Re: [PATCH 1/3] multipathd: monitor new multipath dev even if we can't update it

[Benjamin Marzinski <bmarzins@redhat.com>]

On Tue, Mar 25, 2025 at 11:33:12PM +0100, Martin Wilck wrote:
> On Mon, 2025-03-24 at 16:55 -0400, Benjamin Marzinski wrote:
> > If a multipath device was created by the multipath command,
> > multipathd
> > might not agree with how the device was created. ev_add_map() can
> > reload
> > the device with a different table by calling add_map_without_path() -
> > >
> > update_map(). If this reloading of the map failed, multipathd was
> > simply
> > ignoring the multipath device, even though it still existed.
> > 
> > One way that reloading can fail is if a path that multipathd already
> > has
> > initialized goes offline. If a multipath device is created by the
> > multipath command while the path is offline, it will not use the
> > offline
> > path, since multipath won't be able to get the necessary pathinfo.
> > However, multipathd will already have the pathinfo for the path, and
> > may
> > not even know that it's offline, since the path is an orphan. When it
> > tries to reload the device, it will include the offline path, and the
> > reload will fail.
> 
> Am I understanding correctly that during the reload, bdev_open() failed
> in the kernel because the path was offline?

Yep. It's failing in sd_open() -> scsi_block_when_processing_errors()
see the comment here:
https://github.com/torvalds/linux/blob/5c2a430e85994f4873ea5ec42091baa1153bc731/drivers/scsi/sd.c#L1523
 
> I was thinking about my recent tests [1] where I'd come to the
> conclusion that reload failure is hardly possible. While I'd realized
> that "trying to add a path device that was not previously mapped" might
> fail, I didn't think of the scenario you describe here.
> 
> [1] https://lore.kernel.org/dm-devel/ee6fcbda31fd1f13969653782417fbed748f5bc7.camel@suse.com/
> 
> > 
> > Instead of ignoring the device if it can't reload it, multipathd
> > should
> > just montior it as it is. When the path device is no longer offline,
> > it
> > can be added back to the multipath device by calling
> > "multipathd reconfigure" or "multipathd add path <path>".
> 
> 
> > Signed-off-by: Benjamin Marzinski <bmarzins@redhat.com>
> 
> Reviewed-by: Martin Wilck <mwilck@suse.com>