Re: [PATCH v1 0/2] dma: fix dma_opt_mapping_size() returning bogus value when no backend hint exists

[Damien Le Moal]

On 3/17/26 18:11, John Garry wrote:
> On 16/03/2026 20:39, Ionut Nechita (Wind River) wrote:
>> dma_opt_mapping_size() currently returns min(dma_max_mapping_size(),
>> SIZE_MAX) when neither an IOMMU nor a DMA ops opt_mapping_size callback
>> is present.  That value is the DMA maximum, not an optimal transfer
>> size, yet callers treat it as a genuine optimization hint.
>>
>> The concrete problem shows up on SAS controllers (e.g. mpt3sas) running
>> with IOMMU in passthrough mode.  The bogus value propagates through
>> scsi_transport_sas into Scsi_Host.opt_sectors and then into the block
>> device's optimal_io_size.  mkfs.xfs picks it up, computes
>> swidth=4095 / sunit=2, and fails with:
>>
>>    XFS: SB stripe unit sanity check failed
>>
>> making it impossible to create filesystems during system bootstrap.
> 
> For SAS controllers, don't we limit shost->opt_sectors at 
> shost->max_sectors, and then in sd_revalidate_disk() this value is 
> ignored as sdkp->opt_xfer_blocks would be smaller, right?
> 
> What value are you seeing for max_sectors and opt_sectors? That mpt3sas 
> driver seems to have many methods to set max_sectors.

And mpi3mr is also very similar.

> 
> Thanks,
> John
> 
>>
>> Patch 1 changes dma_opt_mapping_size() to return 0 ("no preference")
>> when no backend provides a real hint.
>>
>> Patch 2 adjusts the only other in-tree caller (nvme-pci) to handle the
>> new 0 return value, falling back to its existing default instead of
>> setting max_hw_sectors to 0.
>>
>> Note: the scsi_transport_sas caller (the one that triggers the XFS
>> issue) already handles 0 safely.  It passes the return value through
>> min_t() into shost->opt_sectors, which becomes 0; sd.c then feeds that
>> into min_not_zero() when computing io_opt, so a zero opt_sectors is
>> correctly treated as "no preference" and ignored.
>>
>> Based on linux-next (next-20260316).
>>
>> Ionut Nechita (2):
>>    dma: return 0 from dma_opt_mapping_size() when no real hint exists
>>    nvme-pci: handle dma_opt_mapping_size() returning 0
>>
>>   drivers/nvme/host/pci.c | 15 ++++++++++-----
>>   kernel/dma/mapping.c    | 13 ++++++++-----
>>   2 files changed, 18 insertions(+), 10 deletions(-)
>>
> 
> 


-- 
Damien Le Moal
Western Digital Research