zloop: fix write pointer calculation in zloop_forget_cache

The write pointer is absolute and in sector units, so we need to
convert it to a relative byte address first.

Fixes: c505448748f7 ("zloop: forget write cache on force removal")
Signed-off-by: Christoph Hellwig <[email protected]>
Reviewed-by: Damien Le Moal <[email protected]>
Reviewed-by: Johannes Thumshirn <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

Author: Christoph Hellwig

drivers/block/zloop.c

@@ -1401,8 +1401,17 @@ static void zloop_forget_cache(struct zloop_device *zlo)
 				zlo->disk->part0, ret);
 			continue;
 		}
-		if (old_wp < zone->wp)
-			zloop_truncate(file, old_wp);
+
+		if (old_wp > zone->wp)
+			continue;
+		/*
+		 * This should not happen, if we recored a full zone, it can't
+		 * be active.
+		 */
+		if (WARN_ON_ONCE(old_wp == ULLONG_MAX))
+			continue;
+
+		zloop_truncate(file, (old_wp - zone->start) << SECTOR_SHIFT);
 	}
 }