Re: [PATCH 1/2] dm: update table type check for dax

[Mike Snitzer]

On Tue, Jun 28 2016 at  4:23pm -0400,
Kani, Toshimitsu <toshi.kani@xxxxxxx> wrote:

> On Tue, 2016-06-28 at 16:07 -0400, Mike Snitzer wrote:
> > On Tue, Jun 28 2016 at  3:37pm -0400,
> > Toshi Kani <toshi.kani@xxxxxxx> wrote:
>  :
> > You said in the 0th header: "Patch 1 solves an error when lvremove is
> > made to a snapshot device."
> > 
> > I'm not seeing why this patch 1 fixes anything specific to snapshot
> > device removal (but I can see why patch 2 makes snapshot creation
> > "work").  I'll apply your 2nd patch and see if I can see what you mean.
> > 
> > I actually see this error, without either of your 2 proposed patches
> > applied, when I try to create a snapshot of a DAX capable LV:
> > 
> > # lvcreate -s -n snap -L 100M pmem/lv
> >   device-mapper: reload ioctl on (253:7) failed: Invalid argument
> >   Failed to lock logical volume pmem/lv.
> >   Aborting. Manual intervention required.
> > Jun 28 15:57:28 rhel-storage-02 kernel: device-mapper: ioctl: can't change
> > device type after initial table load.
> 
> Yes, patch 2 fixes this error.
> 
> I have not looked into why lvremove does this, but lvremove to a snapshot
> device fails to reload DM table of "<dev>-lvsnap" device (which is marked as
> DM_TYPE_BIO_BASED) with DM_TYPE_DAX_BIO_BASED.  Patch 1 fixes this error.

It looks like a strange intermediate state that lvm2 uses during
snapshot removal.

Full listing of snapshot related DM tables (before lvremove):

pmem-lv-real: 0 6086656 linear 259:0 2048
pmem-lv: 0 6086656 snapshot-origin 253:5
pmem-snap-cow: 0 204800 linear 259:0 6088704
pmem-snap: 0 6086656 snapshot 253:5 253:6 P 8

When removing this snapshot we're wanting to be left with:

pmem-lv: 0 6086656 linear 259:0 2048

I augmented the DM core error to be more expressive, resulting in:
  device-mapper: ioctl: 253:7: can't change device type (from 1 to 4) after initial table load.

1 is DM_TYPE_BIO_BASED and 4 is DM_TYPE_DAX_BIO_BASED -- which makes
sense given the linear target is DM_TYPE_DAX_BIO_BASED.

The previous DM table for 253:7 was:
  pmem-snap: 0 6086656 snapshot 253:5 253:6 P 8

The intermediate table that lvm2 is trying to load for 253:7 is:
  0 204800 linear 259:0 6088704

(this linear target was previously pmem-snap-cow)

> I think it also generally makes sense to allow this case.

You're probably right but I need to think about it a little bit more.