[PATCH 1/1] scsi: sas: skip opt_sectors when DMA reports no real optimization hint

[Ionut Nechita (Wind River)]

From: Ionut Nechita <ionut.nechita@xxxxxxxxxxxxx>

sas_host_setup() unconditionally sets shost->opt_sectors from
dma_opt_mapping_size().  When the IOMMU is disabled or in passthrough
mode and no DMA ops provide an opt_mapping_size callback,
dma_opt_mapping_size() returns min(dma_max_mapping_size(), SIZE_MAX)
which equals dma_max_mapping_size() — a hard upper bound, not an
optimization hint.

On a Dell PowerEdge R750 with mpt3sas (Broadcom SAS3816, FW 33.15.00.00)
and intel_iommu=off the following values are observed:

  dma_opt_mapping_size()  = dma_max_mapping_size() (no real hint)
  shost->max_sectors      = 32767
  opt_sectors             = min(32767, huge >> 9) = 32767
  optimal_io_size         = 32767 << 9 = 16776704
                          → round_down(16776704, 4096) = 16773120

The SAS disk (SAMSUNG MZILT800HBHQ0D3) do not report an
Optimal Transfer Length in VPD page B0,so sdkp->opt_xfer_blocks remains 0.
sd_revalidate_disk() then uses min_not_zero(0, opt_sectors) = opt_sectors,
propagating the bogus value into the block device's optimal_io_size
(visible as OPT-IO = 16773120 in lsblk --topology).

mkfs.xfs picks up optimal_io_size and minimum_io_size and computes:

  swidth = 16773120 / 4096 = 4095
  sunit  = 8192 / 4096     = 2

Since 4095 % 2 != 0, XFS rejects the geometry:

  SB stripe unit sanity check failed

This makes it impossible to create XFS filesystems (e.g. for
/var/lib/docker) during system bootstrap.

Fix this by only setting opt_sectors when dma_opt_mapping_size() returns
a value strictly less than dma_max_mapping_size(), which indicates a
genuine DMA optimization constraint from an IOMMU or DMA ops backend.
When they are equal, no backend provided a real hint, so leave
opt_sectors at its default of 0 ("no preference").

Fixes: 4cbfca5f7750 ("scsi: scsi_transport_sas: cap shost opt_sectors according to DMA optimal limit")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Ionut Nechita <ionut.nechita@xxxxxxxxxxxxx>
---
 drivers/scsi/scsi_transport_sas.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/drivers/scsi/scsi_transport_sas.c b/drivers/scsi/scsi_transport_sas.c
index 12124f9d5ccd..6b4de5116feb 100644
--- a/drivers/scsi/scsi_transport_sas.c
+++ b/drivers/scsi/scsi_transport_sas.c
@@ -240,8 +240,20 @@ static int sas_host_setup(struct transport_container *tc, struct device *dev,
 			   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);
+		size_t opt = dma_opt_mapping_size(dma_dev);
+
+		/*
+		 * Only set opt_sectors when the DMA layer reports a
+		 * genuine optimization constraint.  When opt equals
+		 * dma_max_mapping_size() no backend provided a real
+		 * hint — the value is just the DMA maximum, which is
+		 * not useful as an optimal I/O size and can cause
+		 * mkfs.xfs to compute invalid stripe geometry.
+		 */
+		if (opt < dma_max_mapping_size(dma_dev))
+			shost->opt_sectors = min_t(unsigned int,
+					shost->max_sectors,
+					opt >> SECTOR_SHIFT);
 	}
 
 	return 0;
-- 
2.53.0