Re: [dm-devel] [RFC PATCH v2 2/2] dm: add CONFIG_DM_MULTIPATH_SG_IO - failover for SG_IO on dm-multipath

[Martin Wilck <mwilck@suse.com>]

On Thu, 2021-04-29 at 09:32 -0700, Bart Van Assche wrote:
> On 4/29/21 8:50 AM, mwilck@suse.com wrote:
> > +       if (hdr.dxfer_len > (queue_max_hw_sectors(bdev->bd_disk-
> > >queue) << 9))
> > +               return -EIO;
> 
> How about using SECTOR_SHIFT instead of the number 9?

no problem. That line was just copied from the scsi_ioctl code.

> 
> > +               /*
> > +                * Errors resulting from invalid parameters
> > shouldn't be retried
> > +                * on another path.
> > +                */
> > +               switch (rc) {
> > +               case -ENOIOCTLCMD:
> > +               case -EFAULT:
> > +               case -EINVAL:
> > +               case -EPERM:
> > +                       goto out;
> > +               default:
> > +                       break;
> > +               }
> 
> Will -ENOMEM result in an immediate retry? Is that what's desired?

No, I overlooked that case. Thanks for pointing this out. 

> 
> > +               if (rhdr.info & SG_INFO_CHECK) {
> > +                       int result;
> > +                       blk_status_t sts;
> > +
> > +                       __set_status_byte(&result, rhdr.status);
> > +                       __set_msg_byte(&result, rhdr.msg_status);
> > +                       __set_host_byte(&result, rhdr.host_status);
> > +                       __set_driver_byte(&result,
> > rhdr.driver_status);
> > +
> > +                       sts = __scsi_result_to_blk_status(&result,
> > result);
> > +                       rhdr.host_status = host_byte(result);
> > +
> > +                       /* See if this is a target or path error.
> > */
> > +                       if (sts == BLK_STS_OK)
> > +                               rc = 0;
> > +                       else if (blk_path_error(sts))
> > +                               rc = -EIO;
> > +                       else {
> > +                               rc = blk_status_to_errno(sts);
> > +                               goto out;
> > +                       }
> > +               }
> 
> Will SAM_STAT_CHECK_CONDITION be treated as an I/O error? Is that
> what's
> desired? If not, does that mean that scsi_result_to_blk_status()
> shouldn't be used but instead that a custom SCSI result conversion is
> needed?

This mimics the logic for regular SCSI block I/O. By default, CHECK
CONDITION indeed maps to a BLK_STS_IO_ERR, and will be treated as a
path error. As you probably know, there are some exceptions that are
handled in the SCSI mid-layer beforehand. For example, check_sense()
sets DID_TARGET_FAILURE or DID_MEDIUM_ERROR for certain sense codes,
which map to target errors. So yes, I think this is correct.

> If __scsi_result_to_blk_status() is the right function to call, how
> about making that function accept the driver status, host status, msg
> and SAM status as four separate arguments such that the
> __set_*_byte()
> calls can be left out?
> 
> > +                       char *argv[2] = { "fail_path", bdbuf };
> 
> Can the above array be declared static?

How would that work? The function needs to be reentrant, and bdbuf is
on the stack. I don't see an issue here, it's really just two pointers
on the stack.

Regards,
Martin