[dm-devel] [PATCH 7/8] dm: delay registering the gendisk

[Alasdair G Kergon]

On Wed, Aug 04, 2021 at 11:41:46AM +0200, Christoph Hellwig wrote:
> device mapper is currently the only outlier that tries to call
> register_disk after add_disk, leading to fairly inconsistent state
> of these block layer data structures.  

> Note that this introduces a user visible change: the dm kobject is
> now only visible after the initial table has been loaded.

Indeed.  We should try to document the userspace implications of this
change in a bit more detail.  While lvm2 and any other tools that
followed our recommendations about how to use dm should be OK, there's
always the chance that some other less robustly-written code will need
to make adjustments.

Currently to make a dm device, 3 ioctls are called in sequence:

1. DM_DEV_CREATE  - triggers 'add' uevents
2. DM_TABLE_LOAD
3. DM_SUSPEND     - triggers 'change' uevent

After this patch we have:

1. DM_DEV_CREATE  
2. DM_TABLE_LOAD  - triggers 'add' uevents
3. DM_SUSPEND     - triggers 'change' uevent

The equivalent dmsetup commands for a simple test device are
0. udevadm monitor --kernel --env &   # View the uevents as they happen
1. dmsetup create dev1 --notable
2. dmsetup load --table "0 1 error" dev1
3. dmsetup resume dev1

  => Anyone with a udev rule that relies on 'add' needs to check if they
     need to change their code.

The udev rules that lvm2 uses to synchronise what it is doing rely
only on the 'change' event - which is not moving.  The 'add' event
gets ignored.  

When loading tables, our tools also always refer to devices using
the 'major:minor' format, which isn't affected, rather than using
pathnames in /dev which might not exist now after this change if a table
hasn't been loaded into a referenced device yet.  Previously this was
permissible but we always recommended against it to avoid a pointless
pathname lookup that's subject to races and delays.

So again, any tools that followed our recommendations ought to be
unaffected.

Here's an example of poor code that previously worked but will fail now:
  dmsetup create dev1 --notable
  dmsetup create dev2 --notable
  dmsetup ls  <-- get the minor number of dev1 (say it's 1 corresponding
to dm-1)
  dmsetup load dev2 --table '0 1 linear /dev/dm-1 0'
  ...

Peter - have I missed anything?

Alasdair