block: Stack optimal I/O size

When stacking block devices ensure that optimal I/O size is scaled
accordingly.

Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

block/blk-settings.c

@@ -7,6 +7,7 @@
 #include <linux/bio.h>
 #include <linux/blkdev.h>
 #include <linux/bootmem.h>	/* for max_pfn/max_low_pfn */
+#include <linux/gcd.h>
 
 #include "blk.h"
 
@@ -520,6 +521,16 @@ int blk_stack_limits(struct queue_limits *t, struct queue_limits *b,
 		return -1;
 	}
 
+	/* Find lcm() of optimal I/O size */
+	if (t->io_opt && b->io_opt)
+		t->io_opt = (t->io_opt * b->io_opt) / gcd(t->io_opt, b->io_opt);
+	else if (b->io_opt)
+		t->io_opt = b->io_opt;
+
+	/* Verify that optimal I/O size is a multiple of io_min */
+	if (t->io_min && t->io_opt % t->io_min)
+		return -1;
+
 	return 0;
 }
 EXPORT_SYMBOL(blk_stack_limits);