block: Fix partition support for host aware zoned block devices

Commit b72053072c0b ("block: allow partitions on host aware zone
devices") introduced the helper function disk_has_partitions() to check
if a given disk has valid partitions. However, since this function result
directly depends on the disk partition table length rather than the
actual existence of valid partitions in the table, it returns true even
after all partitions are removed from the disk. For host aware zoned
block devices, this results in zone management support to be kept
disabled even after removing all partitions.

Fix this by changing disk_has_partitions() to walk through the partition
table entries and return true if and only if a valid non-zero size
partition is found.

Fixes: b72053072c0b ("block: allow partitions on host aware zone devices")
Cc: [e-mail] # 5.5
Reviewed-by: Damien Le Moal <[e-mail]>
Reviewed-by: Johannes Thumshirn <[e-mail]>
Reviewed-by: Christoph Hellwig <[e-mail]>
Signed-off-by: Shin'ichiro Kawasaki <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

block/genhd.c

···
 }
 EXPORT_SYMBOL_GPL(disk_map_sector_rcu);
 
+/**
+ * disk_has_partitions
+ * @disk: gendisk of interest
+ *
+ * Walk through the partition table and check if valid partition exists.
+ *
+ * CONTEXT:
+ * Don't care.
+ *
+ * RETURNS:
+ * True if the gendisk has at least one valid non-zero size partition.
+ * Otherwise false.
+ */
+bool disk_has_partitions(struct gendisk *disk)
+{
+	struct disk_part_tbl *ptbl;
+	int i;
+	bool ret = false;
+
+	rcu_read_lock();
+	ptbl = rcu_dereference(disk->part_tbl);
+
+	/* Iterate partitions skipping the whole device at index 0 */
+	for (i = 1; i < ptbl->len; i++) {
+		if (rcu_dereference(ptbl->part[i])) {
+			ret = true;
+			break;
+		}
+	}
+
+	rcu_read_unlock();
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(disk_has_partitions);
+
 /*
  * Can be deleted altogether. Later.
  *

include/linux/genhd.h

···
 		!(disk->flags & GENHD_FL_NO_PART_SCAN);
 }
 
-static inline bool disk_has_partitions(struct gendisk *disk)
-{
-	bool ret = false;
-
-	rcu_read_lock();
-	if (rcu_dereference(disk->part_tbl)->len > 1)
-		ret = true;
-	rcu_read_unlock();
-
-	return ret;
-}
-
 static inline dev_t disk_devt(struct gendisk *disk)
 {
 	return MKDEV(disk->major, disk->first_minor);
···
 
 extern struct hd_struct *disk_map_sector_rcu(struct gendisk *disk,
 					     sector_t sector);
+bool disk_has_partitions(struct gendisk *disk);
 
 /*
  * Macros to operate on percpu disk statistics: