--- block/genhd.c

+++ block/genhd.c

@@ -624,6 +624,26 @@ void add_disk(struct gendisk *disk)

"bdi");

WARN_ON(retval);

+ /*

+ * Limit default readahead size for small devices.

+ * disk size readahead size

+ * 1M 8k

+ * 4M 16k

+ * 16M 32k

+ * 64M 64k

+ * 256M 128k

+ * 1G 256k

+ * 4G 512k

+ * 16G 1024k

+ * 64G 2048k

+ * 256G 4096k

+ */

+ if (get_capacity(disk)) {

+ unsigned long size = get_capacity(disk) >> 9;

+ size = 1UL << (ilog2(size) / 2);

+ bdi->ra_pages = min(bdi->ra_pages, size);

+ }

+

disk_add_events(disk);

}

EXPORT_SYMBOL(add_disk);