Am 22.05.2025 um 16:46 hat Stefan Hajnoczi geschrieben: > On Thu, May 22, 2025 at 03:08:03PM +0200, 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 <kw...@redhat.com> > > --- > > v2: > > - Add a comment to explain retry scenarios [Stefan] > > - Handle -EAGAIN returned for suspended devices [Ben]
> > #if defined(__linux__)
> > +#if defined(DM_MPATH_PROBE_PATHS)
> > +static bool sgio_path_error(int ret, sg_io_hdr_t *io_hdr)
> > +{
> > + if (ret < 0) {
> > + switch (ret) {
> > + case -ENODEV:
> > + return true;
> > + case -EAGAIN:
> > + /*
> > + * The device is probably suspended. This happens while the dm
> > table
> > + * is reloaded, e.g. because a path is added or removed. This
> > is an
> > + * operation that should complete within 1ms, so just wait a
> > bit and
> > + * retry.
> > + *
> > + * If the device was suspended for another reason, we'll wait
> > and
> > + * retry SG_IO_MAX_RETRIES times. This is a tolerable delay
> > before
> > + * we return an error and potentially stop the VM.
> > + */
> > + qemu_co_sleep_ns(QEMU_CLOCK_REALTIME, 1000000);
>
> sgio_path_error() is missing coroutine_fn.
>
> Reviewed-by: Stefan Hajnoczi <stefa...@redhat.com>
Thanks, good catch. I already applied the patch and fixed it up to make
sure that I won't forget.
Kevin
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