md: skip redundant raid_disks update when value is unchanged

Calling update_raid_disks() with the same value as the current one
can trigger unnecessary work. For example, RAID1 will reallocate
resources such as the mempool for r1bio.

Signed-off-by: Abd-Alrhman Masalkhi <abd.masalkhi@gmail.com>
Link: https://patch.msgid.link/20260428130524.448063-1-abd.masalkhi@gmail.com
Signed-off-by: Yu Kuai <yukuai@fygo.io>

Author: masalkhi

drivers/md/md.c

@@ -4414,9 +4414,10 @@ raid_disks_store(struct mddev *mddev, const char *buf, size_t len)
 	err = mddev_suspend_and_lock(mddev);
 	if (err)
 		return err;
-	if (mddev->pers)
-		err = update_raid_disks(mddev, n);
-	else if (mddev->reshape_position != MaxSector) {
+	if (mddev->pers) {
+		if (n != mddev->raid_disks)
+			err = update_raid_disks(mddev, n);
+	} else if (mddev->reshape_position != MaxSector) {
 		struct md_rdev *rdev;
 		int olddisks = mddev->raid_disks - mddev->delta_disks;