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

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Date:	Tue, 28 Jun 2016 20:23:24 +0000
From:	"Kani, Toshimitsu" <toshi.kani@....com>
To:	"snitzer@...hat.com" <snitzer@...hat.com>
CC:	"linux-raid@...r.kernel.org" <linux-raid@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"dan.j.williams@...el.com" <dan.j.williams@...el.com>,
	"dm-devel@...hat.com" <dm-devel@...hat.com>,
	"linux-nvdimm@...1.01.org" <linux-nvdimm@...1.01.org>,
	"agk@...hat.com" <agk@...hat.com>
Subject: Re: [PATCH 1/2] dm: update table type check for dax

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

Thanks,
-Toshi