[dm-devel] [PATCH v3 2/2] dm: allow device-mapper to operate without dax support

[Dan Williams]

On Mon, Sep 11, 2017 at 7:41 AM, Mike Snitzer <snitzer at redhat.com> wrote:
> On Wed, Aug 02 2017 at  1:58pm -0400,
> Dan Williams <dan.j.williams at intel.com> wrote:
>
>> Rather than have device-mapper directly 'select DAX', let the fact that
>> BLK_DEV_PMEM selects dax act as a gate for the device-mapper dax
>> support. We arrange for all the dax core routines to compile to nops
>> when CONFIG_DAX=n. With that in place we can simply handle the
>> alloc_dax() error as expected and ifdef out the other device-mapper-dax
>> support code.
>>
>> Now, if dax is provided by a leaf driver that driver may only arrange to
>> compile the dax core as a module. Since device-mapper dax support is
>> consumed by the always-built-in portion of the device-mapper
>> implementation we need to upgrade from DAX=m to DAX=y.
>
> I applied the patches and then got nervous once I dug in.. this last
> paragraph makes little sense to me.  "the always-built-in portion of the
> device-mapper implementation" is why: DM core can happily be compiled as
> a module (dm-mod.ko).
>
> And I'm not sure why you're referencing DAX related
> drivers/md/dm-builtin.c, why are you attachd DM's DAX support to that?
> I'm not seeing where that is actually happening.
>
> I don't see why DM's support for DAX would need to force DAX to be
> builtin rather than just a module.
>
> Sorry I didn't get around to looking at this until now, but it seems you
> went wrong along the way?  Or maybe I'm just missing something?
>

No, my fault, I didn't track BLK_DEV_DM_BUILTIN correctly and we can
safely make DAX a module when CONFIG_BLK_DEV_DM=m. I'll fix that up,
thanks for the catch.