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:
> 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. QEMU can read this information, but it
> doesn't know what path the multipath device send the ioctl down. This
> patch just gives users a way to check the paths in the active pathgroup
> (which all should be able to handle IO) and fail those that can't.
> While QEMU is the driver of this, it's completely general functionality.

As just replied to Martin the problem is that this setup fundamentally
can't work.  Either you pass a SCSI devices through, which should
(mostly, there are a few warts) work.  Or you want host side
multipathing, in which case you must pass through the block device
abstraction and not a SCSI one, or at least do a full emulation of
the SCSI interfaces instead of pretending to pass it through.

The root of all evil here is that dm-multipath tries to pass through
SG_IO, which is dangerous for all but the most trivial commands.