Re: [PATCH] IB/iser: explicitly set shost max_segment_size

[Christoph Hellwig <hch@xxxxxx>]

On Wed, Jun 05, 2019 at 05:02:09PM -0700, Sagi Grimberg wrote:
> if the lld does not explicitly sets this, scsi takes BLK_MAX_SEGMENT_SIZE
> and sets it using dma_set_max_seg_size(). In our case, this will affect
> all the rdma device consumers.
> 
> Fix it by setting shost max_segment_size according to the device
> capability.
> 
> Reported-by: Jason Gunthorpe <jgg@xxxxxxxx>
> Signed-off-by: Sagi Grimberg <sagi@xxxxxxxxxxx>
> ---
> This goes on top of hch patchset:
> "properly communicate queue limits to the DMA layer"
> 
> Normally this should go through the rdma tree, so we can
> either get it through jens with hch patchset. Alternatively
> this is a fix that should go to rc anyways?

I don't think this make sense.

> 
>  drivers/infiniband/ulp/iser/iscsi_iser.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/infiniband/ulp/iser/iscsi_iser.c b/drivers/infiniband/ulp/iser/iscsi_iser.c
> index 56848232eb81..2984a366dd7d 100644
> --- a/drivers/infiniband/ulp/iser/iscsi_iser.c
> +++ b/drivers/infiniband/ulp/iser/iscsi_iser.c
> @@ -653,6 +653,7 @@ iscsi_iser_session_create(struct iscsi_endpoint *ep,
>  						   SHOST_DIX_GUARD_CRC);
>  		}
>  
> +		shost->max_segment_size = ib_dma_max_seg_size(ib_dev);
>  		if (!(ib_dev->attrs.device_cap_flags & IB_DEVICE_SG_GAPS_REG))
>  			shost->virt_boundary_mask = ~MASK_4K;

We only really need this settings in the IB_DEVICE_SG_GAPS_REG case,
as the segement size is unlimited on the PRP-like scheme used by the
other MR types anyway, and set as such by the block layer.  I.e.g this
should become:

 		if (ib_dev->attrs.device_cap_flags & IB_DEVICE_SG_GAPS_REG)
			shost->max_segment_size = ib_dma_max_seg_size(ib_dev);
		else
  			shost->virt_boundary_mask = ~MASK_4K;