Re: [PATCH 0/2] dm mpath: Interface for explicit probing of active paths

[Christoph Hellwig <hch@infradead.org>]

On Tue, May 13, 2025 at 12:29:51PM -0400, Benjamin Marzinski wrote:
> On Mon, May 12, 2025 at 11:49:19PM -0700, Christoph Hellwig wrote:
> > On Tue, May 13, 2025 at 08:32:12AM +0200, Hannes Reinecke wrote:
> > > Reservations and stuff.
> > 
> > They should use the kernel persistent reservation API.
> 
> Currently QEMU isn't sending Persistent Reservation requests to
> multipath devices at all. It's sending those directly to the underlying
> scsi devices. The issue here is with all the other SCSI commands that
> users want to send to their SCSI passthrough device that is actually a
> multipath device on top of a number of SCSI paths. They expect to
> get back the actual sense and status information, so QEMU needs to
> send them via SG_IOs.

And that's the problem.  There is plenty of path (I_T_L) nexus
specific information in SCSI, and just trying to glob it back
together above means you'll get path specific information in something
that doesn't claim to be multi-pathing and will get random and
changing results depending on the underlying path selection.

> Without reading that sense and status information in kernel, the
> multipath target can't know if it needs to fail a path and retry the
> ioctl down a different path.

The blk_status_t has the information to fail over.  But the whole
idea of implementing SG_IO in dm-mpath or any other stacking layer
is just really, really dangerous.