Re: [PATCH 4/4] block: expose devt for GENHD_FL_HIDDEN disks

[Thadeu Lima de Souza Cascardo <cascardo@canonical.com>]

On Thu, Dec 13, 2018 at 10:18:40AM +0100, Hannes Reinecke wrote:
> On 12/6/18 5:48 PM, Thadeu Lima de Souza Cascardo wrote:
> > Without this exposure, lsblk will fail as it tries to find out the
> > device's dev_t numbers. This causes a real problem for nvme multipath
> > devices, as their slaves are hidden.
> > 
> > Exposing them fixes the problem, even though trying to open the devices
> > returns an error in the case of nvme multipath. So, right now, it's the
> > driver's responsibility to return a failure to open hidden devices.
> > 
> > Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
> > ---
> >   block/genhd.c | 9 ++++-----
> >   1 file changed, 4 insertions(+), 5 deletions(-)
> > 
> > diff --git a/block/genhd.c b/block/genhd.c
> > index 7674fce32fca..65a7fa664188 100644
> > --- a/block/genhd.c
> > +++ b/block/genhd.c
> > @@ -687,6 +687,7 @@ static void __device_add_disk(struct device *parent, struct gendisk *disk,
> >   	disk_alloc_events(disk);
> > +	disk_to_dev(disk)->devt = devt;
> >   	if (disk->flags & GENHD_FL_HIDDEN) {
> >   		/*
> >   		 * Don't let hidden disks show up in /proc/partitions,
> > @@ -698,13 +699,12 @@ static void __device_add_disk(struct device *parent, struct gendisk *disk,
> >   		int ret;
> >   		/* Register BDI before referencing it from bdev */
> > -		disk_to_dev(disk)->devt = devt;
> >   		ret = bdi_register_owner(disk->queue->backing_dev_info,
> >   						disk_to_dev(disk));
> >   		WARN_ON(ret);
> > -		blk_register_region(disk_devt(disk), disk->minors, NULL,
> > -				    exact_match, exact_lock, disk);
> >   	}
> > +	blk_register_region(disk_devt(disk), disk->minors, NULL,
> > +			    exact_match, exact_lock, disk);
> >   	register_disk(parent, disk, groups);
> >   	if (register_queue)
> >   		blk_register_queue(disk);
> > @@ -776,8 +776,7 @@ void del_gendisk(struct gendisk *disk)
> >   		WARN_ON(1);
> >   	}
> > -	if (!(disk->flags & GENHD_FL_HIDDEN))
> > -		blk_unregister_region(disk_devt(disk), disk->minors);
> > +	blk_unregister_region(disk_devt(disk), disk->minors);
> >   	kobject_put(disk->part0.holder_dir);
> >   	kobject_put(disk->slave_dir);
> > 
> Welll ... this is not just 'lsblk', but more importantly this will force
> udev to create _block_ device nodes for the hidden devices, essentially
> 'unhide' them.
> 
> Is this what we want?
> Christoph?
> I thought the entire _point_ of having hidden devices is that the are ...
> well ... hidden ...
> 

I knew this would be the most controversial patch. And I had other solutions as
well, but preferred this one. So, the other alternative would be just not use
GENHD_FL_HIDDEN for the nvme devices, which would leave that flag without a
single user in the kernel. That would still fix the two problems
(initramfs-tools and lsblk), and not create any other problem I know of. That
patch would still fail to open the underlying devices when there is a
head/multipath associated with it.

So, the only thing GENHD_FL_HIDDEN gives us is saving some resources, like bdi,
for example. And we could also use it to fail open when blkdev_get* is called.
Of couse, that would still imply that its name should be changed, but as we
already have an attribute named after that, I find it hard to suggest such a
change.

Cascardo.

> Cheers,
> 
> Hannes
> -- 
> Dr. Hannes Reinecke		   Teamlead Storage & Networking
> hare@suse.de			               +49 911 74053 688
> SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
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