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

Thadeu Lima de Souza Cascardo
2018-12-13T16:08:03Z

On Thu, Dec 13, 2018@01:19:25PM +0100, Hannes Reinecke wrote:
> On 12/13/18 12:41 PM, Thadeu Lima de Souza Cascardo wrote:
> > On Thu, Dec 13, 2018@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 at 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.
> >
> The whole point of the native nvme multipath implementation was that the
> block devices behind the multipathed device are _NOT_ visible to the OS.
>

They are still partially visible. And the fact that their relationship to
the block device is not visible makes initramfs-tools break, that is, it
can't find the PCI device and its necessary drivers to get to the root
block device.

Also, when we make those relationships clear, lsblk finds those same
"hidden" devices but breaks because it can't find its devno.

> This patch reverts the whole scheme, making it behaving just like the
> classical device-mapper multipathing did.
>

As you are the expert in multipath, would you say that the behavior is the
same? Do we really only care about showing relationships between devices
and hidden block devices as the only differente between those two
implementations? If so, I don't think it's worth it. From the little I
know, they are two different multipath implementations, and hiding devices
is orthogonal to their differences.

> Why can't we just update lsblk to present the correct output?
>
> nvme-cli (and multipath, for that matter) work happily with the current
> setup, _and_ provide the correct topology:
>
> So why can't we do it with lsblk?

Because old userspace is broken by the patches that introduce slaves and
holders. That is, users upgrade their kernels, and, suddenly, lsblk is
broken, and distros need to offer upgrades.

It's not the same with initramfs-tools. It's broken already. And I am
trying to provide a kernel fix that will fix it without requiring any
updates. And if we go the "paths/" path, when a user upgrade their kernels,
they would still need an updated initramfs-tools, but an outdated one does
not regress because of the kernel upgrade. It has already been broken.

Cascardo.

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