diff --git a/drivers/scsi/scsi_transport_sas.c b/drivers/scsi/scsi_transport_sas.c
index d8f2377b017f..d689b9ed08a6 100644
--- a/drivers/scsi/scsi_transport_sas.c
+++ b/drivers/scsi/scsi_transport_sas.c
@@ -27,6 +27,7 @@
 #include <linux/module.h>
 #include <linux/jiffies.h>
 #include <linux/err.h>
+#include <linux/log2.h>
 #include <linux/slab.h>
 #include <linux/string.h>
 #include <linux/blkdev.h>
@@ -220,12 +221,45 @@ static int sas_bsg_initialize(struct Scsi_Host *shost, struct sas_rphy *rphy)
  * SAS host attributes
  */
 
+/*
+ * Set shost->opt_sectors from the DMA optimal mapping size, but only
+ * when dma_opt_mapping_size() is strictly less than dma_max_mapping_size(),
+ * indicating a genuine optimization hint from an IOMMU or DMA backend.
+ * When the two are equal (e.g. IOMMU disabled / passthrough), no real
+ * hint exists, so leave opt_sectors at 0 to avoid bogus optimal_io_size
+ * values that break filesystem geometry (e.g. mkfs.xfs stripe alignment).
+ */
+static void sas_dma_setup_opt_sectors(struct Scsi_Host *shost)
+{
+	struct device *dma_dev = shost->dma_dev;
+	size_t opt = dma_opt_mapping_size(dma_dev);
+	size_t max = dma_max_mapping_size(dma_dev);
+	unsigned int opt_sectors;
+
+	/* opt >= max means no real hint was provided by the DMA layer */
+	if (opt >= max)
+		return;
+
+	/* Clamp to max_sectors to avoid overflow in sector arithmetic */
+	opt_sectors = min_t(unsigned int, opt >> SECTOR_SHIFT,
+			    shost->max_sectors);
+
+	/* Guard against zero before rounddown_pow_of_two() */
+	if (!opt_sectors)
+		return;
+
+	/*
+	 * Round down to power-of-two so filesystem geometry calculations
+	 * (e.g. XFS stripe width/unit) always produce clean divisors.
+	 */
+	shost->opt_sectors = rounddown_pow_of_two(opt_sectors);
+}
+
 static int sas_host_setup(struct transport_container *tc, struct device *dev,
 			  struct device *cdev)
 {
 	struct Scsi_Host *shost = dev_to_shost(dev);
 	struct sas_host_attrs *sas_host = to_sas_host_attrs(shost);
-	struct device *dma_dev = shost->dma_dev;
 
 	INIT_LIST_HEAD(&sas_host->rphy_list);
 	mutex_init(&sas_host->lock);
@@ -237,10 +271,7 @@ static int sas_host_setup(struct transport_container *tc, struct device *dev,
 		dev_printk(KERN_ERR, dev, "fail to a bsg device %d\n",
 			   shost->host_no);
 
-	if (dma_dev->dma_mask) {
-		shost->opt_sectors = min_t(unsigned int, shost->max_sectors,
-				dma_opt_mapping_size(dma_dev) >> SECTOR_SHIFT);
-	}
+	sas_dma_setup_opt_sectors(shost);
 
 	return 0;
 }