scsi: sas: Skip opt_sectors when DMA reports no real optimization hint

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) does 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 introducing a sas_dma_setup_opt_sectors() helper that sets
opt_sectors only when dma_opt_mapping_size() is strictly less than
dma_max_mapping_size(), indicating a genuine DMA optimization
constraint.

The helper computes min(opt_sectors, max_sectors) first, then rounds
down to a power of two so that filesystem geometry calculations always
produce clean results.

When the two DMA values are equal, no backend provided a real hint, so
opt_sectors stays at 0 ("no preference").

[mkp: implemented hch's suggestion]

Fixes: 4cbfca5f7750 ("scsi: scsi_transport_sas: cap shost opt_sectors according to DMA optimal limit")
Cc: stable@vger.kernel.org
Reviewed-by: John Garry <john.g.garry@oracle.com>
Signed-off-by: Ionut Nechita <ionut.nechita@windriver.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://patch.msgid.link/20260519135238.373784-2-ionut.nechita@windriver.com
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

0 files changed, 0 insertions, 0 deletions