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

[Benjamin Marzinski <bmarzins@redhat.com>]

On Tue, Apr 29, 2025 at 06:50:18PM +0200, Kevin Wolf wrote:
> Multipath cannot directly provide failover for ioctls in the kernel
> because it doesn't know what each ioctl means and which result could
> indicate a path error. Userspace generally knows what the ioctl it
> issued means and if it might be a path error, but neither does it know
> which path the ioctl took nor does it necessarily have the privileges to
> fail a path using the control device.
> 
> In order to allow userspace to address this situation, implement a
> DM_MPATH_PROBE_PATHS ioctl that prompts the dm-mpath driver to probe all
> active paths in the current path group to see whether they still work,
> and fail them if not. If this returns success, userspace can retry the
> ioctl and expect that the previously hit bad path is now failed (or
> working again).
> 
> The immediate motivation for this is the use of SG_IO in QEMU for SCSI
> passthrough. Following a failed SG_IO ioctl, QEMU will trigger probing
> to ensure that all active paths are actually alive, so that retrying
> SG_IO at least has a lower chance of failing due to a path error.
> However, the problem is broader than just SG_IO (it affects any ioctl),
> and if applications need failover support for other ioctls, the same
> probing can be used.
> 
> This is not implemented on the DM control device, but on the DM mpath
> block devices, to allow all users who have access to such a block device
> to make use of this interface, specifically to implement failover for
> ioctls. For the same reason, it is also unprivileged. Its implementation
> is effectively just a bunch of reads, which could already be issued by
> userspace, just without any guarantee that all the rights paths are
> selected.
> 
> The probing implemented here is done fully synchronously path by path;
> probing all paths concurrently is left as an improvement for the future.
> 
> Co-developed-by: Hanna Czenczek <hreitz@redhat.com>
> Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>

Reviewed-by: Benjamin Marzinski <bmarzins@redhat.com>