Re: [PATCH v2] file-posix: Probe paths and retry SG_IO on potential path errors

[Hanna Czenczek <hreitz@redhat.com>]

On 22.05.25 15:08, Kevin Wolf wrote:
> When scsi-block is used on a host multipath device, it runs into the
> problem that the kernel dm-mpath doesn't know anything about SCSI or
> SG_IO and therefore can't decide if a SG_IO request returned an error
> and needs to be retried on a different path. Instead of getting working
> failover, an error is returned to scsi-block and handled according to
> the configured error policy. Obviously, this is not what users want,
> they want working failover.
>
> QEMU can parse the SG_IO result and determine whether this could have
> been a path error, but just retrying the same request could just send it
> to the same failing path again and result in the same error.
>
> With a kernel that supports the DM_MPATH_PROBE_PATHS ioctl on dm-mpath
> block devices (queued in the device mapper tree for Linux 6.16), we can
> tell the kernel to probe all paths and tell us if any usable paths
> remained. If so, we can now retry the SG_IO ioctl and expect it to be
> sent to a working path.
>
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
> v2:
> - Add a comment to explain retry scenarios [Stefan]
> - Handle -EAGAIN returned for suspended devices [Ben]
>
>   block/file-posix.c | 115 ++++++++++++++++++++++++++++++++++++++++++++-
>   1 file changed, 114 insertions(+), 1 deletion(-)

Reviewed-by: Hanna Czenczek <hreitz@redhat.com>