[PATCH v3 0/1] scsi: sas: fix mkfs.xfs failure due to bogus optimal_io_size

[Ionut Nechita (Wind River)]

From: Ionut Nechita <ionut.nechita@xxxxxxxxxxxxx>

v3 (per Christoph Hellwig's review of v2):
  - Extracted the opt_sectors logic into a dedicated sas_dma_opt_sectors()
    helper function, clearly split out from sas_host_setup().
  - Added rounddown_pow_of_two() on the DMA optimal mapping size so that
    the resulting opt_sectors is always a power of two, keeping filesystem
    geometry calculations clean.
  - Added #include <linux/log2.h> for rounddown_pow_of_two().

v2:
  - Dropped the dma_opt_mapping_size() change per Robin Murphy's feedback:
    the DMA core semantics are correct, the bug is in the caller.
  - Dropped the nvme-pci patch (no longer needed).
  - Single patch now fixes the actual bug in scsi_transport_sas.c by
    checking if dma_opt_mapping_size() == dma_max_mapping_size() before
    setting opt_sectors.  When they are equal, no backend provided a
    real hint.
  - Added concrete values from the affected system (Dell PowerEdge R750,
    mpt3sas, SAMSUNG MZILT800HBHQ0D3) to the commit message.

v1 feedback summary:
  - Robin Murphy: dma_opt_mapping_size() semantics are correct; if no
    restriction exists, the largest efficient size IS the largest size.
    Fix the caller, not the common code.
  - John Garry: Asked for concrete max_sectors/opt_sectors values and
    questioned whether sd_revalidate_disk() would override opt_sectors
    via opt_xfer_blocks.
  - Damien Le Moal: Suggested min_not_zero() for nvme-pci (now moot).

Answer to John's question (from v2, still relevant):
  The SAS disks on this system do not report Optimal Transfer Length in
  VPD page B0, so sdkp->opt_xfer_blocks = 0.  sd_revalidate_disk() uses
  min_not_zero(0, opt_sectors) which returns opt_sectors, propagating
  the bogus value.  Observed values:

    shost->max_sectors      = 32767
    opt_sectors             = 32767  (capped at max_sectors)
    optimal_io_size         = 16773120  (visible in lsblk --topology)
    minimum_io_size         = 8192

  mkfs.xfs computes swidth=4095, sunit=2, fails because 4095 % 2 != 0.

  John also noted that io_opt should ideally be a multiple of io_min and
  that blk_validate_limits() should enforce it, but acknowledged that
  doing so would mask problems like this one.

Test environment:
  - Dell PowerEdge R750
  - SAS Controller: Broadcom/LSI mpt3sas (SAS3816, FW 33.15.00.00)
  - Disks: SAMSUNG MZILT800HBHQ0D3 (800GB SCSI SAS SSD)
  - Kernel: 6.12.0-1-amd64 with intel_iommu=off
  - IOMMU: Disabled (DMAR: IOMMU disabled), default domain: Passthrough

Based on linux-next (next-20260318).

Link: https://lore.kernel.org/lkml/20260316203956.64515-1-ionut.nechita@xxxxxxxxxxxxx/
Link: https://lore.kernel.org/all/20260318074314.17372-1-ionut.nechita@xxxxxxxxxxxxx/

Ionut Nechita (1):
  scsi: sas: skip opt_sectors when DMA reports no real optimization hint

 drivers/scsi/scsi_transport_sas.c | 36 +++++++++++++++++++++++++++----
 1 file changed, 32 insertions(+), 4 deletions(-)

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2.43.0