_SUPPRESS_PART_SCAN set
> > -        * permanently.
> > +        * For unprivileged daemons, set GENHD_FL_NO_PART to
> > +        * disable partitions.
> >          */
> > +       if (ub->unprivileged_daemons)
> > +               disk->flags |= GENHD_FL_NO_PART;
> > +
> > +       /* Disable partition scans */
> >         set_bit(GD_SUPPRESS_PART_SCAN, &disk->state);
> >
> >         ublk_get_device(ub);
> > @@ -4429,13 +4429,17 @@ static int ublk_ctrl_start_dev(struct ublk_device *ub,
> >
> >         set_bit(UB_STATE_USED, &ub->state);
> >
> > -       /* Skip partition scan if disabled by user */
> > -       if (ub->dev_info.flags & UBLK_F_NO_AUTO_PART_SCAN) {
> > -               clear_bit(GD_SUPPRESS_PART_SCAN, &disk->state);
> > -       } else {
> > -               /* Schedule async partition scan for trusted daemons */
> > -               if (!ub->unprivileged_daemons)
> > -                       schedule_work(&ub->partition_scan_work);
> > +       if (!ub->unprivileged_daemons) {
> > +               if (!(ub->dev_info.flags & UBLK_F_PARTITIONS)) {
> > +                       /* disable partitions on trusted device */
> > +                       disk->flags |= GENHD_FL_NO_PART;
> > +               } else {
> > +                       /* Enable partition scan on device */
> > +                       clear_bit(GD_SUPPRESS_PART_SCAN, &disk->state);
> > +                       /* Schedule auto partition scan if requested */
> > +                       if (ub->dev_info.flags & UBLK_F_AUTO_PART_SCAN)
> > +                               schedule_work(&ub->partition_scan_work);
> > +               }
> >         }
> >
> >  out_put_cdev:
> > diff --git a/include/uapi/linux/ublk_cmd.h b/include/uapi/linux/ublk_cmd.h
> > index a88876756805..467ecaa20ac1 100644
> > --- a/include/uapi/linux/ublk_cmd.h
> > +++ b/include/uapi/linux/ublk_cmd.h
> > @@ -367,8 +367,11 @@
> >   */
> >  #define UBLK_F_SAFE_STOP_DEV   (1ULL << 17)
> >
> > -/* Disable automatic partition scanning when device is started */
> > -#define UBLK_F_NO_AUTO_PART_SCAN (1ULL << 18)
> > +/* Enable partitions on the device */
> > +#define UBLK_F_PARTITIONS (1ULL << 18)
> 
> This is not backwards compatible. Existing ublk servers expect their
> devices to support partitioning. If you want to introduce a feature
> flag to control this, it needs to be opt-out rather than opt-in.
> UBLK_F_AUTO_PART_SCAN probably also needs to remain as
> UBLK_F_NO_AUTO_PART_SCAN.

UBLK_F_AUTO_PART_SCAN is just merged in 7.0-rc, which isn't released yet,
so it is fine to change it before 7.0 formal release, IMO.


Thanks,
Ming