block, loop: support partitions without scanning

Historically we did distinguish between a flag that surpressed partition
scanning, and a combinations of the minors variable and another flag if
any partitions were supported.  This was generally confusing and doesn't
make much sense, but some corner case uses of the loop driver actually
do want to support manually added partitions on a device that does not
actively scan for partitions.  To make things worsee the loop driver
also wants to dynamically toggle the scanning for partitions on a live
gendisk, which makes the disk->flags updates non-atomic.

Introduce a new GD_SUPPRESS_PART_SCAN bit in disk->state that disables
just scanning for partitions, and toggle that instead of GENHD_FL_NO_PART
in the loop driver.

Fixes: 1ebe2e5f ("block: remove GENHD_FL_EXT_DEVT")
Reported-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20220527055806.1972352-1-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/genhd.c

@@ -385,6 +385,8 @@ int disk_scan_partitions(struct gendisk *disk, fmode_t mode)
 
 	if (disk->flags & (GENHD_FL_NO_PART | GENHD_FL_HIDDEN))
 		return -EINVAL;
+	if (test_bit(GD_SUPPRESS_PART_SCAN, &disk->state))
+		return -EINVAL;
 	if (disk->open_partitions)
 		return -EBUSY;
 

drivers/block/loop.c

@@ -1102,7 +1102,7 @@ static int loop_configure(struct loop_device *lo, fmode_t mode,
 		lo->lo_flags |= LO_FLAGS_PARTSCAN;
 	partscan = lo->lo_flags & LO_FLAGS_PARTSCAN;
 	if (partscan)
-		lo->lo_disk->flags &= ~GENHD_FL_NO_PART;
+		clear_bit(GD_SUPPRESS_PART_SCAN, &lo->lo_disk->state);
 
 	loop_global_unlock(lo, is_loop);
 	if (partscan)
@@ -1198,7 +1198,7 @@ static void __loop_clr_fd(struct loop_device *lo, bool release)
 	 */
 	lo->lo_flags = 0;
 	if (!part_shift)
-		lo->lo_disk->flags |= GENHD_FL_NO_PART;
+		set_bit(GD_SUPPRESS_PART_SCAN, &lo->lo_disk->state);
 	mutex_lock(&lo->lo_mutex);
 	lo->lo_state = Lo_unbound;
 	mutex_unlock(&lo->lo_mutex);
@@ -1308,7 +1308,7 @@ loop_set_status(struct loop_device *lo, const struct loop_info64 *info)
 
 	if (!err && (lo->lo_flags & LO_FLAGS_PARTSCAN) &&
 	     !(prev_lo_flags & LO_FLAGS_PARTSCAN)) {
-		lo->lo_disk->flags &= ~GENHD_FL_NO_PART;
+		clear_bit(GD_SUPPRESS_PART_SCAN, &lo->lo_disk->state);
 		partscan = true;
 	}
 out_unlock:
@@ -2009,7 +2009,7 @@ static int loop_add(int i)
 	 * userspace tools. Parameters like this in general should be avoided.
 	 */
 	if (!part_shift)
-		disk->flags |= GENHD_FL_NO_PART;
+		set_bit(GD_SUPPRESS_PART_SCAN, &disk->state);
 	mutex_init(&lo->lo_mutex);
 	lo->lo_number		= i;
 	spin_lock_init(&lo->lo_lock);

include/linux/blkdev.h

@@ -147,6 +147,7 @@ struct gendisk {
 #define GD_DEAD				2
 #define GD_NATIVE_CAPACITY		3
 #define GD_ADDED			4
+#define GD_SUPPRESS_PART_SCAN		5
 
 	struct mutex open_mutex;	/* open/close mutex */
 	unsigned open_partitions;	/* number of open partitions */