Re: [PATCH v2 1/4] ublk: cleanup partition flags handling and introduce UBLK_F_NO_PARTITONS

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

On Fri, Feb 27, 2026 at 10:29:56AM -0800, Caleb Sander Mateos wrote:
> On Fri, Feb 27, 2026 at 7:07 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.
> >
> > 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 enabled,
> > automatic initial scan can be disabled via UBLK_F_NO_AUTO_PART_SCAN flag.
> > Partitions can be disabled via UBLK_F_NO_PARTITIONS 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_NO_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      | 23 +++++++++--------------
> >  include/uapi/linux/ublk_cmd.h |  3 +++
> >  2 files changed, 12 insertions(+), 14 deletions(-)
> >
> > -v1->v2:
> >   - remove code duplication and extra flag settings
> >   - switch from out-in to out-out to preserve backwards compatibility
> >
> >
> > diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
> > index 3c918db4905c..36da63ba2a57 100644
> > --- a/drivers/block/ublk_drv.c
> > +++ b/drivers/block/ublk_drv.c
> > @@ -81,6 +81,7 @@
> >                 | (IS_ENABLED(CONFIG_BLK_DEV_INTEGRITY) ? UBLK_F_INTEGRITY : 0) \
> >                 | UBLK_F_SAFE_STOP_DEV \
> >                 | UBLK_F_BATCH_IO \
> > +               | UBLK_F_NO_PARTITIONS \
> >                 | UBLK_F_NO_AUTO_PART_SCAN)
> >
> >  #define UBLK_F_ALL_RECOVERY_FLAGS (UBLK_F_USER_RECOVERY \
> > @@ -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);
> >  }
> >
> > @@ -4409,10 +4405,10 @@ 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.
> >          */
> > -       set_bit(GD_SUPPRESS_PART_SCAN, &disk->state);
> > +       disk->flags |= GENHD_FL_NO_PART;
> >
> >         ublk_get_device(ub);
> >         ub->dev_info.state = UBLK_S_DEV_LIVE;
> > @@ -4429,12 +4425,11 @@ 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)
> > +       if (!ub->unprivileged_daemons && !(ub->dev_info.flags & UBLK_F_NO_PARTITIONS)) {
> > +               /* Enable partitions on device */
> > +               disk->flags &= ~GENHD_FL_NO_PART;
> 
> Is it safe to modify disk->flags after the disk has been added? This looks racy.

Yeah, GENHD_FL_NO_PART should cover unprivileged_daemons & UBLK_F_NO_PARTITIONS only
in static way.

Normal partition scanning still need GD_SUPPRESS_PART_SCAN.


Thanks,
Ming