[PATCH 1/2] ublk: add UBLK_F_NO_AUTO_PART_SCAN feature flag

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

Add a new feature flag UBLK_F_NO_AUTO_PART_SCAN to allow users to suppress
automatic partition scanning when starting a ublk device.

This is useful for network-backed devices where partition scanning
can cause issues:
- Partition scan triggers synchronous I/O during device startup
- If userspace server crashes during scan, recovery is problematic
- For remotely-managed devices, partition probing may not be needed

Users can manually trigger partition scanning later when appropriate
using standard tools (e.g., partprobe, blockdev --rereadpt).

Reported-by: Yoav Cohen <yoav@nvidia.com>
Link: https://lore.kernel.org/linux-block/DM4PR12MB63280C5637917C071C2F0D65A9A8A@DM4PR12MB6328.namprd12.prod.outlook.com/
Cc: stable@vger.kernel.org
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---

- suggest to backport to stable, which is useful for avoiding problematic
  recovery, also the change is simple enough

 drivers/block/ublk_drv.c      | 16 +++++++++++++---
 include/uapi/linux/ublk_cmd.h |  8 ++++++++
 2 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
index 78f3e22151b9..ca6ec8ed443f 100644
--- a/drivers/block/ublk_drv.c
+++ b/drivers/block/ublk_drv.c
@@ -73,7 +73,8 @@
 		| UBLK_F_AUTO_BUF_REG \
 		| UBLK_F_QUIESCE \
 		| UBLK_F_PER_IO_DAEMON \
-		| UBLK_F_BUF_REG_OFF_DAEMON)
+		| UBLK_F_BUF_REG_OFF_DAEMON \
+		| UBLK_F_NO_AUTO_PART_SCAN)
 
 #define UBLK_F_ALL_RECOVERY_FLAGS (UBLK_F_USER_RECOVERY \
 		| UBLK_F_USER_RECOVERY_REISSUE \
@@ -2930,8 +2931,13 @@ static int ublk_ctrl_start_dev(struct ublk_device *ub,
 
 	ublk_apply_params(ub);
 
-	/* don't probe partitions if any daemon task is un-trusted */
-	if (ub->unprivileged_daemons)
+	/*
+	 * Don't probe partitions if:
+	 * - any daemon task is un-trusted, or
+	 * - user explicitly requested to suppress partition scan
+	 */
+	if (ub->unprivileged_daemons ||
+	    (ub->dev_info.flags & UBLK_F_NO_AUTO_PART_SCAN))
 		set_bit(GD_SUPPRESS_PART_SCAN, &disk->state);
 
 	ublk_get_device(ub);
@@ -2947,6 +2953,10 @@ static int ublk_ctrl_start_dev(struct ublk_device *ub,
 	if (ret)
 		goto out_put_cdev;
 
+	/* allow user to probe partitions from userspace */
+	if (!ub->unprivileged_daemons &&
+	    (ub->dev_info.flags & UBLK_F_NO_AUTO_PART_SCAN))
+		clear_bit(GD_SUPPRESS_PART_SCAN, &disk->state);
 	set_bit(UB_STATE_USED, &ub->state);
 
 out_put_cdev:
diff --git a/include/uapi/linux/ublk_cmd.h b/include/uapi/linux/ublk_cmd.h
index ec77dabba45b..0827db14a215 100644
--- a/include/uapi/linux/ublk_cmd.h
+++ b/include/uapi/linux/ublk_cmd.h
@@ -311,6 +311,14 @@
  */
 #define UBLK_F_BUF_REG_OFF_DAEMON (1ULL << 14)
 
+/*
+ * If this feature is set, the kernel will not automatically scan for partitions
+ * when the device is started. This is useful for network-backed devices where
+ * partition scanning can cause deadlocks if the userspace server crashes during
+ * the scan. Users can manually trigger partition scanning later when appropriate.
+ */
+#define UBLK_F_NO_AUTO_PART_SCAN (1ULL << 15)
+
 /* device state */
 #define UBLK_S_DEV_DEAD	0
 #define UBLK_S_DEV_LIVE	1
-- 
2.47.0