block: Improve blk_zone_write_plug_bio_merged()

Improve blk_zone_write_plug_bio_merged() to check that we succefully get
a reference on the zone write plug of the merged BIO, as expected since
for a merge we already have at least one request and one BIO referencing
the zone write plug. Comments in this function are also improved to
better explain the references to the BIO zone write plug.
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Link: https://lore.kernel.org/r/20240501110907.96950-11-dlemoal@kernel.orgSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-zoned.c

@@ -885,11 +885,16 @@ void blk_zone_write_plug_bio_merged(struct bio *bio)
 	bio_set_flag(bio, BIO_ZONE_WRITE_PLUGGING);
 
 	/*
-	 * Increase the plug reference count and advance the zone write
-	 * pointer offset.
+	 * Get a reference on the zone write plug of the target zone and advance
+	 * the zone write pointer offset. Given that this is a merge, we already
+	 * have at least one request and one BIO referencing the zone write
+	 * plug. So this should not fail.
 	 */
 	zwplug = disk_get_zone_wplug(bio->bi_bdev->bd_disk,
 				     bio->bi_iter.bi_sector);
+	if (WARN_ON_ONCE(!zwplug))
+		return;
+
 	spin_lock_irqsave(&zwplug->lock, flags);
 	zwplug->wp_offset += bio_sectors(bio);
 	spin_unlock_irqrestore(&zwplug->lock, flags);